What s the strangest corner case you ve seen in C or NET

Question

I collect a few corner cases and brain teasers and would always like to hear more  The page only really covers C  language bits and bobs  but I also find core  NET things interesting too  For example  here s one which isn t on the page  but which I find incredible   string x   new string new char 0    string y   new string new char 0    Console WriteLine object ReferenceEquals x  y      I d expect that to print False - after all   new   with a reference type  always creates a new object  doesn t it  The specs for both C  and the CLI indicate that it should   Well  not in this particular case  It prints True  and has done on every version of the framework I ve tested it with   I haven t tried it on Mono  admittedly      Just to be clear  this is only an example of the kind of thing I m looking for - I wasn t particularly looking for discussion explanation of this oddity   It s not the same as normal string interning  in particular  string interning doesn t normally happen when a constructor is called   I was really asking for similar odd behaviour   Any other gems lurking out there

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I think the answer to the question is because .net uses string interning something that might cause equal strings to point to the same object (since a strings are mutable this is not a problem)

(I'm not talking about the overridden equality operator on the string class)

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This one had me truly puzzled  I apologise for the length but it s WinForm   I posted it in the newsgroups a while back      I ve come across an interesting bug  I   have workarounds but i d like to know   the root of the problem  I ve stripped   it down into a short file and hope   someone might have an idea about   what s going on       It s a simple program that loads a   control onto a form and binds  Foo    against a combobox   SelectedItem     for it s  Bar  property and a   datetimepicker   Value   for it s    DateTime  property  The   DateTimePicker Visible value is set to   false  Once it s loaded up  select the   combobox and then attempt to deselect   it by selecting the checkbox  This is   rendered impossible by the combobox   retaining the focus  you cannot even   close the form  such is it s grasp on   the focus       I have found three ways of fixing this   problem       a  Remove the binding to Bar  a bit   obvious        b  Remove the binding to   DateTime       c  Make the DateTimePicker   visible          I m currently running Win2k  And  NET   2 00  I think 1 1 has the same problem  Code is below    using System  using System Collections  using System Windows Forms   namespace WindowsApplication6       public class Bar               public Bar                                  public class Foo               private Bar m Bar   new Bar            private DateTime m DateTime   DateTime Now           public Foo                                public Bar Bar                       get                               return m Bar                            set                               m Bar   value                                   public DateTime DateTime                       get                               return m DateTime                            set                               m DateTime   value                                     public class TestBugControl   UserControl               public TestBugControl                         InitializeComponent                       public void InitializeData IList types                        this cBoxType DataSource   types                     public void BindFoo Foo foo                        this cBoxType DataBindings Add  SelectedItem   foo   Bar                this dtStart DataBindings Add  Value   foo   DateTime                            lt summary gt              Required designer variable               lt  summary gt          private System ComponentModel IContainer components   null                lt summary gt              Clean up any resources being used               lt  summary gt               lt param name  disposing  gt true if managed resources should be disposed  otherwise  false  lt  param gt          protected override void Dispose bool disposing                        if  disposing  amp  amp   components    null                                 components Dispose                              base Dispose disposing                       region Component Designer generated code               lt summary gt              Required method for Designer support - do not modify             the contents of this method with the code editor               lt  summary gt          private void InitializeComponent                         this checkBox1   new System Windows Forms CheckBox                this cBoxType   new System Windows Forms ComboBox                this dtStart   new System Windows Forms DateTimePicker                this SuspendLayout                                  checkBox1                            this checkBox1 AutoSize   true              this checkBox1 Location   new System Drawing Point 14  5               this checkBox1 Name    checkBox1               this checkBox1 Size   new System Drawing Size 97  20               this checkBox1 TabIndex   0              this checkBox1 Text    checkBox1               this checkBox1 UseVisualStyleBackColor   true                                cBoxType                            this cBoxType FormattingEnabled   true              this cBoxType Location   new System Drawing Point 117  3               this cBoxType Name    cBoxType               this cBoxType Size   new System Drawing Size 165  24               this cBoxType TabIndex   1                                dtStart                            this dtStart Location   new System Drawing Point 117  40               this dtStart Name    dtStart               this dtStart Size   new System Drawing Size 165  23               this dtStart TabIndex   2              this dtStart Visible   false                                TestBugControl                            this AutoScaleDimensions   new System Drawing SizeF 8F  16F               this AutoScaleMode   System Windows Forms AutoScaleMode Font              this Controls Add this dtStart               this Controls Add this cBoxType               this Controls Add this checkBox1               this Font   new System Drawing Font  Verdana   9 75F              System Drawing FontStyle Regular  System Drawing GraphicsUnit Point                byte  0                 this Margin   new System Windows Forms Padding 4               this Name    TestBugControl               this Size   new System Drawing Size 285  66               this ResumeLayout false               this PerformLayout                         endregion          private System Windows Forms CheckBox checkBox1          private System Windows Forms ComboBox cBoxType          private System Windows Forms DateTimePicker dtStart             public class Form1   Form               public Form1                         InitializeComponent                this Load    new EventHandler Form1 Load                      void Form1 Load object sender  EventArgs e                        InitializeControl                       public void InitializeControl                         TestBugControl control   new TestBugControl                IList list   new ArrayList                for  int i   0  i  lt  10  i                                  list Add new Bar                               control InitializeData list               control BindFoo new Foo                 this Controls Add control                           lt summary gt              Required designer variable               lt  summary gt          private System ComponentModel IContainer components   null                lt summary gt              Clean up any resources being used               lt  summary gt               lt param name  disposing  gt true if managed resources should be disposed  otherwise  false  lt  param gt          protected override void Dispose bool disposing                        if  disposing  amp  amp   components    null                                 components Dispose                              base Dispose disposing                       region Windows Form Designer generated code               lt summary gt              Required method for Designer support - do not modify             the contents of this method with the code editor               lt  summary gt          private void InitializeComponent                         this components   new System ComponentModel Container                this AutoScaleMode   System Windows Forms AutoScaleMode Font              this Text    Form1                       endregion            static class Program                    lt summary gt              The main entry point for the application               lt  summary gt           STAThread          static void Main                         Application EnableVisualStyles                Application SetCompatibleTextRenderingDefault false               Application Run new Form1

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In an API we re using  methods that return a domain object might return a special  null object   In the implementation of this  the comparison operator and the Equals   method are overridden to return true if it is compared with null   So a user of this API might have some code like this   return test    null   test   GetDefault      or perhaps a bit more verbose  like this   if  test    null      return GetDefault    return test    where GetDefault   is a method returning some default value that we want to use instead of null  The surprise hit me when I was using ReSharper and following it s recommendation to rewrite either of this to the following   return test    GetDefault      If the test object is a null object returned from the API instead of a proper null  the behavior of the code has now changed  as the null coalescing operator actually checks for null  not running operator  or Equals

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Bankers  Rounding   This one is not so much a compiler bug or malfunction  but certainly a strange corner case     The  Net Framework employs a scheme or rounding known as Banker s Rounding   In Bankers  Rounding the 0 5 numbers are rounded to the nearest even number  so  Math Round -0 5     0 Math Round 0 5     0 Math Round 1 5     2 Math Round 2 5     2 etc      This can lead to some unexpected bugs in financial calculations based on the more well known Round-Half-Up rounding   This is also true of Visual Basic

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Bankers  Rounding   This one is not so much a compiler bug or malfunction  but certainly a strange corner case     The  Net Framework employs a scheme or rounding known as Banker s Rounding   In Bankers  Rounding the 0 5 numbers are rounded to the nearest even number  so  Math Round -0 5     0 Math Round 0 5     0 Math Round 1 5     2 Math Round 2 5     2 etc      This can lead to some unexpected bugs in financial calculations based on the more well known Round-Half-Up rounding   This is also true of Visual Basic

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From a question I asked not long ago   Conditional operator cannot cast implicitly   Given   Bool aBoolValue    Where aBoolValue is assigned either True or False   The following will not compile   Byte aByteValue   aBoolValue   1   0    But this would   Int anIntValue   aBoolValue   1   0    The answer provided is pretty good too

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Here s one I only found out about recently     interface IFoo      string Message  get         IFoo obj   new IFoo  abc    Console WriteLine obj Message     The above looks crazy at first glance  but is actually legal No  really  although I ve missed out a key part  but it isn t anything hacky like  add a class called IFoo  or  add a using alias to point IFoo at a class      See if you can figure out why  then  Who says you can   t instantiate an interface

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Public Class Item    Public ID As Guid    Public Text As String     Public Sub New ByVal id As Guid  ByVal name As String        Me ID   id       Me Text   name    End Sub End Class  Public Sub Load sender As Object  e As EventArgs  Handles Me Load    Dim box As New ComboBox    Me Controls Add box            Sorry I forgot this line the first time      Dim h As IntPtr   box Handle   Im not sure you need this but you might      Try       box Items Add New Item Guid Empty  Nothing      Catch ex As Exception       MsgBox ex ToString       End Try End Sub   The output is  Attempted to read protected memory  This is an indication that other memory is corrupt

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When is a Boolean neither True nor False   Bill discovered that you can hack a boolean so that if A is True and B is True   A and B  is False   Hacked Booleans

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Here is an example of how you can create a struct that causes the error message  Attempted to read or write protected memory  This is often an indication that other memory is corrupt   The difference between success and failure is very subtle   The following unit test demonstrates the problem   See if you can work out what went wrong        Test      public void Test                 var bar   new MyClass                       Foo   500                    bar Foo    500           Assert That bar Foo Value Amount  Is EqualTo 1000               private class MyClass               public MyStruct  Foo   get  set               private struct MyStruct               public decimal Amount   get  private set             public MyStruct decimal amount    this                         Amount   amount                     public static MyStruct operator   MyStruct x  MyStruct y                        return new MyStruct x Amount   y Amount                      public static MyStruct operator   MyStruct x  decimal y                        return new MyStruct x Amount   y                      public static implicit operator MyStruct int value                        return new MyStruct value                      public static implicit operator MyStruct decimal value                        return new MyStruct value

