What is a NullReferenceException and how do I fix it

Question

I have some code and when it executes  it throws a NullReferenceException  saying      Object reference not set to an instance of an object    What does this mean  and what can I do to fix this error

User · Answer

On the matter of  quot what should I do about it quot   there can be many answers  A more  quot formal quot  way of preventing such error conditions while developing is applying design by contract in your code  This means you need to set class invariants  and or even function method preconditions and postconditions on your system  while developing  In short  class invariants ensure that there will be some constraints in your class that will not get violated in normal use  and therefore  the class will not get in an inconsistent state   Preconditions mean that data given as input to a function method must follow some constraints set and never violate them  and postconditions mean that a function method output must follow the set constraints again without ever violating them  Contract conditions should never be violated during execution of a bug-free program  therefore design by contract is checked in practice in debug mode  while being disabled in releases  to maximize the developed system performance  This way  you can avoid NullReferenceException cases that are results of violation of the constraints set  For example  if you use an object property X in a class and later try to invoke one of its methods and X has a null value  then this will lead to NullReferenceException  public X   get  set     public void InvokeX         X DoSomething       if X value is null  you will get a NullReferenceException    But if you set  quot property X must never have a null value quot  as method precondition  then you can prevent the scenario described before    Using code contracts   ContractInvariantMethod  protected void ObjectInvariant          Contract Invariant X    null                For this cause  Code Contracts project exists for  NET applications  Alternatively  design by contract can be applied using assertions  UPDATE  It is worth mentioning that the term was coined by Bertrand Meyer in connection with his design of the Eiffel programming language

User · Answer

An example of this exception being thrown is  When you are trying to check something  that is null   For example   string testString   null    Because it doesn t have a value  i e  it s null   Length  cannot do what it needs to do   if  testString Length    0     Throws a nullreferenceexception         Do something      The  NET runtime will throw a NullReferenceException when you attempt to perform an action on something which hasn t been instantiated i e  the code above   In comparison to an ArgumentNullException which is typically thrown as a defensive measure if a method expects that what is being passed to it is not null   More information is in C  NullReferenceException and Null Parameter

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Be aware that regardless of the scenario  the cause is always the same in  NET   You are trying to use a reference variable whose value is Nothing null   When the value is Nothing null for the reference variable  that means it is not actually holding a reference to an instance of any object that exists on the heap  You either never assigned something to the variable  never created an instance of the value assigned to the variable  or you set the variable equal to Nothing null manually  or you called a function that set the variable to Nothing null for you

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Simon Mourier gave this example   object o   null  DateTime d    DateTime o      NullReferenceException   where an unboxing conversion  cast  from object  or from one of the classes System ValueType or System Enum  or from an interface type  to a value type  other than Nullable lt  gt   in itself gives the NullReferenceException   In the other direction  a boxing conversion from a Nullable lt  gt  which has HasValue equal to false to a reference type  can give a null reference which can then later lead to a NullReferenceException  The classic example is   DateTime  d   null  var s   d ToString        OK  no exception  no boxing   returns    var t   d GetType         Bang  d is boxed  NullReferenceException   Sometimes the boxing happens in another way  For example with this non-generic extension method   public static void MyExtension this object x      x ToString        the following code will be problematic   DateTime  d   null  d MyExtension        Leads to boxing  NullReferenceException occurs inside the body of the called method  not here    These cases arise because of the special rules the runtime uses when boxing Nullable lt  gt  instances

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A NullReferenceException is thrown when we are trying to access Properties of a null object or when a string value becomes empty and we are trying to access string methods   For example     When a string method of an empty string accessed   string str   string Empty  str ToLower       throw null reference exception  When a property of a null object accessed   Public Class Person       public string Name   get  set      Person objPerson  objPerson Name      throw Null refernce Exception

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It means that the variable in question is pointed at nothing  I could generate this like so   SqlConnection connection   null  connection Open      That will throw the error because while I ve declared the variable  connection   it s not pointed to anything  When I try to call the member  Open   there s no reference for it to resolve  and it will throw the error   To avoid this error    Always initialize your objects before you try to do anything with them  If you re not sure whether the object is null  check it with object    null    JetBrains  Resharper tool will identify every place in your code that has the possibility of a null reference error  allowing you to put in a null check  This error is the number one source of bugs  IMHO

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You can fix NullReferenceException in a clean way using Null-conditional Operators in c 6 and write less code to handle null checks   It s used to test for null before performing a member access      or index      operation   Example    var name   p  Spouse  FirstName    is equivalent to       if  p    null                if  p Spouse    null                        name   p Spouse FirstName                    The result is that the name will be null when p is null or when p Spouse is null    Otherwise  the variable name will be assigned the value of the p Spouse FirstName    For More details   Null-conditional Operators

User · Answer

What can you do about it   There is a lot of good answers here explaining what a null reference is and how to debug it  But there is very little on how to prevent the issue or at least make it easier to catch   Check arguments  For example  methods can check the different arguments to see if they are null and throw an ArgumentNullException  an exception obviously created for this exact purpose    The constructor for the ArgumentNullException even takes the name of the parameter and a message as arguments so you can tell the developer exactly what the problem is   public void DoSomething MyObject obj        if obj    null                 throw new ArgumentNullException  obj    Need a reference to obj               Use Tools  There are also several libraries that can help   Resharper  for example can provide you with warnings while you are writing code  especially if you use their attribute  NotNullAttribute  There s  Microsoft Code Contracts  where you use syntax like Contract Requires obj    null  which gives you runtime and compile checking  Introducing Code Contracts   There s also  PostSharp  which will allow you to just use attributes like this    public void DoSometing  NotNull  obj    By doing that and making PostSharp part of your build process obj will be checked for null at runtime  See  PostSharp null check  Plain Code Solution  Or you can always code your own approach using plain old code  For example here is a struct that you can use to catch null references  It s modeled after the same concept as Nullable lt T gt     System Diagnostics DebuggerNonUserCode  public struct NotNull lt T gt  where T  class       private T  value       public T Value               get                       if   value    null                                throw new Exception  null value not allowed                               return  value                    set                       if  value    null                                throw new Exception  null value not allowed                                 value   value                       public static implicit operator T NotNull lt T gt  notNullValue                return notNullValue Value             public static implicit operator NotNull lt T gt  T value                return new NotNull lt T gt    Value   value              You would use very similar to the same way you would use Nullable lt T gt   except with the goal of accomplishing exactly the opposite - to not allow null  Here are some examples   NotNull lt Person gt  person   null     throws exception NotNull lt Person gt  person   new Person       OK NotNull lt Person gt  person   GetPerson       throws exception if GetPerson   returns null   NotNull lt T gt  is implicitly cast to and from T so you can use it just about anywhere you need it  For example  you can pass a Person object to a method that takes a NotNull lt Person gt    Person person   new Person   Name    John     WriteName person    public static void WriteName NotNull lt Person gt  person        Console WriteLine person Value Name       As you can see above as with nullable you would access the underlying value through the Value property  Alternatively  you can use an explicit or implicit cast  you can see an example with the return value below   Person person   GetPerson     public static NotNull lt Person gt  GetPerson         return new Person   Name    John         Or you can even use it when the method just returns T  in this case Person  by doing a cast  For example  the following code would just like the code above   Person person    NotNull lt Person gt  GetPerson     public static Person GetPerson         return new Person   Name    John         Combine with Extension  Combine NotNull lt T gt  with an extension method and you can cover even more situations  Here is an example of what the extension method can look like    System Diagnostics DebuggerNonUserCode  public static class NotNullExtension       public static T NotNull lt T gt  this T  this  where T  class               if   this    null                        throw new Exception  null value not allowed                       return  this            And here is an example of how it could be used   var person   GetPerson   NotNull      GitHub  For your reference I made the code above available on GitHub  you can find it at   https   github com luisperezphd NotNull  Related Language Feature  C  6 0 introduced the  null-conditional operator  that helps with this a little  With this feature  you can reference nested objects and if any one of them is null the whole expression returns null   This reduces the number of null checks you have to do in some cases  The syntax is to put a question mark before each dot  Take the following code for example   var address   country  State  County  City    Imagine that country is an object of type Country that has a property called State and so on  If country  State  County  or City is null then address will benull  Therefore you only have to check whetheraddressisnull    It s a great feature  but it gives you less information  It doesn t make it obvious which of the 4 is null   Built-in like Nullable   C  has a nice shorthand for Nullable lt T gt   you can make something nullable by putting a question mark after the type like so int     It would be nice if C  had something like the NotNull lt T gt  struct above and had a similar shorthand  maybe the exclamation point     so that you could write something like  public void WriteName Person  person