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PropertyInfo SetValue   can assign ints to enums  ints to nullable ints  enums to nullable enums  but not ints to nullable enums   enumProperty SetValue obj  1  null     works nullableIntProperty SetValue obj  1  null     works nullableEnumProperty SetValue obj  MyEnum Foo  null     works nullableEnumProperty SetValue obj  1  null      throws an exception       Full description here

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Just found a nice little thing today   public class Base      public virtual void Initialize dynamic stuff                     public class Derived Base      public override void Initialize dynamic stuff       base Initialize stuff                     This throws compile error     The call to method  Initialize  needs to be dynamically dispatched  but cannot be because it is part of a base access expression  Consider casting the dynamic arguments or eliminating the base access   If I write base Initialize stuff as object   it works perfectly  however this seems to be a  magic word  here  since it does exactly the same  everything is still recieved as dynamic

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I m not sure if you d say this is a Windows Vista 7 oddity or a  Net oddity but it had me scratching my head for a while   string filename     c  program files my folder test txt   System IO File WriteAllText filename   Hello world     bool exists   System IO File Exists filename      returns true  string text   System IO File ReadAllText filename      Returns  Hello world     In Windows Vista 7 the file will actually be written to C  Users  lt username gt  Virtual Store Program Files my folder test txt

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What if you have a generic class that has methods that could be made ambiguous depending on the type arguments  I ran into this situation recently writing a two-way dictionary  I wanted to write symmetric Get   methods that would return the opposite of whatever argument was passed  Something like this   class TwoWayRelationship lt T1  T2 gt        public T2 Get T1 key                    public T1 Get T2 key                    All is well good if you make an instance where T1 and T2 are different types   var r1   new TwoWayRelationship lt int  string gt     r1 Get 1   r1 Get  a      But if T1 and T2 are the same  and probably if one was a subclass of another   it s a compiler error   var r2   new TwoWayRelationship lt int  int gt     r2 Get 1        The call is ambiguous       Interestingly  all other methods in the second case are still usable  it s only calls to the now-ambiguous method that causes a compiler error  Interesting case  if a little unlikely and obscure

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Few years ago  when working on loyality program  we had an issue with the amount of points given to customers  The issue was related to casting converting double to int   In code below   double d   13 6   int i1   Convert ToInt32 d   int i2    int d    does i1    i2    It turns out that i1    i2  Because of different rounding policies in Convert and cast operator the actual values are   i1    14 i2    13   It s always better to call Math Ceiling   or Math Floor    or Math Round with MidpointRounding that meets our requirements   int i1   Convert ToInt32  Math Ceiling d     int i2    int  Math Ceiling d

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What will this function do if called as Rec 0   not under the debugger    static void Rec int i        Console WriteLine i       if  i  lt  int MaxValue                Rec i   1             Answer    On 32-bit JIT it should result in a StackOverflowException On 64-bit JIT it should print all the numbers to int MaxValue   This is because the 64-bit JIT compiler applies tail call optimisation  whereas the 32-bit JIT does not    Unfortunately I haven t got a 64-bit machine to hand to verify this  but the method does meet all the conditions for tail-call optimisation  If anybody does have one I d be interested to see if it s true

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Consider this weird case   public interface MyInterface     void Method      public class Base     public void Method         public class Derived   Base  MyInterface       If Base and Derived are declared in the same assembly  the compiler will make Base  Method virtual and sealed  in the CIL   even though Base doesn t implement the interface    If Base and Derived are in different assemblies  when compiling the Derived assembly  the compiler won t change the other assembly  so it will introduce a member in Derived that will be an explicit implementation for MyInterface  Method that will just delegate the call to Base  Method   The compiler has to do this in order to support polymorphic dispatch with regards to the interface  i e  it has to make that method virtual

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VB NET  nullables and the ternary operator   Dim i As Integer    If True  Nothing  5    This took me some time to debug  since I expected i to contain Nothing   What does i really contain  0   This is surprising but actually  correct  behavior  Nothing in VB NET is not exactly the same as null in CLR  Nothing can either mean null or default T  for a value type T  depending on the context  In the above case  If infers Integer as the common type of Nothing and 5  so  in this case  Nothing means 0

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here are a few of mine    this can be null when calling an instance method with out a NullReferenceException being thrown a default enumeration value doesn t have to be defined for the enumeration   Simple one first       enum NoZero                               Number   1                        public bool ReturnsFalse                           The default value is not defined              return Enum IsDefined typeof  NoZero   default NoZero                The below code can actually print true    internal sealed class Strange       public void Foo                 Console WriteLine this    null             A simple piece of client code that will result in that is    delegate void HelloDelegate Strange bar    public class Program        STAThread        public static void Main string   args                Strange bar   null          var hello   new DynamicMethod  ThisIsNull               typeof void   new     typeof Strange              typeof Strange  Module           ILGenerator il   hello GetILGenerator 256           il Emit OpCodes Ldarg 0           var foo   typeof Strange  GetMethod  Foo            il Emit OpCodes Call  foo           il Emit OpCodes Ret           var print    HelloDelegate hello CreateDelegate typeof HelloDelegate            print bar           Console ReadLine              this is actually true in most languages as long as the instance method when called doesn t use the state of the object  this is only dereferenced when the state of the object is accessed

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here are a few of mine    this can be null when calling an instance method with out a NullReferenceException being thrown a default enumeration value doesn t have to be defined for the enumeration   Simple one first       enum NoZero                               Number   1                        public bool ReturnsFalse                           The default value is not defined              return Enum IsDefined typeof  NoZero   default NoZero                The below code can actually print true    internal sealed class Strange       public void Foo                 Console WriteLine this    null             A simple piece of client code that will result in that is    delegate void HelloDelegate Strange bar    public class Program        STAThread        public static void Main string   args                Strange bar   null          var hello   new DynamicMethod  ThisIsNull               typeof void   new     typeof Strange              typeof Strange  Module           ILGenerator il   hello GetILGenerator 256           il Emit OpCodes Ldarg 0           var foo   typeof Strange  GetMethod  Foo            il Emit OpCodes Call  foo           il Emit OpCodes Ret           var print    HelloDelegate hello CreateDelegate typeof HelloDelegate            print bar           Console ReadLine              this is actually true in most languages as long as the instance method when called doesn t use the state of the object  this is only dereferenced when the state of the object is accessed

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Here s one I only found out about recently     interface IFoo      string Message  get         IFoo obj   new IFoo  abc    Console WriteLine obj Message     The above looks crazy at first glance  but is actually legal No  really  although I ve missed out a key part  but it isn t anything hacky like  add a class called IFoo  or  add a using alias to point IFoo at a class      See if you can figure out why  then  Who says you can   t instantiate an interface

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The following prints False instead of throwing an overflow exception   Console WriteLine   0    yep int MaxValue       private bool yep  int val         return   0  lt  val   2

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The following prints False instead of throwing an overflow exception   Console WriteLine   0    yep int MaxValue       private bool yep  int val         return   0  lt  val   2

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Public Class Item    Public ID As Guid    Public Text As String     Public Sub New ByVal id As Guid  ByVal name As String        Me ID   id       Me Text   name    End Sub End Class  Public Sub Load sender As Object  e As EventArgs  Handles Me Load    Dim box As New ComboBox    Me Controls Add box            Sorry I forgot this line the first time      Dim h As IntPtr   box Handle   Im not sure you need this but you might      Try       box Items Add New Item Guid Empty  Nothing      Catch ex As Exception       MsgBox ex ToString       End Try End Sub   The output is  Attempted to read protected memory  This is an indication that other memory is corrupt

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This one had me truly puzzled  I apologise for the length but it s WinForm   I posted it in the newsgroups a while back      I ve come across an interesting bug  I   have workarounds but i d like to know   the root of the problem  I ve stripped   it down into a short file and hope   someone might have an idea about   what s going on       It s a simple program that loads a   control onto a form and binds  Foo    against a combobox   SelectedItem     for it s  Bar  property and a   datetimepicker   Value   for it s    DateTime  property  The   DateTimePicker Visible value is set to   false  Once it s loaded up  select the   combobox and then attempt to deselect   it by selecting the checkbox  This is   rendered impossible by the combobox   retaining the focus  you cannot even   close the form  such is it s grasp on   the focus       I have found three ways of fixing this   problem       a  Remove the binding to Bar  a bit   obvious        b  Remove the binding to   DateTime       c  Make the DateTimePicker   visible          I m currently running Win2k  And  NET   2 00  I think 1 1 has the same problem  Code is below    using System  using System Collections  using System Windows Forms   namespace WindowsApplication6       public class Bar               public Bar                                  public class Foo               private Bar m Bar   new Bar            private DateTime m DateTime   DateTime Now           public Foo                                public Bar Bar                       get                               return m Bar                            set                               m Bar   value                                   public DateTime DateTime                       get                               return m DateTime                            set                               m DateTime   value                                     public class TestBugControl   UserControl               public TestBugControl                         InitializeComponent                       public void InitializeData IList types                        this cBoxType DataSource   types                     public void BindFoo Foo foo                        this cBoxType DataBindings Add  SelectedItem   foo   Bar                this dtStart DataBindings Add  Value   foo   DateTime                            lt summary gt              Required designer variable               lt  summary gt          private System ComponentModel IContainer components   null                lt summary gt              Clean up any resources being used               lt  summary gt               lt param name  disposing  gt true if managed resources should be disposed  otherwise  false  lt  param gt          protected override void Dispose bool disposing                        if  disposing  amp  amp   components    null                                 components Dispose                              base Dispose disposing                       region Component Designer generated code               lt summary gt              Required method for Designer support - do not modify             the contents of this method with the code editor               lt  summary gt          private void InitializeComponent                         this checkBox1   new System Windows Forms CheckBox                this cBoxType   new System Windows Forms ComboBox                this dtStart   new System Windows Forms DateTimePicker                this SuspendLayout                                  checkBox1                            this checkBox1 AutoSize   true              this checkBox1 Location   new System Drawing Point 14  5               this checkBox1 Name    checkBox1               this checkBox1 Size   new System Drawing Size 97  20               this checkBox1 TabIndex   0              this checkBox1 Text    checkBox1               this checkBox1 UseVisualStyleBackColor   true                                cBoxType                            this cBoxType FormattingEnabled   true              this cBoxType Location   new System Drawing Point 117  3               this cBoxType Name    cBoxType               this cBoxType Size   new System Drawing Size 165  24               this cBoxType TabIndex   1                                dtStart                            this dtStart Location   new System Drawing Point 117  40               this dtStart Name    dtStart               this dtStart Size   new System Drawing Size 165  23               this dtStart TabIndex   2              this dtStart Visible   false                                TestBugControl                            this AutoScaleDimensions   new System Drawing SizeF 8F  16F               this AutoScaleMode   System Windows Forms AutoScaleMode Font              this Controls Add this dtStart               this Controls Add this cBoxType               this Controls Add this checkBox1               this Font   new System Drawing Font  Verdana   9 75F              System Drawing FontStyle Regular  System Drawing GraphicsUnit Point                byte  0                 this Margin   new System Windows Forms Padding 4               this Name    TestBugControl               this Size   new System Drawing Size 285  66               this ResumeLayout false               this PerformLayout                         endregion          private System Windows Forms CheckBox checkBox1          private System Windows Forms ComboBox cBoxType          private System Windows Forms DateTimePicker dtStart             public class Form1   Form               public Form1                         InitializeComponent                this Load    new EventHandler Form1 Load                      void Form1 Load object sender  EventArgs e                        InitializeControl                       public void InitializeControl                         TestBugControl control   new TestBugControl                IList list   new ArrayList                for  int i   0  i  lt  10  i                                  list Add new Bar                               control InitializeData list               control BindFoo new Foo                 this Controls Add control                           lt summary gt              Required designer variable               lt  summary gt          private System ComponentModel IContainer components   null                lt summary gt              Clean up any resources being used               lt  summary gt               lt param name  disposing  gt true if managed resources should be disposed  otherwise  false  lt  param gt          protected override void Dispose bool disposing                        if  disposing  amp  amp   components    null                                 components Dispose                              base Dispose disposing                       region Windows Form Designer generated code               lt summary gt              Required method for Designer support - do not modify             the contents of this method with the code editor               lt  summary gt          private void InitializeComponent                         this components   new System ComponentModel Container                this AutoScaleMode   System Windows Forms AutoScaleMode Font              this Text    Form1                       endregion            static class Program                    lt summary gt              The main entry point for the application               lt  summary gt           STAThread          static void Main                         Application EnableVisualStyles                Application SetCompatibleTextRenderingDefault false               Application Run new Form1