User · Answer

I have a different perspective to answering this  This sort of answers  what else can I do to avoid it    When working across different layers  for example in an MVC application  a controller needs services to call business operations  In such scenarios Dependency Injection Container can be used to initialize the services to avoid the NullReferenceException  So that means you don t need to worry about checking for null and just call the services from the controller as though they will always to available  and initialized  as either a singleton or a prototype   public class MyController       private ServiceA serviceA      private ServiceB serviceB       public MyController ServiceA serviceA  ServiceB serviceB                this serviceA   serviceA          this serviceB   serviceB             public void MyMethod                    We don t need to check null because the dependency injection container             injects it  provided you took care of bootstrapping it          var someObject   serviceA DoThis

User · Answer

You are using the object that contains the null value reference  So it s giving a null exception  In the example the string value is null and when checking its length  the exception occurred  Example  string value   null  if  value Length    0      lt -- Causes exception       Console WriteLine value       lt -- Never reached    The exception error is   Unhandled Exception  System NullReferenceException  Object reference not set to an instance of an object  at Program Main

User · Answer

NullReferenceException or Object reference not set to an instance of an object occurs when an object of the class you are trying to use is not instantiated  For example   Assume that you have a class named Student   public class Student       private string FirstName      private string LastName      public string GetFullName                 return FirstName   LastName            Now  consider another class where you are trying to retrieve the student s full name   public class StudentInfo             public string GetStudentName                 Student s          string fullname   s GetFullName            return fullname                    As seen in the above code  the statement  Student s - only declares the variable of type Student  note that the Student class is not instantiated at this point  Hence  when the statement s GetFullName   gets executed  it will throw the NullReferenceException

User · Answer

The error line  Object reference not set to an instance of an object    states that you have not assigned instance object to a object reference and still you are accessing properies methods of that object    for example  let say you have a class called myClass and it contains one property prop1    public Class myClass      public int prop1  get set       Now you are accessing this prop1 in some other class just like below   public class Demo        public void testMethod                  myClass ref   null          ref prop1   1     This line throws error            above line throws error because reference of class myClass is declared but not instantiated or an instance of object is not assigned to referecne of that class    To fix this you have to instantiate  assign object to reference of that class    public class Demo        public void testMethod                  myClass ref   null          ref   new myClass            ref prop1   1

User · Answer

It means your code used an object reference variable that was set to null  i e  it did not reference an actual object instance    To prevent the error  objects that could be null should be tested for null before being used   if  myvar    null           Go ahead and use myvar     myvar property         else          Whoops  myvar is null and cannot be used without first        assigning it to an instance reference        Attempting to use myvar here will result in NullReferenceException

User · Answer

TL DR  Try using Html Partial instead of Renderpage    I was getting Object reference not set to an instance of an object when I tried to render a View within a View by sending it a Model  like this          MyEntity M   new MyEntity       RenderPage   MyOtherView cshtml   M      error in  MyOtherView  the Model was Null   Debugging showed the model was Null inside MyOtherView  Until I changed it to          MyEntity M   new MyEntity       Html Partial   MyOtherView cshtml   M     And it worked   Furthermore  the reason I didn t have Html Partial to begin with was because Visual Studio sometimes throws error-looking squiggly lines under Html Partial if it s inside a differently constructed foreach loop  even though it s not really an error    inherits System Web Mvc WebViewPage        ViewBag Title    Entity Index       List lt MyEntity gt  MyEntities   new List lt MyEntity gt         MyEntities Add new MyEntity         MyEntities Add new MyEntity         MyEntities Add new MyEntity        lt div gt                 foreach var M in MyEntities                           Squiggly lines below  Hovering says  cannot convert method group  partial  to non-delegate type Object  did you intend to envoke the Method               Html Partial  MyOtherView cshtml                     lt  div gt    But I was able to run the application with no problems with this  error   I was able to get rid of the error by changing the structure of the foreach loop to look like this    foreach var M in MyEntities               Although I have a feeling it was because Visual Studio was misreading the ampersands and brackets

User · Answer

If one is getting this message during saving or compiling the build  just close all the files and then open any file to compile and save    For me the reason was that I had rename the file and old file was still open

User · Answer

Well  in simple terms   You are trying to access an object that isn t created or currently not in memory   So how to tackle this    Debug and let the debugger break    It will directly take you to the variable that is broken    Now your task is to simply fix this   Using the new keyword in the appropriate place  If it is caused on some database commands because the object isn t present then all you need to do is do a null check and handle it   if  i    null           Handle this    The hardest one    if the GC collected the object already    This generally occurs if you are trying to find an object using strings    That is  finding it by name of the object then it may happen that the GC might already cleaned it up    This is hard to find and will become quite a problem    A better way to tackle this is do null checks wherever necessary during the development process  This will save you a lot of time    By finding by name I mean some framework allow you to FIndObjects using strings and the code might look like this  FindObject  ObjectName

User · Answer

Literally the easiest way to fix a NullReferenceExeption has two ways  If you have a GameObject for example with a script attached and a variable named rb  rigidbody  this variable will start null when you start your game  This is why you get a NullReferenceExeption because the computer does not have data stored in that variable     I ll be using a RigidBody variable as an example  We can add data really easily actually in a few ways      Add a RigidBody to your object with AddComponent   Physics   Rigidbody Then go into your script and type rb   GetComponent lt Rigidbody gt     This line of code works best under your Start   or Awake   functions    You can add a component programmatically and assign the variable at the same time with one line of code  rb   AddComponent lt RigidBody gt         Further Notes  If you want unity to add a component to your object and you might have forgotten to add one  you can type  RequireComponent typeof RigidBody    above your class declaration  the space below all of your usings   Enjoy and have fun making games