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Interesting - when I first looked at that I assumed it was something the C  compiler was checking for  but even if you emit the IL directly to remove any chance of interference it still happens  which means it really is the newobj op-code that s doing the checking   var method   new DynamicMethod  Test   null  null   var il   method GetILGenerator     il Emit OpCodes Ldc I4 0   il Emit OpCodes Newarr  typeof char    il Emit OpCodes Newobj  typeof string  GetConstructor new     typeof char          il Emit OpCodes Ldc I4 0   il Emit OpCodes Newarr  typeof char    il Emit OpCodes Newobj  typeof string  GetConstructor new     typeof char          il Emit OpCodes Call  typeof object  GetMethod  ReferenceEquals     il Emit OpCodes Box  typeof bool    il Emit OpCodes Call  typeof Console  GetMethod  WriteLine   new     typeof object        il Emit OpCodes Ret    method Invoke null  null     It also equates to true if you check against string Empty which means this op-code must have special behaviour to intern empty strings

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If you have the extension method   public static bool  ToBoolean this string s        bool result       if  bool TryParse s  out result           return result      else         return null      and this code   string nullStr   null  var res   nullStr ToBoolean      This will not throw an exception because it is an extension method  and really HelperClass ToBoolean null   and not an instance method  This can be confusing

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Bankers  Rounding   This one is not so much a compiler bug or malfunction  but certainly a strange corner case     The  Net Framework employs a scheme or rounding known as Banker s Rounding   In Bankers  Rounding the 0 5 numbers are rounded to the nearest even number  so  Math Round -0 5     0 Math Round 0 5     0 Math Round 1 5     2 Math Round 2 5     2 etc      This can lead to some unexpected bugs in financial calculations based on the more well known Round-Half-Up rounding   This is also true of Visual Basic

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The following might be general knowledge I was just simply lacking  but eh  Some time ago  we had a bug case which included virtual properties  Abstracting the context a bit  consider the following code  and apply breakpoint to specified area    class Program       static void Main string   args                Derived d   new Derived            d Property    AWESOME            class Base       string  baseProp      public virtual string Property                 get                        return  BASE      baseProp                    set                        baseProp   value                do work with the base property which might                not be exposed to derived types               here             Console Out WriteLine   baseProp is BASE     value ToString                        class Derived   Base       string  prop      public override string Property                get   return  prop            set                          prop   value               base Property   value               lt - put a breakpoint here then mouse over BaseProperty                  and then mouse over the base Property call inside it             public string BaseProperty   get   return base Property    private set           While in the Derived object context  you can get the same behavior when adding base Property as a watch  or typing base Property into the quickwatch    Took me some time to realize what was going on  In the end I was enlightened by the Quickwatch  When going into the Quickwatch and exploring the Derived object d  or from the object s context  this  and selecting the field base  the edit field on top of the Quickwatch displays the following cast     TestProject1 Base  d     Which means that if base is replaced as such  the call would be  public string BaseProperty   get   return   TestProject1 Base  d   Property    private set         for the Watches  Quickwatch and the debugging mouse-over tooltips  and it would then make sense for it to display  AWESOME  instead of  BASE AWESOME  when considering polymorphism  I m still unsure why it would transform it into a cast  one hypothesis is that call might not be available from those modules  context  and only callvirt   Anyhow  that obviously doesn t alter anything in terms of functionality  Derived BaseProperty will still really return  BASE AWESOME   and thus this was not the root of our bug at work  simply a confusing component  I did however find it interesting how it could mislead developpers which would be unaware of that fact during their debug sessions  specially if Base is not exposed in your project but rather referenced as a 3rd party DLL  resulting in Devs just saying        Oi  wait  what   omg that DLL is   like    doing something funny

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I think I showed you this one before  but I like the fun here - this took some debugging to track down   the original code was obviously more complex and subtle          static void Foo lt T gt    where T   new                 T t   new T            Console WriteLine t ToString        works fine         Console WriteLine t GetHashCode        works fine         Console WriteLine t Equals t       works fine             so it looks like an object and smells like an object                but this throws a NullReferenceException            Console WriteLine t GetType             So what was T     Answer  any Nullable lt T gt  - such as int   All the methods are overridden  except GetType   which can t be  so it is cast  boxed  to object  and hence to null  to call object GetType      which calls on null  -p    Update  the plot thickens    Ayende Rahien threw down a similar challenge on his blog  but with a where T   class  new     private static void Main         CanThisHappen lt MyFunnyType gt        public static void CanThisHappen lt T gt    where T   class  new         var instance   new T       new   on a ref-type  should be non-null  then     Debug Assert instance    null   How did we break the CLR         But it can be defeated  Using the same indirection used by things like remoting  warning - the following is pure evil   class MyFunnyProxyAttribute   ProxyAttribute       public override MarshalByRefObject CreateInstance Type serverType            return null           MyFunnyProxy  class MyFunnyType   ContextBoundObject       With this in place  the new   call is redirected to the proxy  MyFunnyProxyAttribute   which returns null  Now go and wash your eyes

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This is one of the most unusual i ve seen so far  aside from the ones here of course     public class Turtle lt T gt  where T   Turtle lt T gt        It lets you declare it but has no real use  since it will always ask you to wrap whatever class you stuff in the center with another Turtle    joke  I guess it s turtles all the way down      joke

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The scoping in c  is truly bizarre at times  Lets me give you one example   if  true       OleDbCommand command   SQLServer CreateCommand       OleDbCommand command   SQLServer CreateCommand      This fails to compile  because command is redeclared  There are some interested guesswork as to why it works that way in this thread on stackoverflow and in my blog

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VB NET  nullables and the ternary operator   Dim i As Integer    If True  Nothing  5    This took me some time to debug  since I expected i to contain Nothing   What does i really contain  0   This is surprising but actually  correct  behavior  Nothing in VB NET is not exactly the same as null in CLR  Nothing can either mean null or default T  for a value type T  depending on the context  In the above case  If infers Integer as the common type of Nothing and 5  so  in this case  Nothing means 0

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C  supports conversions between arrays and lists as long as the arrays are not multidimensional and there is an inheritance relation between the types and the types are reference types  object   oArray   new string      one    two    three     string   sArray    string   oArray      Also works for IList  and IEnumerable  ICollection  IList lt string gt  sList    IList lt string gt  oArray  IList lt object gt  oList   new string      one    two    three       Note that this does not work   object   oArray2   new int     1  2  3       Error  Cannot implicitly convert type  int    to  object    int   iArray    int   oArray2                Error  Cannot convert type  object    to  int

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The following doesn t work   if  something      doit    else     var v   1   2    But this works   if  something      doit    else       var v   1   2

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What will this function do if called as Rec 0   not under the debugger    static void Rec int i        Console WriteLine i       if  i  lt  int MaxValue                Rec i   1             Answer    On 32-bit JIT it should result in a StackOverflowException On 64-bit JIT it should print all the numbers to int MaxValue   This is because the 64-bit JIT compiler applies tail call optimisation  whereas the 32-bit JIT does not    Unfortunately I haven t got a 64-bit machine to hand to verify this  but the method does meet all the conditions for tail-call optimisation  If anybody does have one I d be interested to see if it s true

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I m arriving a bit late to the party  but I ve got three four five    If you poll InvokeRequired on a control that hasn t been loaded shown  it will say false - and blow up in your face if you try to change it from another thread  the solution is to reference this Handle in the creator of the control   Another one which tripped me up is that given an assembly with   enum MyEnum       Red      Blue      if you calculate MyEnum Red ToString   in another assembly  and in between times someone has recompiled your enum to   enum MyEnum       Black      Red      Blue      at runtime  you will get  Black   I had a shared assembly with some handy constants in  My predecessor had left a load of ugly-looking get-only properties  I thought I d get rid of the clutter and just use public const  I was more than a little surprised when VS compiled them to their values  and not references  If you implement a new method of an interface from another assembly  but you rebuild referencing the old version of that assembly  you get a TypeLoadException  no implementation of  NewMethod    even though you have implemented it  see here   Dictionary lt      The order in which the items are returned is undefined   This is horrible  because it can bite you sometimes  but work others  and if you ve just blindly assumed that Dictionary is going to play nice   why shouldn t it  I thought  List does    you really have to have your nose in it before you finally start to question your assumption