User · Answer

While what causes a NullReferenceExceptions and approaches to avoid fix such an exception have been addressed in other answers  what many programmers haven t learned yet is how to independently debug such exceptions during development   In Visual Studio this is usually easy thanks to the Visual Studio Debugger     First  make sure that the correct error is going to be caught - see How do I allow breaking on   39 System NullReferenceException  39  in VS2010  Note1  Then either Start with Debugging  F5  or Attach  the VS Debugger  to Running Process  On occasion it may be useful to use Debugger Break  which will prompt to launch the debugger   Now  when the NullReferenceException is thrown  or unhandled  the debugger will stop  remember the rule set above   on the line on which the exception occurred  Sometimes the error will be easy to spot   For instance  in the following line the only code that can cause the exception is if myString evaluates to null  This can be verified by looking at the Watch Window or running expressions in the Immediate Window   var x   myString Trim      In more advanced cases  such as the following  you ll need to use one of the techniques above  Watch or Immediate Windows  to inspect the expressions to determine if str1 was null or if str2 was null   var x   str1 Trim     str2 Trim      Once where the exception is throw has been located  it s usually trivial to reason backwards to find out where the null value was  incorrectly  introduced --  Take the time required to understand the cause of the exception  Inspect for null expressions  Inspect the previous expressions which could have resulted in such null expressions  Add breakpoints and step through the program as appropriate  Use the debugger     1  If Break on Throws is too aggressive and the debugger stops on an NPE in the  NET or 3rd-party library  Break on User-Unhandled can be used to limit the exceptions caught  Additionally  VS2012 introduces Just My Code which I recommend enabling as well      If you are debugging with Just My Code enabled  the behavior is slightly different  With Just My Code enabled  the debugger ignores first-chance common language runtime  CLR  exceptions that are thrown outside of My Code and do not pass through My Code

User · Answer

Interestingly  none of the answers on this page mention the two edge cases  hope no one minds if I add them  Edge case  1  concurrent access to a Dictionary Generic dictionaries in  NET are not thread-safe and they sometimes might throw a NullReference or even  more frequent  a KeyNotFoundException when you try to access a key from two concurrent threads  The exception is quite misleading in this case  Edge case  2  unsafe code If a NullReferenceException is thrown by unsafe code  you might look at your pointer variables  and check them for IntPtr Zero or something  Which is the same thing   quot null pointer exception quot    but in unsafe code  variables are often cast to value-types arrays  etc   and you bang your head against the wall  wondering how a value-type can throw this exception   Another reason for non-using unsafe code unless you need it  by the way  Edge case  3  Visual Studio multi monitor setup with secondary monitor s  that has different DPI setting than the primary monitor This edge case is software-specific and pertains to Visual Studio 2019 IDE  and possibly earlier versions   Method to reproduce the problem  drag any component from the Toolbox to a Windows form on a non-primary monitor with different DPI setting than the primary monitor  and you get a pop-up with    Object reference not set to an instance of an object     According to this thread this issue has been known for quite some time and at the time of writing it still hasn t been fixed

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If we consider common scenarios where this exception can be thrown  accessing properties withing object at the top   Ex    string postalcode Customer Address PostalCode     if customer or address is null   this will through exeption   in here   if address is null   then you will get NullReferenceException   So  as a practice we should always use null check  before accessing properties in such objects  specially in generic   string postalcode Customer  Address  PostalCode    if customer or address is null   this will return null  without through a exception

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This is basically is a Null reference exception  As Microsoft states-     A NullReferenceException exception is thrown when you try to access a   member of a type whose value is null    What does that mean   That means if any member which doesn   t hold any value and we are making that member to perform certain task then the system will undoubtedly toss a message and say-     Hey wait  that member has no values so it can   t perform the task which you are handing it over      The exception itself says that something is being referred but whose value is not being set  So this denotes that it only occurs while using reference types as Value types are non-nullable   NullReferenceException won t occur if we are using Value type members   class Program       static void Main string   args                string str   null          Console WriteLine str Length           Console ReadLine              The above code shows simple string which is assigned with a null value   Now  when I try to print the length of the string str  I do get An unhandled exception of type    System NullReferenceException    occurred message because member str is pointing to null and there can   t be any length of null      NullReferenceException    also occurs when we forget to instantiate a reference type    Suppose I have a class and member method in it  I have not instantiated my class but only named my class  Now if I try to use the method  the compiler will throw an error or issue a warning  depending on the compiler    class Program       static void Main string   args                MyClass1 obj          obj foo       Use of unassigned local variable  obj           public class MyClass1       internal void foo                 Console WriteLine  hello from foo               Compiler for the above code raises an error that variable obj is unassigned which signifies that our variable has null values or nothing  Compiler for the above code raises an error that variable obj is unassigned which signifies that our variable has null values or nothing   Why it occurs    NullReferenceException arises due to our fault for not checking the object   s value  We often leave the object values unchecked in the code development  It also arises when we forget to instantiate our objects  Using methods  properties  collections etc  which can return or set null values can also be the cause of this exception    How can it be avoided   There are various ways and methods to avoid this renowned exception    Explicit Checking  We should adhere to the tradition of checking the objects  properties  methods  arrays  and collections whether they are null  This can be simply implemented using conditional statements like if-else if-else etc  Exception handling  One of the important ways of managing this exception  Using simple try-catch-finally blocks we can control this exception and also maintain a log of it  This can be very useful when your application is on production stage  Null operators  Null Coalescing operator and null conditional operators can also be used in handy while setting values to objects  variables  properties and fields  Debugger  For developers  we have the big weapon of Debugging with us  If have we face NullReferenceException during the development face we can use the debugger to get to the source of the exception  In-built method  System methods such as GetValueOrDefault   IsNullOrWhiteSpace   and IsNullorEmpty    checks for nulls and assign the default value if there is a null value    There are many good answers already here  You can also check more detailed description with examples on my blog   Hope this helps too