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What if you have a generic class that has methods that could be made ambiguous depending on the type arguments  I ran into this situation recently writing a two-way dictionary  I wanted to write symmetric Get   methods that would return the opposite of whatever argument was passed  Something like this   class TwoWayRelationship lt T1  T2 gt        public T2 Get T1 key                    public T1 Get T2 key                    All is well good if you make an instance where T1 and T2 are different types   var r1   new TwoWayRelationship lt int  string gt     r1 Get 1   r1 Get  a      But if T1 and T2 are the same  and probably if one was a subclass of another   it s a compiler error   var r2   new TwoWayRelationship lt int  int gt     r2 Get 1        The call is ambiguous       Interestingly  all other methods in the second case are still usable  it s only calls to the now-ambiguous method that causes a compiler error  Interesting case  if a little unlikely and obscure

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They should have made 0 an integer even when there s an enum function overload   I knew C  core team rationale for mapping 0 to enum  but still  it is not as orthogonal as it should be   Example from Npgsql   Test example   namespace Craft       enum Symbol   Alpha   1  Beta   2  Gamma   3  Delta   4         class Mate               static void Main string   args                         JustTest Symbol Alpha      enum             JustTest 0      why enum             JustTest  int 0      why still enum              int i   0               JustTest Convert ToInt32 0       have to use Convert ToInt32 to convince the compiler to make the call site use the object version              JustTest i      it s ok from down here and below             JustTest 1               JustTest  string                JustTest Guid NewGuid                 JustTest new DataTable                  Console ReadLine                       static void JustTest Symbol a                        Console WriteLine  Enum                       static void JustTest object o                        Console WriteLine  Object

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When is a Boolean neither True nor False   Bill discovered that you can hack a boolean so that if A is True and B is True   A and B  is False   Hacked Booleans

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Have you ever thought the C  compiler could generate invalid CIL  Run this and you ll get a TypeLoadException   interface I lt T gt      T M T p     abstract class A lt T gt    I lt T gt      public abstract T M T p     abstract class B lt T gt    A lt T gt   I lt int gt      public override T M T p    return p      public int M int p    return p   2      class C   B lt int gt       class Program     static void Main string   args        Console WriteLine new C   M 42            I don t know how it fares in the C  4 0 compiler though   EDIT  this is the output from my system   C  Temp gt type Program cs using System  using System Collections Generic  using System Linq  using System Text   namespace ConsoleApplication1      interface I lt T gt        T M T p         abstract class A lt T gt    I lt T gt        public abstract T M T p         abstract class B lt T gt    A lt T gt   I lt int gt        public override T M T p    return p        public int M int p    return p   2          class C   B lt int gt         class Program       static void Main string   args          Console WriteLine new C   M 11                 C  Temp gt csc Program cs Microsoft  R  Visual C  2008 Compiler version 3 5 30729 1 for Microsoft  R   NET Framework version 3 5 Copyright  C  Microsoft Corporation  All rights reserved    C  Temp gt Program  Unhandled Exception  System TypeLoadException  Could not load type  ConsoleAppli cation1 C  from assembly  Program  Version 0 0 0 0  Culture neutral  PublicKeyTo ken null      at ConsoleApplication1 Program Main String   args   C  Temp gt peverify Program exe  Microsoft  R   NET Framework PE Verifier   Version  3 5 30729 1 Copyright  c  Microsoft Corporation   All rights reserved    token  0x02000005  Type load failed   IL   Error   C  Temp Program exe   ConsoleApplication1 Program  Main  offset 0x 00000001  Unable to resolve token  2 Error s  Verifying Program exe  C  Temp gt ver  Microsoft Windows XP  Version 5 1 2600

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This is the strangest I ve encountered by accident   public class DummyObject       public override string ToString                 return null            Used as follows   DummyObject obj   new DummyObject    Console WriteLine  The text      obj GetType       is     obj     Will throw a NullReferenceException  Turns out the multiple additions are compiled by the C  compiler to a call to String Concat object     Prior to  NET 4  there is a bug in just that overload of Concat where the object is checked for null  but not the result of ToString     object obj2   args i   string text    obj2    null    obj2 ToString     string Empty     if obj2 is non-null  but obj2 ToString   returns null  then text  null int length   text Length    This is a bug by ECMA-334   14 7 4      The binary   operator performs string concatenation when one or both operands are of type string  If an operand of string concatenation is null  an empty string is substituted  Otherwise  any non-string operand is converted to its string representation by invoking the virtual ToString method inherited from type object  If ToString returns null  an empty string is substituted

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Interesting - when I first looked at that I assumed it was something the C  compiler was checking for  but even if you emit the IL directly to remove any chance of interference it still happens  which means it really is the newobj op-code that s doing the checking   var method   new DynamicMethod  Test   null  null   var il   method GetILGenerator     il Emit OpCodes Ldc I4 0   il Emit OpCodes Newarr  typeof char    il Emit OpCodes Newobj  typeof string  GetConstructor new     typeof char          il Emit OpCodes Ldc I4 0   il Emit OpCodes Newarr  typeof char    il Emit OpCodes Newobj  typeof string  GetConstructor new     typeof char          il Emit OpCodes Call  typeof object  GetMethod  ReferenceEquals     il Emit OpCodes Box  typeof bool    il Emit OpCodes Call  typeof Console  GetMethod  WriteLine   new     typeof object        il Emit OpCodes Ret    method Invoke null  null     It also equates to true if you check against string Empty which means this op-code must have special behaviour to intern empty strings

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When is a Boolean neither True nor False   Bill discovered that you can hack a boolean so that if A is True and B is True   A and B  is False   Hacked Booleans

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What will this function do if called as Rec 0   not under the debugger    static void Rec int i        Console WriteLine i       if  i  lt  int MaxValue                Rec i   1             Answer    On 32-bit JIT it should result in a StackOverflowException On 64-bit JIT it should print all the numbers to int MaxValue   This is because the 64-bit JIT compiler applies tail call optimisation  whereas the 32-bit JIT does not    Unfortunately I haven t got a 64-bit machine to hand to verify this  but the method does meet all the conditions for tail-call optimisation  If anybody does have one I d be interested to see if it s true

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C  supports conversions between arrays and lists as long as the arrays are not multidimensional and there is an inheritance relation between the types and the types are reference types  object   oArray   new string      one    two    three     string   sArray    string   oArray      Also works for IList  and IEnumerable  ICollection  IList lt string gt  sList    IList lt string gt  oArray  IList lt object gt  oList   new string      one    two    three       Note that this does not work   object   oArray2   new int     1  2  3       Error  Cannot implicitly convert type  int    to  object    int   iArray    int   oArray2                Error  Cannot convert type  object    to  int

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Public Class Item    Public ID As Guid    Public Text As String     Public Sub New ByVal id As Guid  ByVal name As String        Me ID   id       Me Text   name    End Sub End Class  Public Sub Load sender As Object  e As EventArgs  Handles Me Load    Dim box As New ComboBox    Me Controls Add box            Sorry I forgot this line the first time      Dim h As IntPtr   box Handle   Im not sure you need this but you might      Try       box Items Add New Item Guid Empty  Nothing      Catch ex As Exception       MsgBox ex ToString       End Try End Sub   The output is  Attempted to read protected memory  This is an indication that other memory is corrupt

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Assign This     This is one that I like to ask at parties  which is probably why I don t get invited anymore    Can you make the following piece of code compile       public void Foo                 this   new Teaser            An easy cheat could be   string cheat          public void Foo                 this   new Teaser               But the real solution is this   public struct Teaser       public void Foo                 this   new Teaser              So it s a little know fact that value types  structs  can reassign their this variable

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This is the strangest I ve encountered by accident   public class DummyObject       public override string ToString                 return null            Used as follows   DummyObject obj   new DummyObject    Console WriteLine  The text      obj GetType       is     obj     Will throw a NullReferenceException  Turns out the multiple additions are compiled by the C  compiler to a call to String Concat object     Prior to  NET 4  there is a bug in just that overload of Concat where the object is checked for null  but not the result of ToString     object obj2   args i   string text    obj2    null    obj2 ToString     string Empty     if obj2 is non-null  but obj2 ToString   returns null  then text  null int length   text Length    This is a bug by ECMA-334   14 7 4      The binary   operator performs string concatenation when one or both operands are of type string  If an operand of string concatenation is null  an empty string is substituted  Otherwise  any non-string operand is converted to its string representation by invoking the virtual ToString method inherited from type object  If ToString returns null  an empty string is substituted

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This one is pretty straightforward but I still find it somewhat interesting  What would be the value of x after the call to Foo   static int x   0   public static void Foo         try   return        finally   x   1       static void Main     Foo

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From a question I asked not long ago   Conditional operator cannot cast implicitly   Given   Bool aBoolValue    Where aBoolValue is assigned either True or False   The following will not compile   Byte aByteValue   aBoolValue   1   0    But this would   Int anIntValue   aBoolValue   1   0    The answer provided is pretty good too

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C  Accessibility Puzzler  The following derived class is accessing a private field from its base class  and the compiler silently looks to the other side  public class Derived   Base       public int BrokenAccess                 return base m basePrivateField           The field is indeed private  private int m basePrivateField   0   Care to guess how we can make such code compile                Answer  The trick is to declare Derived as an inner class of Base  public class Base       private int m basePrivateField   0       public class Derived   Base               public int BrokenAccess                         return base m basePrivateField                     Inner classes are given full access to the outer class members  In this case the inner class also happens to derive from the outer class  This allows us to  quot break quot  the encapsulation of  private members

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The following prints False instead of throwing an overflow exception   Console WriteLine   0    yep int MaxValue       private bool yep  int val         return   0  lt  val   2