User · Answer

NullReference Exception     Visual Basic The NullReference Exception for Visual Basic is no different from the one in C   After all  they are both reporting the same exception defined in the  NET Framework which they both use  Causes unique to Visual Basic are rare  perhaps only one   This answer will use Visual Basic terms  syntax  and context  The examples used come from a large number of past Stack   Overflow questions  This is to maximize relevance by using the kinds of situations often seen in posts  A bit more explanation is also provided for those who might need it  An example similar to yours is very likely listed here  Note   This is concept-based  there is no code for you to paste into your project  It is intended to help you understand what causes a NullReferenceException  NRE   how to find it  how to fix it  and how to avoid it  An NRE can be caused many ways so this is unlikely to be your sole encounter  The examples  from Stack   Overflow posts  do not always show the best way to do something in the first place  Typically  the simplest remedy is used   Basic Meaning The message  quot Object not set to an instance of Object quot  means you are trying to use an object which has not been initialized  This boils down to one of these   Your code declared an object variable  but it did not initialize it  create an instance or  instantiate  it  Something which your code assumed would initialize an object  did not Possibly  other code prematurely invalidated an object still in use  Finding The Cause Since the problem is an object reference which is Nothing  the answer is to examine them to find out which one  Then determine why it is not initialized  Hold the mouse over the various variables and Visual Studio  VS  will show their values - the culprit will be Nothing   You should also remove any Try Catch blocks from the relevant code  especially ones where there is nothing in the Catch block  This will cause your code to crash when it tries to use an object which is Nothing  This is what you want because it will identify the exact location of the problem  and allow you to identify the object causing it  A MsgBox in the Catch which displays Error while    will be of little help  This method also leads to very bad Stack   Overflow questions  because you can t describe the actual exception  the object involved or even the line of code where it happens  You can also use the Locals Window  Debug - gt  Windows - gt  Locals  to examine your objects  Once you know what and where the problem is  it is usually fairly easy to fix and faster than posting a new question  See also   Breakpoints MSDN  How to  Use the Try Catch Block to Catch Exceptions MSDN  Best Practices for Exceptions  Examples and Remedies Class Objects   Creating an Instance  Dim reg As CashRegister     TextBox1 Text   reg Amount           NRE  The problem is that Dim does not create a CashRegister object  it only declares a variable named reg of that Type  Declaring an object variable and creating an instance are two different things  Remedy The New operator can often be used to create the instance when you declare it  Dim reg As New CashRegister           New  creates instance  invokes the constructor    Longer  more explicit form  Dim reg As CashRegister   New CashRegister  When it is only appropriate to create the instance later  Private reg As CashRegister           Declare       reg   New CashRegister                Create instance  Note  Do not use Dim again in a procedure  including the constructor  Sub New   Private reg As CashRegister       Public Sub New              Dim reg As New CashRegister End Sub  This will create a local variable  reg  which exists only in that context  sub   The reg variable with module level Scope which you will use everywhere else remains Nothing   Missing the New operator is the  1 cause of NullReference Exceptions seen in the Stack   Overflow questions reviewed  Visual Basic tries to make the process clear repeatedly using New  Using the New Operator creates a new object and calls Sub New -- the constructor -- where your object can perform any other initialization   To be clear  Dim  or Private  only declares a variable and its Type    The Scope of the variable - whether it exists for the entire module class or is local to a procedure - is determined by where it is declared  Private   Friend   Public defines the access level  not Scope  For more information  see   New Operator Scope in Visual Basic Access Levels in Visual Basic Value Types and Reference Types   Arrays Arrays must also be instantiated  Private arr as String    This array has only been declared  not created  There are several ways to initialize an array  Private arr as String     New String 10      or Private arr   As String   New String 10       For a local array  in a procedure  and using  Option Infer   Dim arr   New String 10      Note  Beginning with VS 2010  when initializing a local array using a literal and Option Infer  the As  lt Type gt  and New elements are optional  Dim myDbl As Double      1 5  2  9 9  18  3 14  Dim myDbl   New Double    1 5  2  9 9  18  3 14  Dim myDbl      1 5  2  9 9  18  3 14   The data Type and array size are inferred from the data being assigned  Class Module level declarations still require As  lt Type gt  with Option Strict  Private myDoubles As Double      1 5  2  9 9  18  3 14   Example  Array of class objects Dim arrFoo 5  As Foo  For i As Integer   0 To arrFoo Count - 1    arrFoo i  Bar   i   10         Exception Next  The array has been created  but the Foo objects in it have not  Remedy For i As Integer   0 To arrFoo Count - 1     arrFoo i    New Foo             Create Foo instance     arrFoo i  Bar   i   10 Next  Using a List Of T  will make it quite difficult to have an element without a valid object  Dim FooList As New List Of Foo        List created  but it is empty Dim f As Foo                          Temporary variable for the loop  For i As Integer   0 To 5     f   New Foo                        Foo instance created     f Bar    i   10     FooList Add f                      Foo object added to list Next  For more information  see   Option Infer Statement Scope in Visual Basic Arrays in Visual Basic   Lists and Collections  NET collections  of which there are many varieties - Lists  Dictionary  etc   must also be instantiated or created  Private myList As List Of String     myList Add  quot ziggy quot               NullReference  You get the same exception for the same reason - myList was only declared  but no instance created  The remedy is the same  myList   New List Of String     Or create an instance when declared  Private myList As New List Of String   A common oversight is a class which uses a collection Type  Public Class Foo     Private barList As List Of Bar       Friend Function BarCount As Integer         Return barList Count     End Function      Friend Sub AddItem newBar As Bar          If barList Contains newBar    False Then             barList Add newBar          End If     End Function  Either procedure will result in an NRE  because barList is only declared  not instantiated  Creating an instance of Foo will not also create an instance of the internal barList  It may have been the intent to do this in the constructor  Public Sub New           Constructor       Stuff to do when a new Foo is created        barList   New List Of Bar  End Sub  As before  this is incorrect  Public Sub New         Creates another barList local to this procedure      Dim barList As New List Of Bar  End Sub  For more information  see List Of T  Class   Data Provider Objects Working with databases presents many opportunities for a NullReference because there can be many objects  Command  Connection  Transaction  Dataset  DataTable  DataRows      in use at once   Note  It does not matter which data provider you are using -- MySQL  SQL Server  OleDB  etc  -- the concepts are the same  Example 1 Dim da As OleDbDataAdapter Dim ds As DataSet Dim MaxRows As Integer  con Open   Dim sql    quot SELECT   FROM tblfoobar List quot  da   New OleDbDataAdapter sql  con  da Fill ds   quot foobar quot   con Close    MaxRows   ds Tables  quot foobar quot   Rows Count        Error  As before  the ds Dataset object was declared  but an instance was never created  The DataAdapter will fill an existing DataSet  not create one  In this case  since ds is a local variable  the IDE warns you that this might happen   When declared as a module class level variable  as appears to be the case with con  the compiler can t know if the object was created by an upstream procedure  Do not ignore warnings  Remedy Dim ds As New DataSet  Example 2 ds   New DataSet da   New OleDBDataAdapter sql  con  da Fill ds   quot Employees quot    txtID Text   ds Tables  quot Employee quot   Rows 0  Item 1  txtID Name   ds Tables  quot Employee quot   Rows 0  Item 2   A typo is a problem here  Employees vs Employee  There was no DataTable named  quot Employee quot  created  so a NullReferenceException results trying to access it  Another potential problem is assuming there will be Items which may not be so when the SQL includes a WHERE clause  Remedy Since this uses one table  using Tables 0  will avoid spelling errors   Examining Rows Count can also help  If ds Tables 0  Rows Count  gt  0 Then     txtID Text   ds Tables 0  Rows 0  Item 1      txtID Name   ds Tables 0  Rows 0  Item 2  End If  Fill is a function returning the number of Rows affected which can also be tested  If da Fill ds   quot Employees quot    gt  0 Then     Example 3 Dim da As New OleDb OleDbDataAdapter  quot SELECT TICKET TICKET NO          TICKET CUSTOMER ID      FROM TICKET RESERVATION AS TICKET INNER JOIN         FLIGHT DETAILS AS FLIGHT     WHERE  TICKET TICKET NO       quot   con  Dim ds As New DataSet da Fill ds   If ds Tables  quot TICKET RESERVATION quot   Rows Count  gt  0 Then  The DataAdapter will provide TableNames as shown in the previous example  but it does not parse names from the SQL or database table  As a result  ds Tables  quot TICKET RESERVATION quot   references a non-existent table  The Remedy is the same  reference the table by index  If ds Tables 0  Rows Count  gt  0 Then  See also DataTable Class   Object Paths   Nested If myFoo Bar Items IsNot Nothing Then         The code is only testing Items while both myFoo and Bar may also be Nothing  The remedy is to test the entire chain or path of objects one at a time  If  myFoo IsNot Nothing  AndAlso      myFoo Bar IsNot Nothing  AndAlso      myFoo Bar Items IsNot Nothing  Then           AndAlso is important  Subsequent tests will not be performed once the first False condition is encountered  This allows the code to safely  drill  into the object s  one  level  at a time  evaluating myFoo Bar only after  and if  myFoo is determined to be valid  Object chains or paths can get quite long when coding complex objects  myBase myNodes 3  Layer SubLayer Foo Files Add  quot somefilename quot    It is not possible to reference anything  downstream  of a null object   This also applies to controls  myWebBrowser Document GetElementById  quot formfld1 quot   InnerText    quot some value quot   Here  myWebBrowser or Document could be Nothing or the formfld1 element may not exist   UI Controls Dim cmd5 As New SqlCommand  quot select Cartons  Pieces  Foobar  quot          amp   quot FROM Invoice where invoice no     quot   amp         Me ComboBox5 SelectedItem ToString Trim  amp   quot   And category     quot   amp         Me ListBox1 SelectedItem ToString Trim  amp   quot   And item name     quot   amp         Me ComboBox2 SelectedValue ToString Trim  amp   quot   And expiry date     quot   amp         Me expiry Text  amp   quot   quot   con   Among other things  this code does not anticipate that the user may not have selected something in one or more UI controls   ListBox1 SelectedItem may well be Nothing  so ListBox1 SelectedItem ToString will result in an NRE  Remedy Validate data before using it  also use Option Strict and SQL parameters   Dim expiry As DateTime           for text date validation If  ComboBox5 SelectedItems Count  gt  0  AndAlso      ListBox1 SelectedItems Count  gt  0  AndAlso      ComboBox2 SelectedItems Count  gt  0  AndAlso      DateTime TryParse expiry Text  expiry  Then           do stuff Else     MessageBox Show    error message     End If  Alternatively  you can use  ComboBox5 SelectedItem IsNot Nothing  AndAlso     Visual Basic Forms Public Class Form1      Private NameBoxes   New TextBox 5   Controls  quot TextBox1 quot                         Controls  quot TextBox2 quot    Controls  quot TextBox3 quot                         Controls  quot TextBox4 quot    Controls  quot TextBox5 quot                         Controls  quot TextBox6 quot           same thing in a different format      Private boxList As New List Of TextBox  From  TextBox1  TextBox2  TextBox3             Immediate NRE      Private somevar As String   Me Controls  quot TextBox1 quot   Text  This is a fairly common way to get an NRE  In C   depending on how it is coded  the IDE will report that Controls does not exist in the current context  or  quot cannot reference non-static member quot   So  to some extent  this is a VB-only situation  It is also complex because it can result in a failure cascade  The arrays and collections cannot be initialized this way  This initialization code will run before the constructor creates the Form or the Controls  As a result   Lists and Collection will simply be empty The Array will contain five elements of Nothing The somevar assignment will result in an immediate NRE because Nothing doesn t have a  Text property  Referencing array elements later will result in an NRE  If you do this in Form Load  due to an odd bug  the IDE may not report the exception when it happens  The exception will pop up later when your code tries to use the array  This  quot silent exception quot  is detailed in this post  For our purposes  the key is that when something catastrophic happens while creating a form  Sub New or Form Load event   exceptions may go unreported  the code exits the procedure and just displays the form  Since no other code in your Sub New or Form Load event will run after the NRE  a great many other things can be left uninitialized  Sub Form Load                 Dim name As String   NameBoxes 2  Text          NRE               More code  which will likely not be executed           End Sub  Note this applies to any and all control and component references making these illegal where they are  Public Class Form1      Private myFiles   As String   Me OpenFileDialog1 FileName  amp          Private dbcon As String   OpenFileDialog1 FileName  amp   quot  Jet Oledb    quot      Private studentName As String   TextBox13 Text  Partial Remedy It is curious that VB does not provide a warning  but the remedy is to declare the containers at the form level  but initialize them in form load event handler when the controls do exist  This can be done in Sub New as long as your code is after the InitializeComponent call    Module level declaration Private NameBoxes as TextBox   Private studentName As String    Form Load  Form Shown or Sub New      Using the OP s approach  illegal using OPTION STRICT  NameBoxes   New TextBox    Me Controls  quot TextBox1 quot    Me Controls  quot TestBox2 quot         studentName   TextBox32 Text             For simple control references  The array code may not be out of the woods yet  Any controls which are in a container control  like a GroupBox or Panel  will not be found in Me Controls  they will be in the Controls collection of that Panel or GroupBox  Nor will a control be returned when the control name is misspelled   quot TeStBox2 quot    In such cases  Nothing will again be stored in those