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I found a second really strange corner case that beats my first one by a long shot   String Equals Method  String  String  StringComparison  is not actually side effect free   I was working on a block of code that had this on a line by itself at the top of some function   stringvariable1 Equals stringvariable2  StringComparison InvariantCultureIgnoreCase     Removing that line lead to a stack overflow somewhere else in the program   The code turned out to be installing a handler for what was in essence a BeforeAssemblyLoad event and trying to do  if  assemblyfilename EndsWith  someparticular dll   StringComparison InvariantCultureIgnoreCase         assemblyfilename    someparticular modified dll       By now I shouldn t have to tell you  Using a culture that hasn t been used before in a string comparison causes an assembly load  InvariantCulture is not an exception to this

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I think I showed you this one before  but I like the fun here - this took some debugging to track down   the original code was obviously more complex and subtle          static void Foo lt T gt    where T   new                 T t   new T            Console WriteLine t ToString        works fine         Console WriteLine t GetHashCode        works fine         Console WriteLine t Equals t       works fine             so it looks like an object and smells like an object                but this throws a NullReferenceException            Console WriteLine t GetType             So what was T     Answer  any Nullable lt T gt  - such as int   All the methods are overridden  except GetType   which can t be  so it is cast  boxed  to object  and hence to null  to call object GetType      which calls on null  -p    Update  the plot thickens    Ayende Rahien threw down a similar challenge on his blog  but with a where T   class  new     private static void Main         CanThisHappen lt MyFunnyType gt        public static void CanThisHappen lt T gt    where T   class  new         var instance   new T       new   on a ref-type  should be non-null  then     Debug Assert instance    null   How did we break the CLR         But it can be defeated  Using the same indirection used by things like remoting  warning - the following is pure evil   class MyFunnyProxyAttribute   ProxyAttribute       public override MarshalByRefObject CreateInstance Type serverType            return null           MyFunnyProxy  class MyFunnyType   ContextBoundObject       With this in place  the new   call is redirected to the proxy  MyFunnyProxyAttribute   which returns null  Now go and wash your eyes

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Assign This     This is one that I like to ask at parties  which is probably why I don t get invited anymore    Can you make the following piece of code compile       public void Foo                 this   new Teaser            An easy cheat could be   string cheat          public void Foo                 this   new Teaser               But the real solution is this   public struct Teaser       public void Foo                 this   new Teaser              So it s a little know fact that value types  structs  can reassign their this variable

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This is one of the most unusual i ve seen so far  aside from the ones here of course     public class Turtle lt T gt  where T   Turtle lt T gt        It lets you declare it but has no real use  since it will always ask you to wrap whatever class you stuff in the center with another Turtle    joke  I guess it s turtles all the way down      joke

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This one had me truly puzzled  I apologise for the length but it s WinForm   I posted it in the newsgroups a while back      I ve come across an interesting bug  I   have workarounds but i d like to know   the root of the problem  I ve stripped   it down into a short file and hope   someone might have an idea about   what s going on       It s a simple program that loads a   control onto a form and binds  Foo    against a combobox   SelectedItem     for it s  Bar  property and a   datetimepicker   Value   for it s    DateTime  property  The   DateTimePicker Visible value is set to   false  Once it s loaded up  select the   combobox and then attempt to deselect   it by selecting the checkbox  This is   rendered impossible by the combobox   retaining the focus  you cannot even   close the form  such is it s grasp on   the focus       I have found three ways of fixing this   problem       a  Remove the binding to Bar  a bit   obvious        b  Remove the binding to   DateTime       c  Make the DateTimePicker   visible          I m currently running Win2k  And  NET   2 00  I think 1 1 has the same problem  Code is below    using System  using System Collections  using System Windows Forms   namespace WindowsApplication6       public class Bar               public Bar                                  public class Foo               private Bar m Bar   new Bar            private DateTime m DateTime   DateTime Now           public Foo                                public Bar Bar                       get                               return m Bar                            set                               m Bar   value                                   public DateTime DateTime                       get                               return m DateTime                            set                               m DateTime   value                                     public class TestBugControl   UserControl               public TestBugControl                         InitializeComponent                       public void InitializeData IList types                        this cBoxType DataSource   types                     public void BindFoo Foo foo                        this cBoxType DataBindings Add  SelectedItem   foo   Bar                this dtStart DataBindings Add  Value   foo   DateTime                            lt summary gt              Required designer variable               lt  summary gt          private System ComponentModel IContainer components   null                lt summary gt              Clean up any resources being used               lt  summary gt               lt param name  disposing  gt true if managed resources should be disposed  otherwise  false  lt  param gt          protected override void Dispose bool disposing                        if  disposing  amp  amp   components    null                                 components Dispose                              base Dispose disposing                       region Component Designer generated code               lt summary gt              Required method for Designer support - do not modify             the contents of this method with the code editor               lt  summary gt          private void InitializeComponent                         this checkBox1   new System Windows Forms CheckBox                this cBoxType   new System Windows Forms ComboBox                this dtStart   new System Windows Forms DateTimePicker                this SuspendLayout                                  checkBox1                            this checkBox1 AutoSize   true              this checkBox1 Location   new System Drawing Point 14  5               this checkBox1 Name    checkBox1               this checkBox1 Size   new System Drawing Size 97  20               this checkBox1 TabIndex   0              this checkBox1 Text    checkBox1               this checkBox1 UseVisualStyleBackColor   true                                cBoxType                            this cBoxType FormattingEnabled   true              this cBoxType Location   new System Drawing Point 117  3               this cBoxType Name    cBoxType               this cBoxType Size   new System Drawing Size 165  24               this cBoxType TabIndex   1                                dtStart                            this dtStart Location   new System Drawing Point 117  40               this dtStart Name    dtStart               this dtStart Size   new System Drawing Size 165  23               this dtStart TabIndex   2              this dtStart Visible   false                                TestBugControl                            this AutoScaleDimensions   new System Drawing SizeF 8F  16F               this AutoScaleMode   System Windows Forms AutoScaleMode Font              this Controls Add this dtStart               this Controls Add this cBoxType               this Controls Add this checkBox1               this Font   new System Drawing Font  Verdana   9 75F              System Drawing FontStyle Regular  System Drawing GraphicsUnit Point                byte  0                 this Margin   new System Windows Forms Padding 4               this Name    TestBugControl               this Size   new System Drawing Size 285  66               this ResumeLayout false               this PerformLayout                         endregion          private System Windows Forms CheckBox checkBox1          private System Windows Forms ComboBox cBoxType          private System Windows Forms DateTimePicker dtStart             public class Form1   Form               public Form1                         InitializeComponent                this Load    new EventHandler Form1 Load                      void Form1 Load object sender  EventArgs e                        InitializeControl                       public void InitializeControl                         TestBugControl control   new TestBugControl                IList list   new ArrayList                for  int i   0  i  lt  10  i                                  list Add new Bar                               control InitializeData list               control BindFoo new Foo                 this Controls Add control                           lt summary gt              Required designer variable               lt  summary gt          private System ComponentModel IContainer components   null                lt summary gt              Clean up any resources being used               lt  summary gt               lt param name  disposing  gt true if managed resources should be disposed  otherwise  false  lt  param gt          protected override void Dispose bool disposing                        if  disposing  amp  amp   components    null                                 components Dispose                              base Dispose disposing                       region Windows Form Designer generated code               lt summary gt              Required method for Designer support - do not modify             the contents of this method with the code editor               lt  summary gt          private void InitializeComponent                         this components   new System ComponentModel Container                this AutoScaleMode   System Windows Forms AutoScaleMode Font              this Text    Form1                       endregion            static class Program                    lt summary gt              The main entry point for the application               lt  summary gt           STAThread          static void Main                         Application EnableVisualStyles                Application SetCompatibleTextRenderingDefault false               Application Run new Form1

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They should have made 0 an integer even when there s an enum function overload   I knew C  core team rationale for mapping 0 to enum  but still  it is not as orthogonal as it should be   Example from Npgsql   Test example   namespace Craft       enum Symbol   Alpha   1  Beta   2  Gamma   3  Delta   4         class Mate               static void Main string   args                         JustTest Symbol Alpha      enum             JustTest 0      why enum             JustTest  int 0      why still enum              int i   0               JustTest Convert ToInt32 0       have to use Convert ToInt32 to convince the compiler to make the call site use the object version              JustTest i      it s ok from down here and below             JustTest 1               JustTest  string                JustTest Guid NewGuid                 JustTest new DataTable                  Console ReadLine                       static void JustTest Symbol a                        Console WriteLine  Enum                       static void JustTest object o                        Console WriteLine  Object

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C  Accessibility Puzzler  The following derived class is accessing a private field from its base class  and the compiler silently looks to the other side  public class Derived   Base       public int BrokenAccess                 return base m basePrivateField           The field is indeed private  private int m basePrivateField   0   Care to guess how we can make such code compile                Answer  The trick is to declare Derived as an inner class of Base  public class Base       private int m basePrivateField   0       public class Derived   Base               public int BrokenAccess                         return base m basePrivateField                     Inner classes are given full access to the outer class members  In this case the inner class also happens to derive from the outer class  This allows us to  quot break quot  the encapsulation of  private members

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Few years ago  when working on loyality program  we had an issue with the amount of points given to customers  The issue was related to casting converting double to int   In code below   double d   13 6   int i1   Convert ToInt32 d   int i2    int d    does i1    i2    It turns out that i1    i2  Because of different rounding policies in Convert and cast operator the actual values are   i1    14 i2    13   It s always better to call Math Ceiling   or Math Floor    or Math Round with MidpointRounding that meets our requirements   int i1   Convert ToInt32  Math Ceiling d     int i2    int  Math Ceiling d

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I think the answer to the question is because .net uses string interning something that might cause equal strings to point to the same object (since a strings are mutable this is not a problem)

(I'm not talking about the overridden equality operator on the string class)