array elements and an NRE will result when you attempt to reference it  These should be easy to find now that you know what you are looking for    quot Button2 quot  resides on a Panel Remedy Rather than indirect references by name using the form s Controls collection  use the control reference    Declaration Private NameBoxes As TextBox      Initialization -  simple and easy to read  hard to botch  NameBoxes   New TextBox    TextBox1  TextBox2          Initialize a List NamesList   New List Of TextBox   TextBox1  TextBox2  TextBox3        or NamesList   New List Of TextBox  NamesList AddRange  TextBox1  TextBox2  TextBox3        Function Returning Nothing Private bars As New List Of Bars           Declared and created  Public Function BarList   As List Of Bars      bars Clear     If someCondition Then         For n As Integer   0 to someValue             bars Add GetBar n           Next n     Else         Exit Function     End If      Return bars End Function  This is a case where the IDE will warn you that  not all paths return a value and a NullReferenceException may result   You can suppress the warning  by replacing Exit Function with Return Nothing  but that does not solve the problem  Anything which tries to use the return when someCondition   False will result in an NRE  bList   myFoo BarList   For Each b As Bar in bList        EXCEPTION            Remedy Replace Exit Function in the function with Return bList  Returning an empty List is not the same as returning Nothing  If there is a chance that a returned object can be Nothing  test before using it   bList   myFoo BarList    If bList IsNot Nothing Then      Poorly Implemented Try Catch A badly implemented Try Catch can hide where the problem is and result in new ones  Dim dr As SqlDataReader Try     Dim lnk As LinkButton   TryCast sender  LinkButton      Dim gr As GridViewRow   DirectCast lnk NamingContainer  GridViewRow      Dim eid As String   GridView1 DataKeys gr RowIndex  Value ToString       ViewState  quot username quot     eid     sqlQry    quot select FirstName  Surname  DepartmentName  ExtensionName  jobTitle               Pager  mailaddress  from employees1 where username   quot   amp  eid  amp   quot   quot      If connection State  lt  gt  ConnectionState Open Then         connection Open       End If     command   New SqlCommand sqlQry  connection        More code fooing and barring      dr   command ExecuteReader       If dr Read   Then         lblFirstName Text   Convert ToString dr  quot FirstName quot                    End If     mpe Show   Catch  Finally     command Dispose       dr Close                  lt -- NRE     connection Close   End Try  This is a case of an object not being created as expected  but also demonstrates the counter usefulness of an empty Catch  There is an extra comma in the SQL  after  mailaddress   which results in an exception at  ExecuteReader  After the Catch does nothing  Finally tries to perform clean up  but since you cannot Close a null DataReader object  a brand new NullReferenceException results  An empty Catch block is the devil s playground  This OP was baffled why he was getting an NRE in the Finally block  In other situations  an empty Catch may result in something else much further downstream going haywire and cause you to spend time looking at the wrong things in the wrong place for the problem   The  quot silent exception quot  described above provides the same entertainment value   Remedy Don t use empty Try Catch blocks - let the code crash so you can a  identify the cause b  identify the location and c  apply a proper remedy   Try Catch blocks are not intended to hide exceptions from the person uniquely qualified to fix them - the developer   DBNull is not the same as Nothing For Each row As DataGridViewRow In dgvPlanning Rows     If Not IsDBNull row Cells 0  Value  Then              The IsDBNull function is used to test if a value equals System DBNull   From MSDN   The System DBNull value indicates that the Object represents missing or non-existent data  DBNull is not the same as Nothing  which indicates that a variable has not yet been initialized   Remedy If row Cells 0  IsNot Nothing Then      As before  you can test for Nothing  then for a specific value  If  row Cells 0  IsNot Nothing  AndAlso  IsDBNull row Cells 0  Value    False  Then  Example 2 Dim getFoo    From f In dbContext FooBars                Where f something   something                Select f  FirstOrDefault  If Not IsDBNull getFoo  Then     If IsDBNull getFoo user id  Then         txtFirst Text   getFoo first name     Else             FirstOrDefault returns the first item or the default value  which is Nothing for reference types and never DBNull  If getFoo IsNot Nothing Then      Controls Dim chk As CheckBox  chk   CType Me Controls chkName   CheckBox  If chk Checked Then     Return chk End If  If a CheckBox with chkName can t be found  or exists in a GroupBox   then chk will be Nothing and be attempting to reference any property will result in an exception  Remedy If  chk IsNot Nothing  AndAlso  chk Checked  Then      The DataGridView The DGV has a few quirks seen periodically  dgvBooks DataSource   loan Books dgvBooks Columns  quot ISBN quot   Visible   True         NullReferenceException dgvBooks Columns  quot Title quot   DefaultCellStyle Format    quot C quot  dgvBooks Columns  quot Author quot   DefaultCellStyle Format    quot C quot  dgvBooks Columns  quot Price quot   DefaultCellStyle Format    quot C quot   If dgvBooks has AutoGenerateColumns   True  it will create the columns  but it does not name them  so the above code fails when it references them by name  Remedy Name the columns manually  or reference by index  dgvBooks Columns 0  Visible   True  Example 2     Beware of the NewRow xlWorkSheet   xlWorkBook Sheets  quot sheet1 quot    For i   0 To myDGV RowCount - 1     For j   0 To myDGV ColumnCount - 1         For k As Integer   1 To myDGV Columns Count             xlWorkSheet Cells 1  k    myDGV Columns k - 1  HeaderText             xlWorkSheet Cells i   2  j   1    myDGV j  i  Value ToString           Next     Next Next  When your DataGridView has AllowUserToAddRows as True  the default   the Cells in the blank new row at the bottom will all contain Nothing   Most attempts to use the contents  for example  ToString  will result in an NRE  Remedy Use a For Each loop and test the IsNewRow property to determine if it is that last row  This works whether AllowUserToAddRows is true or not  For Each r As DataGridViewRow in myDGV Rows     If r IsNewRow   False Then            ok to use this row  If you do use a For n loop  modify the row count or use Exit For when IsNewRow is true   My Settings  StringCollection  Under certain circumstances  trying to use an item from My Settings which is a StringCollection can result in a NullReference the first time you use it  The solution is the same  but not as obvious  Consider  My Settings FooBars Add  quot ziggy quot             foobars is a string collection  Since VB is managing Settings for you  it is reasonable to expect it to initialize the collection  It will  but only if you have previously added an initial entry to the collection  in the Settings editor   Since the collection is  apparently  initialized when an item is added  it remains Nothing when there are no items in the Settings editor to add  Remedy Initialize the settings collection in the form s Load event handler  if when needed  If My Settings FooBars Is Nothing Then     My Settings FooBars   New System Collections Specialized StringCollection End If  Typically  the Settings collection will only need to be initialized the first time the application runs  An alternate remedy is to add an initial value to your collection in Project - gt  Settings   FooBars  save the project  then remove the fake value   Key Points You probably forgot the New operator  or Something yo

User · Answer

Another case where NullReferenceExceptions can happen is the  incorrect  use of the as operator   class Book       public string Name   get  set      class Car      Car mycar   new Car    Book mybook   mycar as Book       Incompatible conversion -- gt  mybook   null  Console WriteLine mybook Name        NullReferenceException   Here  Book and Car are incompatible types  a Car cannot be converted cast to a Book   When this cast fails  as returns null   Using mybook after this causes a NullReferenceException   In general  you should use a cast or as  as follows   If you are expecting the type conversion to always succeed  ie  you know what the object should be ahead of time   then you should use a cast   ComicBook cb    ComicBook specificBook    If you are unsure of the type  but you want to try to use it as a specific type  then use as   ComicBook cb   specificBook as ComicBook  if  cb    null

User · Answer

What is the cause  Bottom Line You are trying to use something that is null  or Nothing in VB NET   This means you either set it to null  or you never set it to anything at all  Like anything else  null gets passed around  If it is null in method  quot A quot   it could be that method  quot B quot  passed a null to method  quot A quot   null can have different meanings   Object variables which are uninitialized and hence point to nothing  In this case  if you access properties or methods of such objects  it causes a NullReferenceException  The developer is using null intentionally to indicate there is no meaningful value available  Note that C  has the concept of nullable datatypes for variables  like database tables can have nullable fields  - you can assign null to them to indicate there is no value stored in it  for example int  a   null  where the question mark indicates it is allowed to store null in variable a  You can check that either with if  a HasValue        or with if  a  null         Nullable variables  like a this example  allow to access the value via a Value explicitly  or just as normal via a  Note that accessing it via a Value throws an InvalidOperationException instead of a NullReferenceException if a is null - you should do the check beforehand  i e  if you have another on-nullable variable int b  then you should do assignments like if  a HasValue    b   a Value    or shorter if  a    null    b   a      The rest of this article goes into more detail and shows mistakes that many programmers often make which can lead to a NullReferenceException  More Specifically The runtime throwing a NullReferenceException always means the same thing  you are trying to use a reference  and the reference is not initialized  or it was once initialized  but is no longer initialized   This means the reference is null  and you cannot access members  such as methods  through a null reference  The simplest case  string foo   null  foo ToUpper     This will throw a NullReferenceException at the second line because you can t call the instance method ToUpper   on a string reference pointing to null  Debugging How do you find the source of a NullReferenceException  Apart from looking at the exception itself  which will be thrown exactly at the location where it occurs  the general rules of debugging in Visual Studio apply  place strategic breakpoints and inspect your variables  either by hovering the mouse over their names  opening a  Quick Watch window or using the various debugging panels like Locals and Autos  If you want to find out where the reference is or isn t set  right-click its name and select  quot Find All References quot   You can then place a breakpoint at every found location and run your program with the debugger attached  Every time the debugger breaks on such a breakpoint  you need to determine whether you expect the reference to be non-null  inspect the variable  and verify that it points to an instance when you expect it to  By following the program flow this way  you can find the location where the instance should not be null  and why it isn t properly set  Examples Some common scenarios where the exception can be thrown  Generic ref1 ref2 ref3 member  If ref1 or ref2 or ref3 is null  then you ll get a NullReferenceException  If you want to solve the problem  then find out which one is null by rewriting the expression to its simpler equivalent  var r1   ref1  var r2   r1 ref2  var r3   r2 ref3  r3 member  Specifically  in HttpContext Current User Identity Name  the HttpContext Current could be null  or the User property could be null  or the Identity property could be null  Indirect public class Person        public int Age   get  set      public class Book        public Person Author   get  set      public class Example        public void Foo                  Book b1   new Book            int authorAge   b1 Author Age     You never initialized the Author property                                            there is no Person to get an Age from           If you want to avoid the child  Person  null reference  you could initialize it in the parent  Book  object s constructor  Nested Object Initializers The same applies to nested object initializers  Book b1   new Book        Author     Age   45        This translates to  Book b1   new Book    b1 Author Age   45   While the new keyword is used  it only creates a new instance of Book  but not a new instance of Person  so the Author the property is still null  Nested Collection Initializers public class Person        public ICollection lt Book gt  Books   get  set      public class Book        public string Title   get  set       The nested collection Initializers behave the same  Person p1   new Person        Books              new Book   Title    quot Title1 quot              new Book   Title    quot Title2 quot               This translates to  Person p1   new Person    p1 Books Add new Book   Title    quot Title1 quot      p1 Books Add new Book   Title    quot Title2 quot       The new Person only creates an instance of Person  but the Books collection is still null  The collection Initializer syntax does not create a collection for p1 Books  it only translates to the p1 Books Add      statements  Array int   numbers   null  int n   numbers 0      numbers is null  There is no array to index   Array Elements Person   people   new Person 5   people 0  Age   20    people 0  is null  The array was allocated but not                       initialized  There is no Person to set the Age for   Jagged Arrays long     array   new long 1     array 0  0    3     is null because only the first dimension is yet initialized                      Use array 0    new long 2   first   Collection List Dictionary Dictionary lt string  int gt  agesForNames   null  int age   agesForNames  quot Bob quot       agesForNames is null                                    There is no Dictionary to perform the lookup   Range Variable  Indirect Deferred  public class Person        public string Name   get  set      var people   new List lt Person gt     people Add null   var names   from p in people select p Name  string firstName   names First       Exception is thrown here  but actually occurs                                      on the line above    quot p quot  is null because the                                      first element we added to the list is null   Events  C   public class Demo       public event EventHandler StateChanged           protected virtual void OnStateChanged EventArgs e                        StateChanged this  e      Exception is thrown here                                    if no event handlers have been attached                                   to StateChanged event           Note  The VB NET compiler inserts null checks for event usage  so it s not necessary to check events for Nothing in VB NET   Bad Naming Conventions  If you named fields differently from locals  you might have realized that you never initialized the field  public class Form1       private Customer customer           private void Form1 Load object sender  EventArgs e                 Customer customer   new Customer            customer Name    quot John quot                  private void Button Click object sender  EventArgs e                MessageBox Show customer Name            This can be solved by following the convention to prefix fields with an underscore      private Customer  customer   ASP NET Page Life cycle  public partial class Issues Edit   System Web UI Page       protected TestIssue myIssue       protected void Page Load object sender  EventArgs e                if   IsPostBack                            Only called on first load  not when button clicked              myIssue   new TestIssue                                  protected void SaveButton Click object sender  EventArgs e                myIssue Entry    quot NullReferenceException here  quot            ASP NET Session Values    if the  quot FirstName quot  session value has not yet been set     then this line will throw a NullReferenceException string firstName   Session  quot FirstName quot   ToString     ASP NET MVC empty view models If the exception occurs when referencing a property of  Model in an ASP NET MVC View  you need to understand that the Model gets set in your action method  when you return a view  When you return an empty model  or model property  from your controller  the exception occurs when the views access it     Controller public class Restaurant Controller       public ActionResult Search                 return View        Forgot the provide a Model here              Razor view   foreach  var restaurantSearch in Model RestaurantSearch      Throws            lt p gt  Model somePropertyName lt  p gt   lt  -- Also throws -- gt   WPF Control Creation Order and Events WPF controls are created during the call to InitializeComponent in the order they appear in the visual tree   A NullReferenceException will be raised in the case of early-created controls with event handlers  etc    that fire during InitializeComponent which reference late-created controls  For example   lt Grid gt       lt  -- Combobox declared first -- gt       lt ComboBox Name  quot comboBox1 quot                 Margin  quot 10 quot                SelectedIndex  quot 0 quot                 SelectionChanged  quot comboBox1 SelectionChanged quot  gt          lt ComboBoxItem Content  quot Item 1 quot    gt          lt ComboBoxItem Content  quot Item 2 quot    gt          lt ComboBoxItem Content  quot Item 3 quot    gt       lt  ComboBox gt                lt  -- Label declared later -- gt       lt Label Name  quot label1 quot              Content  quot Label quot             Margin  quot 10 quot    gt   lt  Grid gt   Here comboBox1 is created before label1  If comboBox1 SelectionChanged attempts to reference  label1  it will not yet have been created  private void comboBox1 SelectionChanged object sender  SelectionChangedEventArgs e        label1 Content   comboBox1 SelectedIndex ToString       