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I m arriving a bit late to the party  but I ve got three four five    If you poll InvokeRequired on a control that hasn t been loaded shown  it will say false - and blow up in your face if you try to change it from another thread  the solution is to reference this Handle in the creator of the control   Another one which tripped me up is that given an assembly with   enum MyEnum       Red      Blue      if you calculate MyEnum Red ToString   in another assembly  and in between times someone has recompiled your enum to   enum MyEnum       Black      Red      Blue      at runtime  you will get  Black   I had a shared assembly with some handy constants in  My predecessor had left a load of ugly-looking get-only properties  I thought I d get rid of the clutter and just use public const  I was more than a little surprised when VS compiled them to their values  and not references  If you implement a new method of an interface from another assembly  but you rebuild referencing the old version of that assembly  you get a TypeLoadException  no implementation of  NewMethod    even though you have implemented it  see here   Dictionary lt      The order in which the items are returned is undefined   This is horrible  because it can bite you sometimes  but work others  and if you ve just blindly assumed that Dictionary is going to play nice   why shouldn t it  I thought  List does    you really have to have your nose in it before you finally start to question your assumption

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Public Class Item    Public ID As Guid    Public Text As String     Public Sub New ByVal id As Guid  ByVal name As String        Me ID   id       Me Text   name    End Sub End Class  Public Sub Load sender As Object  e As EventArgs  Handles Me Load    Dim box As New ComboBox    Me Controls Add box            Sorry I forgot this line the first time      Dim h As IntPtr   box Handle   Im not sure you need this but you might      Try       box Items Add New Item Guid Empty  Nothing      Catch ex As Exception       MsgBox ex ToString       End Try End Sub   The output is  Attempted to read protected memory  This is an indication that other memory is corrupt

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When is a Boolean neither True nor False   Bill discovered that you can hack a boolean so that if A is True and B is True   A and B  is False   Hacked Booleans

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Consider this weird case   public interface MyInterface     void Method      public class Base     public void Method         public class Derived   Base  MyInterface       If Base and Derived are declared in the same assembly  the compiler will make Base  Method virtual and sealed  in the CIL   even though Base doesn t implement the interface    If Base and Derived are in different assemblies  when compiling the Derived assembly  the compiler won t change the other assembly  so it will introduce a member in Derived that will be an explicit implementation for MyInterface  Method that will just delegate the call to Base  Method   The compiler has to do this in order to support polymorphic dispatch with regards to the interface  i e  it has to make that method virtual

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There is something really exciting about C   the way it handles closures   Instead of copying the stack variable values to the closure free variable  it does that preprocessor magic wrapping all occurences of the variable into an object and thus moves it out of stack - straight to the heap      I guess  that makes C  even more functionally-complete  or lambda-complete huh   language than ML itself  which uses stack value copying AFAIK   F  has that feature too  as C  does    That does bring much delight to me  thank you MS guys   It s not an oddity or corner case though    but something really unexpected from a stack-based VM language

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The following prints False instead of throwing an overflow exception   Console WriteLine   0    yep int MaxValue       private bool yep  int val         return   0  lt  val   2

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PropertyInfo SetValue   can assign ints to enums  ints to nullable ints  enums to nullable enums  but not ints to nullable enums   enumProperty SetValue obj  1  null     works nullableIntProperty SetValue obj  1  null     works nullableEnumProperty SetValue obj  MyEnum Foo  null     works nullableEnumProperty SetValue obj  1  null      throws an exception       Full description here

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Just found a nice little thing today   public class Base      public virtual void Initialize dynamic stuff                     public class Derived Base      public override void Initialize dynamic stuff       base Initialize stuff                     This throws compile error     The call to method  Initialize  needs to be dynamically dispatched  but cannot be because it is part of a base access expression  Consider casting the dynamic arguments or eliminating the base access   If I write base Initialize stuff as object   it works perfectly  however this seems to be a  magic word  here  since it does exactly the same  everything is still recieved as dynamic

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In an API we re using  methods that return a domain object might return a special  null object   In the implementation of this  the comparison operator and the Equals   method are overridden to return true if it is compared with null   So a user of this API might have some code like this   return test    null   test   GetDefault      or perhaps a bit more verbose  like this   if  test    null      return GetDefault    return test    where GetDefault   is a method returning some default value that we want to use instead of null  The surprise hit me when I was using ReSharper and following it s recommendation to rewrite either of this to the following   return test    GetDefault      If the test object is a null object returned from the API instead of a proper null  the behavior of the code has now changed  as the null coalescing operator actually checks for null  not running operator  or Equals

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I m arriving a bit late to the party  but I ve got three four five    If you poll InvokeRequired on a control that hasn t been loaded shown  it will say false - and blow up in your face if you try to change it from another thread  the solution is to reference this Handle in the creator of the control   Another one which tripped me up is that given an assembly with   enum MyEnum       Red      Blue      if you calculate MyEnum Red ToString   in another assembly  and in between times someone has recompiled your enum to   enum MyEnum       Black      Red      Blue      at runtime  you will get  Black   I had a shared assembly with some handy constants in  My predecessor had left a load of ugly-looking get-only properties  I thought I d get rid of the clutter and just use public const  I was more than a little surprised when VS compiled them to their values  and not references  If you implement a new method of an interface from another assembly  but you rebuild referencing the old version of that assembly  you get a TypeLoadException  no implementation of  NewMethod    even though you have implemented it  see here   Dictionary lt      The order in which the items are returned is undefined   This is horrible  because it can bite you sometimes  but work others  and if you ve just blindly assumed that Dictionary is going to play nice   why shouldn t it  I thought  List does    you really have to have your nose in it before you finally start to question your assumption

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I think I showed you this one before  but I like the fun here - this took some debugging to track down   the original code was obviously more complex and subtle          static void Foo lt T gt    where T   new                 T t   new T            Console WriteLine t ToString        works fine         Console WriteLine t GetHashCode        works fine         Console WriteLine t Equals t       works fine             so it looks like an object and smells like an object                but this throws a NullReferenceException            Console WriteLine t GetType             So what was T     Answer  any Nullable lt T gt  - such as int   All the methods are overridden  except GetType   which can t be  so it is cast  boxed  to object  and hence to null  to call object GetType      which calls on null  -p    Update  the plot thickens    Ayende Rahien threw down a similar challenge on his blog  but with a where T   class  new     private static void Main         CanThisHappen lt MyFunnyType gt        public static void CanThisHappen lt T gt    where T   class  new         var instance   new T       new   on a ref-type  should be non-null  then     Debug Assert instance    null   How did we break the CLR         But it can be defeated  Using the same indirection used by things like remoting  warning - the following is pure evil   class MyFunnyProxyAttribute   ProxyAttribute       public override MarshalByRefObject CreateInstance Type serverType            return null           MyFunnyProxy  class MyFunnyType   ContextBoundObject       With this in place  the new   call is redirected to the proxy  MyFunnyProxyAttribute   which returns null  Now go and wash your eyes

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Bankers  Rounding   This one is not so much a compiler bug or malfunction  but certainly a strange corner case     The  Net Framework employs a scheme or rounding known as Banker s Rounding   In Bankers  Rounding the 0 5 numbers are rounded to the nearest even number  so  Math Round -0 5     0 Math Round 0 5     0 Math Round 1 5     2 Math Round 2 5     2 etc      This can lead to some unexpected bugs in financial calculations based on the more well known Round-Half-Up rounding   This is also true of Visual Basic

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I think the answer to the question is because .net uses string interning something that might cause equal strings to point to the same object (since a strings are mutable this is not a problem)

(I'm not talking about the overridden equality operator on the string class)

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Here is an example of how you can create a struct that causes the error message  Attempted to read or write protected memory  This is often an indication that other memory is corrupt   The difference between success and failure is very subtle   The following unit test demonstrates the problem   See if you can work out what went wrong        Test      public void Test                 var bar   new MyClass                       Foo   500                    bar Foo    500           Assert That bar Foo Value Amount  Is EqualTo 1000               private class MyClass               public MyStruct  Foo   get  set               private struct MyStruct               public decimal Amount   get  private set             public MyStruct decimal amount    this                         Amount   amount                     public static MyStruct operator   MyStruct x  MyStruct y                        return new MyStruct x Amount   y Amount                      public static MyStruct operator   MyStruct x  decimal y                        return new MyStruct x Amount   y                      public static implicit operator MyStruct int value                        return new MyStruct value                      public static implicit operator MyStruct decimal value                        return new MyStruct value

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I m not sure if you d say this is a Windows Vista 7 oddity or a  Net oddity but it had me scratching my head for a while   string filename     c  program files my folder test txt   System IO File WriteAllText filename   Hello world     bool exists   System IO File Exists filename      returns true  string text   System IO File ReadAllText filename      Returns  Hello world     In Windows Vista 7 the file will actually be written to C  Users  lt username gt  Virtual Store Program Files my folder test txt

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C  supports conversions between arrays and lists as long as the arrays are not multidimensional and there is an inheritance relation between the types and the types are reference types  object   oArray   new string      one    two    three     string   sArray    string   oArray      Also works for IList  and IEnumerable  ICollection  IList lt string gt  sList    IList lt string gt  oArray  IList lt object gt  oList   new string      one    two    three       Note that this does not work   object   oArray2   new int     1  2  3       Error  Cannot implicitly convert type  int    to  object    int   iArray    int   oArray2                Error  Cannot convert type  object    to  int

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There is something really exciting about C   the way it handles closures   Instead of copying the stack variable values to the closure free variable  it does that preprocessor magic wrapping all occurences of the variable into an object and thus moves it out of stack - straight to the heap      I guess  that makes C  even more functionally-complete  or lambda-complete huh   language than ML itself  which uses stack value copying AFAIK   F  has that feature too  as C  does    That does bring much delight to me  thank you MS guys   It s not an oddity or corner case though    but something really unexpected from a stack-based VM language

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What will this function do if called as Rec 0   not under the debugger    static void Rec int i        Console WriteLine i       if  i  lt  int MaxValue                Rec i   1             Answer    On 32-bit JIT it should result in a StackOverflowException On 64-bit JIT it should print all the numbers to int MaxValue   This is because the 64-bit JIT compiler applies tail call optimisation  whereas the 32-bit JIT does not    Unfortunately I haven t got a 64-bit machine to hand to verify this  but the method does meet all the conditions for tail-call optimisation  If anybody does have one I d be interested to see if it s true

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Interesting - when I first looked at that I assumed it was something the C  compiler was checking for  but even if you emit the IL directly to remove any chance of interference it still happens  which means it really is the newobj op-code that s doing the checking   var method   new DynamicMethod  Test   null  null   var il   method GetILGenerator     il Emit OpCodes Ldc I4 0   il Emit OpCodes Newarr  typeof char    il Emit OpCodes Newobj  typeof string  GetConstructor new     typeof char          il Emit OpCodes Ldc I4 0   il Emit OpCodes Newarr  typeof char    il Emit OpCodes Newobj  typeof string  GetConstructor new     typeof char          il Emit OpCodes Call  typeof object  GetMethod  ReferenceEquals     il Emit OpCodes Box  typeof bool    il Emit OpCodes Call  typeof Console  GetMethod  WriteLine   new     typeof object        il Emit OpCodes Ret    method Invoke null  null     It also equates to true if you check against string Empty which means this op-code must have special behaviour to intern empty strings

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The following might be general knowledge I was just simply lacking  but eh  Some time ago  we had a bug case which included virtual properties  Abstracting the context a bit  consider the following code  and apply breakpoint to specified area    class Program       static void Main string   args                Derived d   new Derived            d Property    AWESOME            class Base       string  baseProp      public virtual string Property                 get                        return  BASE      baseProp                    set                        baseProp   value                do work with the base property which might                not be exposed to derived types               here             Console Out WriteLine   baseProp is BASE     value ToString                        class Derived   Base       string  prop      public override string Property                get   return  prop            set                          prop   value               base Property   value               lt - put a breakpoint here then mouse over BaseProperty                  and then mouse over the base Property call inside it             public string BaseProperty   get   return base Property    private set           While in the Derived object context  you can get the same behavior when adding base Property as a watch  or typing base Property into the quickwatch    Took me some time to realize what was going on  In the end I was enlightened by the Quickwatch  When going into the Quickwatch and exploring the Derived object d  or from the object s context  this  and selecting the field base  the edit field on top of the Quickwatch displays the following cast     TestProject1 Base  d     Which means that if base is replaced as such  the call would be  public string BaseProperty   get   return   TestProject1 Base  d   Property    private set         for the Watches  Quickwatch and the debugging mouse-over tooltips  and it would then make sense for it to display  AWESOME  instead of  BASE AWESOME  when considering polymorphism  I m still unsure why it would transform it into a cast  one hypothesis is that call might not be available from those modules  context  and only callvirt   Anyhow  that obviously doesn t alter anything in terms of functionality  Derived BaseProperty will still really return  BASE AWESOME   and thus this was not the root of our bug at work  simply a confusing component  I did however find it interesting how it could mislead developpers which would be unaware of that fact during their debug sessions  specially if Base is not exposed in your project but rather referenced as a 3rd party DLL  resulting in Devs just saying        Oi  wait  what   omg that DLL is   like    doing something funny

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The following doesn t work   if  something      doit    else     var v   1   2    But this works   if  something      doit    else       var v   1   2

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This one s pretty hard to top   I ran into it while I was trying to build a RealProxy implementation that truly supports Begin EndInvoke  thanks MS for making this impossible to do without horrible hacks    This example is basically a bug in the CLR  the unmanaged code path for BeginInvoke doesn t validate that the return message from RealProxy PrivateInvoke  and my Invoke override  is returning an instance of an IAsyncResult   Once it s returned  the CLR gets incredibly confused and loses any idea of whats going on  as demonstrated by the tests at the bottom   using System  using System Collections Generic  using System Linq  using System Text  using System Runtime Remoting Proxies  using System Reflection  using System Runtime Remoting Messaging   namespace BrokenProxy       class NotAnIAsyncResult               public string SomeProperty   get  set               class BrokenProxy   RealProxy               private void HackFlags                         var flagsField   typeof RealProxy  GetField   flags   BindingFlags NonPublic   BindingFlags Instance               int val    int flagsField GetValue this               val    1     1   RemotingProxy  check out System Runtime Remoting Proxies RealProxyFlags             flagsField SetValue this  val                      public BrokenProxy Type t                base t                        HackFlags                       public override IMessage Invoke IMessage msg                        var naiar   new NotAnIAsyncResult                naiar SomeProperty    o noes               return new ReturnMessage naiar  null  0  null   IMethodCallMessage msg                        interface IRandomInterface               int DoSomething               class Program               static void Main string   args                        BrokenProxy bp   new BrokenProxy typeof IRandomInterface                var instance    IRandomInterface bp GetTransparentProxy                Func lt int gt  doSomethingDelegate   instance DoSomething              IAsyncResult notAnIAsyncResult   doSomethingDelegate BeginInvoke null  null                var interfaces   notAnIAsyncResult GetType   GetInterfaces                Console WriteLine  interfaces Any      No interfaces on notAnIAsyncResult     Interfaces                Console WriteLine notAnIAsyncResult is IAsyncResult      Should be false  is it               Console WriteLine   NotAnIAsyncResult notAnIAsyncResult  SomeProperty               Console WriteLine   IAsyncResult notAnIAsyncResult  IsCompleted      No way this works                      Output   No interfaces on notAnIAsyncResult True o noes  Unhandled Exception  System EntryPointNotFoundException  Entry point was not found     at System IAsyncResult get IsCompleted      at BrokenProxy Program Main String   args

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The scoping in c  is truly bizarre at times  Lets me give you one example   if  true       OleDbCommand command   SQLServer CreateCommand       OleDbCommand command   SQLServer CreateCommand      This fails to compile  because command is redeclared  There are some interested guesswork as to why it works that way in this thread on stackoverflow and in my blog

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C  supports conversions between arrays and lists as long as the arrays are not multidimensional and there is an inheritance relation between the types and the types are reference types  object   oArray   new string      one    two    three     string   sArray    string   oArray      Also works for IList  and IEnumerable  ICollection  IList lt string gt  sList    IList lt string gt  oArray  IList lt object gt  oList   new string      one    two    three       Note that this does not work   object   oArray2   new int     1  2  3       Error  Cannot implicitly convert type  int    to  object    int   iArray    int   oArray2                Error  Cannot convert type  object    to  int

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I think I showed you this one before  but I like the fun here - this took some debugging to track down   the original code was obviously more complex and subtle          static void Foo lt T gt    where T   new                 T t   new T            Console WriteLine t ToString        works fine         Console WriteLine t GetHashCode        works fine         Console WriteLine t Equals t       works fine             so it looks like an object and smells like an object                but this throws a NullReferenceException            Console WriteLine t GetType             So what was T     Answer  any Nullable lt T gt  - such as int   All the methods are overridden  except GetType   which can t be  so it is cast  boxed  to object  and hence to null  to call object GetType      which calls on null  -p    Update  the plot thickens    Ayende Rahien threw down a similar challenge on his blog  but with a where T   class  new     private static void Main         CanThisHappen lt MyFunnyType gt        public static void CanThisHappen lt T gt    where T   class  new         var instance   new T       new   on a ref-type  should be non-null  then     Debug Assert instance    null   How did we break the CLR         But it can be defeated  Using the same indirection used by things like remoting  warning - the following is pure evil   class MyFunnyProxyAttribute   ProxyAttribute       public override MarshalByRefObject CreateInstance Type serverType            return null           MyFunnyProxy  class MyFunnyType   ContextBoundObject       With this in place  the new   call is redirected to the proxy  MyFunnyProxyAttribute   which returns null  Now go and wash your eyes

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I think the answer to the question is because .net uses string interning something that might cause equal strings to point to the same object (since a strings are mutable this is not a problem)

(I'm not talking about the overridden equality operator on the string class)

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This one is pretty straightforward but I still find it somewhat interesting  What would be the value of x after the call to Foo   static int x   0   public static void Foo         try   return        finally   x   1       static void Main     Foo

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This one had me truly puzzled  I apologise for the length but it s WinForm   I posted it in the newsgroups a while back      I ve come across an interesting bug  I   have workarounds but i d like to know   the root of the problem  I ve stripped   it down into a short file and hope   someone might have an idea about   what s going on       It s a simple program that loads a   control onto a form and binds  Foo    against a combobox   SelectedItem     for it s  Bar  property and a   datetimepicker   Value   for it s    DateTime  property  The   DateTimePicker Visible value is set to   false  Once it s loaded up  select the   combobox and then attempt to deselect   it by selecting the checkbox  This is   rendered impossible by the combobox   retaining the focus  you cannot even   close the form  such is it s grasp on   the focus       I have found three ways of fixing this   problem       a  Remove the binding to Bar  a bit   obvious        b  Remove the binding to   DateTime       c  Make the DateTimePicker   visible          I m currently running Win2k  And  NET   2 00  I think 1 1 has the same problem  Code is below    using System  using System Collections  using System Windows Forms   namespace WindowsApplication6       public class Bar               public Bar                                  public class Foo               private Bar m Bar   new Bar            private DateTime m DateTime   DateTime Now           public Foo                                public Bar Bar                       get                               return m Bar                            set                               m Bar   value                                   public DateTime DateTime                       get                               return m DateTime                            set                               m DateTime   value                                     public class TestBugControl   UserControl               public TestBugControl                         InitializeComponent                       public void InitializeData IList types                        this cBoxType DataSource   types                     public void BindFoo Foo foo                        this cBoxType DataBindings Add  SelectedItem   foo   Bar                this dtStart DataBindings Add  Value   foo   DateTime                            lt summary gt              Required designer variable               lt  summary gt          private System ComponentModel IContainer components   null                lt summary gt              Clean up any resources being used               lt  summary gt               lt param name  disposing  gt true if managed resources should be disposed  otherwise  false  lt  param gt          protected override void Dispose bool disposing                        if  disposing  amp  amp   components    null                                 components Dispose                              base Dispose disposing                       region Component Designer generated code               lt summary gt              Required method for Designer support - do not modify             the contents of this method with the code editor               lt  summary gt          private void InitializeComponent                         this checkBox1   new System Windows Forms CheckBox                this cBoxType   new System Windows Forms ComboBox                this dtStart   new System Windows Forms DateTimePicker                this SuspendLayout                                  checkBox1                            this checkBox1 AutoSize   true              this checkBox1 Location   new System Drawing Point 14  5               this checkBox1 Name    checkBox1               this checkBox1 Size   new System Drawing Size 97  20               this checkBox1 TabIndex   0              this checkBox1 Text    checkBox1               this checkBox1 UseVisualStyleBackColor   true                                cBoxType                            this cBoxType FormattingEnabled   true              this cBoxType Location   new System Drawing Point 117  3               this cBoxType Name    cBoxType               this cBoxType Size   new System Drawing Size 165  24               this cBoxType TabIndex   1                                dtStart                            this dtStart Location   new System Drawing Point 117  40               this dtStart Name    dtStart               this dtStart Size   new System Drawing Size 165  23               this dtStart TabIndex   2              this dtStart Visible   false                                TestBugControl                            this AutoScaleDimensions   new System Drawing SizeF 8F  16F               this AutoScaleMode   System Windows Forms AutoScaleMode Font              this Controls Add this dtStart               this Controls Add this cBoxType               this Controls Add this checkBox1               this Font   new System Drawing Font  Verdana   9 75F              System Drawing FontStyle Regular  System Drawing GraphicsUnit Point                byte  0                 this Margin   new System Windows Forms Padding 4               this Name    TestBugControl               this Size   new System Drawing Size 285  66               this ResumeLayout false               this PerformLayout                         endregion          private System Windows Forms CheckBox checkBox1          private System Windows Forms ComboBox cBoxType          private System Windows Forms DateTimePicker dtStart             public class Form1   Form               public Form1                         InitializeComponent                this Load    new EventHandler Form1 Load                      void Form1 Load object sender  EventArgs e                        InitializeControl                       public void InitializeControl                         TestBugControl control   new TestBugControl                IList list   new ArrayList                for  int i   0  i  lt  10  i                                  list Add new Bar                               control InitializeData list               control BindFoo new Foo                 this Controls Add control                           lt summary gt              Required designer variable               lt  summary gt          private System ComponentModel IContainer components   null                lt summary gt              Clean up any resources being used               lt  summary gt               lt param name  disposing  gt true if managed resources should be disposed  otherwise  false  lt  param gt          protected override void Dispose bool disposing                        if  disposing  amp  amp   components    null                                 components Dispose                              base Dispose disposing                       region Windows Form Designer generated code               lt summary gt              Required method for Designer support - do not modify             the contents of this method with the code editor               lt  summary gt          private void InitializeComponent                         this components   new System ComponentModel Container                this AutoScaleMode   System Windows Forms AutoScaleMode Font              this Text    Form1                       endregion            static class Program                    lt summary gt              The main entry point for the application               lt  summary gt           STAThread          static void Main                         Application EnableVisualStyles                Application SetCompatibleTextRenderingDefault false               Application Run new Form1

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I found a second really strange corner case that beats my first one by a long shot   String Equals Method  String  String  StringComparison  is not actually side effect free   I was working on a block of code that had this on a line by itself at the top of some function   stringvariable1 Equals stringvariable2  StringComparison InvariantCultureIgnoreCase     Removing that line lead to a stack overflow somewhere else in the program   The code turned out to be installing a handler for what was in essence a BeforeAssemblyLoad event and trying to do  if  assemblyfilename EndsWith  someparticular dll   StringComparison InvariantCultureIgnoreCase         assemblyfilename    someparticular modified dll       By now I shouldn t have to tell you  Using a culture that hasn t been used before in a string comparison causes an assembly load  InvariantCulture is not an exception to this

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This one s pretty hard to top   I ran into it while I was trying to build a RealProxy implementation that truly supports Begin EndInvoke  thanks MS for making this impossible to do without horrible hacks    This example is basically a bug in the CLR  the unmanaged code path for BeginInvoke doesn t validate that the return message from RealProxy PrivateInvoke  and my Invoke override  is returning an instance of an IAsyncResult   Once it s returned  the CLR gets incredibly confused and loses any idea of whats going on  as demonstrated by the tests at the bottom   using System  using System Collections Generic  using System Linq  using System Text  using System Runtime Remoting Proxies  using System Reflection  using System Runtime Remoting Messaging   namespace BrokenProxy       class NotAnIAsyncResult               public string SomeProperty   get  set               class BrokenProxy   RealProxy               private void HackFlags                         var flagsField   typeof RealProxy  GetField   flags   BindingFlags NonPublic   BindingFlags Instance               int val    int flagsField GetValue this               val    1     1   RemotingProxy  check out System Runtime Remoting Proxies RealProxyFlags             flagsField SetValue this  val                      public BrokenProxy Type t                base t                        HackFlags                       public override IMessage Invoke IMessage msg                        var naiar   new NotAnIAsyncResult                naiar SomeProperty    o noes               return new ReturnMessage naiar  null  0  null   IMethodCallMessage msg                        interface IRandomInterface               int DoSomething               class Program               static void Main string   args                        BrokenProxy bp   new BrokenProxy typeof IRandomInterface                var instance    IRandomInterface bp GetTransparentProxy                Func lt int gt  doSomethingDelegate   instance DoSomething              IAsyncResult notAnIAsyncResult   doSomethingDelegate BeginInvoke null  null                var interfaces   notAnIAsyncResult GetType   GetInterfaces                Console WriteLine  interfaces Any      No interfaces on notAnIAsyncResult     Interfaces                Console WriteLine notAnIAsyncResult is IAsyncResult      Should be false  is it               Console WriteLine   NotAnIAsyncResult notAnIAsyncResult  SomeProperty               Console WriteLine   IAsyncResult notAnIAsyncResult  IsCompleted      No way this works                      Output   No interfaces on notAnIAsyncResult True o noes  Unhandled Exception  System EntryPointNotFoundException  Entry point was not found     at System IAsyncResult get IsCompleted      at BrokenProxy Program Main String   args

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I m arriving a bit late to the party  but I ve got three four five    If you poll InvokeRequired on a control that hasn t been loaded shown  it will say false - and blow up in your face if you try to change it from another thread  the solution is to reference this Handle in the creator of the control   Another one which tripped me up is that given an assembly with   enum MyEnum       Red      Blue      if you calculate MyEnum Red ToString   in another assembly  and in between times someone has recompiled your enum to   enum MyEnum       Black      Red      Blue      at runtime  you will get  Black   I had a shared assembly with some handy constants in  My predecessor had left a load of ugly-looking get-only properties  I thought I d get rid of the clutter and just use public const  I was more than a little surprised when VS compiled them to their values  and not references  If you implement a new method of an interface from another assembly  but you rebuild referencing the old version of that assembly  you get a TypeLoadException  no implementation of  NewMethod    even though you have implemented it  see here   Dictionary lt      The order in which the items are returned is undefined   This is horrible  because it can bite you sometimes  but work others  and if you ve just blindly assumed that Dictionary is going to play nice   why shouldn t it  I thought  List does    you really have to have your nose in it before you finally start to question your assumption

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If you have the extension method   public static bool  ToBoolean this string s        bool result       if  bool TryParse s  out result           return result      else         return null      and this code   string nullStr   null  var res   nullStr ToBoolean      This will not throw an exception because it is an extension method  and really HelperClass ToBoolean null   and not an instance method  This can be confusing

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Have you ever thought the C  compiler could generate invalid CIL  Run this and you ll get a TypeLoadException   interface I lt T gt      T M T p     abstract class A lt T gt    I lt T gt      public abstract T M T p     abstract class B lt T gt    A lt T gt   I lt int gt      public override T M T p    return p      public int M int p    return p   2      class C   B lt int gt       class Program     static void Main string   args        Console WriteLine new C   M 42            I don t know how it fares in the C  4 0 compiler though   EDIT  this is the output from my system   C  Temp gt type Program cs using System  using System Collections Generic  using System Linq  using System Text   namespace ConsoleApplication1      interface I lt T gt        T M T p         abstract class A lt T gt    I lt T gt        public abstract T M T p         abstract class B lt T gt    A lt T gt   I lt int gt        public override T M T p    return p        public int M int p    return p   2          class C   B lt int gt         class Program       static void Main string   args          Console WriteLine new C   M 11                 C  Temp gt csc Program cs Microsoft  R  Visual C  2008 Compiler version 3 5 30729 1 for Microsoft  R   NET Framework version 3 5 Copyright  C  Microsoft Corporation  All rights reserved    C  Temp gt Program  Unhandled Exception  System TypeLoadException  Could not load type  ConsoleAppli cation1 C  from assembly  Program  Version 0 0 0 0  Culture neutral  PublicKeyTo ken null      at ConsoleApplication1 Program Main String   args   C  Temp gt peverify Program exe  Microsoft  R   NET Framework PE Verifier   Version  3 5 30729 1 Copyright  c  Microsoft Corporation   All rights reserved    token  0x02000005  Type load failed   IL   Error   C  Temp Program exe   ConsoleApplication1 Program  Main  offset 0x 00000001  Unable to resolve token  2 Error s  Verifying Program exe  C  Temp gt ver  Microsoft Windows XP  Version 5 1 2600

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Interesting - when I first looked at that I assumed it was something the C  compiler was checking for  but even if you emit the IL directly to remove any chance of interference it still happens  which means it really is the newobj op-code that s doing the checking   var method   new DynamicMethod  Test   null  null   var il   method GetILGenerator     il Emit OpCodes Ldc I4 0   il Emit OpCodes Newarr  typeof char    il Emit OpCodes Newobj  typeof string  GetConstructor new     typeof char          il Emit OpCodes Ldc I4 0   il Emit OpCodes Newarr  typeof char    il Emit OpCodes Newobj  typeof string  GetConstructor new     typeof char          il Emit OpCodes Call  typeof object  GetMethod  ReferenceEquals     il Emit OpCodes Box  typeof bool    il Emit OpCodes Call  typeof Console  GetMethod  WriteLine   new     typeof object        il Emit OpCodes Ret    method Invoke null  null     It also equates to true if you check against string Empty which means this op-code must have special behaviour to intern empty strings

[c#] What's the strangest corner case you've seen in C# or .NET?

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