NullReference here      Changing the order of the declarations in the XAML  i e   listing label1 before comboBox1  ignoring issues of design philosophy  would at least resolve the NullReferenceException here  Cast with as var myThing   someObject as Thing   This doesn t throw an InvalidCastException but returns a null when the cast fails  and when someObject is itself null   So be aware of that  LINQ FirstOrDefault   and SingleOrDefault   The plain versions First   and Single   throw exceptions when there is nothing  The  quot OrDefault quot  versions return null in that case  So be aware of that  foreach foreach throws when you try to iterate null collection  Usually caused by unexpected null result from methods that return collections  List lt int gt  list   null      foreach var v in list         exception  More realistic example - select nodes from XML document  Will throw if nodes are not found but initial debugging shows that all properties valid  foreach  var node in myData MyXml DocumentNode SelectNodes  quot   Data quot      Ways to Avoid Explicitly check for null and ignore null values  If you expect the reference sometimes to be null  you can check for it being null before accessing instance members  void PrintName Person p        if  p    null                 Console WriteLine p Name            Explicitly check for null and provide a default value  Methods call you expect to return an instance can return null  for example when the object being sought cannot be found  You can choose to return a default value when this is the case  string GetCategory Book b         if  b    null          return  quot Unknown quot       return b Category     Explicitly check for null from method calls and throw a custom exception  You can also throw a custom exception  only to catch it in the calling code  string GetCategory string bookTitle         var book   library FindBook bookTitle       This may return null     if  book    null          throw new BookNotFoundException bookTitle       Your custom exception     return book Category     Use Debug Assert if a value should never be null  to catch the problem earlier than the exception occurs  When you know during development that a method maybe can  but never should return null  you can use Debug Assert   to break as soon as possible when it does occur  string GetTitle int knownBookID            You know this should never return null      var book   library GetBook knownBookID             Exception will occur on the next line instead of at the end of this method      Debug Assert book    null   quot Library didn t return a book for known book ID  quot            Some other code      return book Title     Will never throw NullReferenceException in Debug mode     Though this check will not end up in your release build  causing it to throw the NullReferenceException again when book    null at runtime in release mode  Use GetValueOrDefault   for nullable value types to provide a default value when they are null  DateTime  appointment   null  Console WriteLine appointment GetValueOrDefault DateTime Now       Will display the default value provided  DateTime Now   because appointment is null   appointment   new DateTime 2022  10  20   Console WriteLine appointment GetValueOrDefault DateTime Now       Will display the appointment date  not the default  Use the null coalescing operator      C   or If    VB   The shorthand to providing a default value when a null is encountered  IService CreateService ILogger log  Int32  frobPowerLevel       var serviceImpl   new MyService log    NullLog Instance           Note that the above  quot GetValueOrDefault   quot  can also be rewritten to use       the coalesce operator     serviceImpl FrobPowerLevel   frobPowerLevel    5     Use the null condition operator     or   x  for arrays  available in C  6 and VB NET 14   This is also sometimes called the safe navigation or Elvis  after its shape  operator  If the expression on the left side of the operator is null  then the right side will not be evaluated  and null is returned instead  That means cases like this  var title   person Title ToUpper     If the person does not have a title  this will throw an exception because it is trying to call ToUpper on a property with a null value  In C  5 and below  this can be guarded with  var title   person Title    null   null   person Title ToUpper     Now the title variable will be null instead of throwing an exception  C  6 introduces a shorter syntax for this  var title   person Title  ToUpper     This will result in the title variable being null  and the call to ToUpper is not made if person Title is null  Of course  you still have to check title for null or use the null condition operator together with the null coalescing operator      to supply a default value     regular null check int titleLength   0  if  title    null      titleLength   title Length     If title is null  this would throw NullReferenceException         combining the     and the      operator int titleLength   title  Length    0   Likewise  for arrays you can use   i  as follows  int   myIntArray   null  var i   5  int  elem   myIntArray  i   if   elem HasValue  Console WriteLine  quot No value quot     This will do the following  If myIntArray is null  the expression returns null and you can safely check it  If it contains an array  it will do the same as  elem   myIntArray i   and returns the i lt sup gt th lt  sup gt  element  Use null context  available in C  8   Introduced in C  8 there null context s and nullable reference types perform static analysis on variables and provides a compiler warning if a value can be potentially null or have been set to null  The nullable reference types allows types to be explicitly allowed to be null  The nullable annotation context and nullable warning context can be set for a project using the Nullable element in your csproj file  This element configures how the compiler interprets the nullability of types and what warnings are generated  Valid settings are   enable  The nullable annotation context is enabled  The nullable warning context is enabled  Variables of a reference type  string for example  are non-nullable  All nullability warnings are enabled  disable  The nullable annotation context is disabled  The nullable warning context is disabled  Variables of a reference type are oblivious  just like earlier versions of C   All nullability warnings are disabled  safeonly  The nullable annotation context is enabled  The nullable warning context is safeonly  Variables of a reference type are nonnullable  All safety nullability warnings are enabled  warnings  The nullable annotation context is disabled  The nullable warning context is enabled  Variables of a reference type are oblivious  All nullability warnings are enabled  safeonlywarnings  The nullable annotation context is disabled  The nullable warning context is safeonly  Variables of a reference type are oblivious  All safety nullability warnings are enabled   A nullable reference type is noted using the same syntax as nullable value types  a   is appended to the type of the variable  Special techniques for debugging and fixing null derefs in iterators C  supports  quot iterator blocks quot   called  quot generators quot  in some other popular languages    Null dereference exceptions can be particularly tricky to debug in iterator blocks because of deferred execution  public IEnumerable lt Frob gt  GetFrobs FrobFactory f  int count        for  int i   0  i  lt  count    i      yield return f MakeFrob          FrobFactory factory   whatever  IEnumerable lt Frobs gt  frobs   GetFrobs        foreach Frob frob in frobs           If whatever results in null then MakeFrob will throw   Now  you might think that the right thing to do is this     DON T DO THIS public IEnumerable lt Frob gt  GetFrobs FrobFactory f  int count       if  f    null         throw new ArgumentNullException  quot f quot    quot factory must not be null quot       for  int i   0  i  lt  count    i        yield return f MakeFrob       Why is this wrong   Because the iterator block does not actually run until the foreach   The call to GetFrobs simply returns an object which when iterated will run the iterator block  By writing a null check like this you prevent the null dereference  but you move the null argument exception to the point of the iteration  not to the point of the call  and that is very confusing to debug  The correct fix is     DO THIS public IEnumerable lt Frob gt  GetFrobs FrobFactory f  int count          No yields in a public method that throws     if  f    null          throw new ArgumentNullException  quot f quot    quot factory must not be null quot       return GetFrobsForReal f  count     private IEnumerable lt Frob gt  GetFrobsForReal FrobFactory f  int count          Yields in a private method    Debug Assert f    null      for  int i   0  i  lt  count    i          yield return f MakeFrob       That is  make a private helper method that has the iterator block logic  and a public surface method that does the null check and returns the iterator   Now when GetFrobs is called  the null check happens immediately  and then GetFrobsForReal executes when the sequence is iterated  If you examine the reference source for LINQ to Objects you will see that this technique is used throughout  It is slightly more clunky to write  but it makes debugging nullity errors much easier   Optimize your code for the convenience of the caller  not the convenience of the author  A note on null dereferences in unsafe code C  has an  quot unsafe quot  mode which is  as the name implies  extremely dangerous because the normal safety mechanisms which provide memory safety and type safety are not enforced  You should not be writing unsafe code unless you have a thorough and deep understanding of how memory works  In unsafe mode  you should be aware of two important facts   dereferencing a null pointer produces the same exception as dereferencing a null reference dereferencing an invalid non-null pointer can produce that exception in some circumstances  To understand why that is  it helps to understand how  NET produces null dereference exceptions in the first place   These details apply to  NET running on Windows  other operating systems use similar mechanisms   Memory is virtualized in Windows  each process gets a virtual memory space of many  quot pages quot  of memory that are tracked by the operating system  Each page of memory has flags set on it which determine how it may be used  read from  written to  executed  and so on   The lowest page is marked as  quot produce an error if ever used in any way quot   Both a null pointer and a null reference in C  are internally represented as the number zero  and so any attempt to dereference it into its corresponding memory storage causes the operating system to produce an error  The  NET runtime then detects this error and turns it into the null dereference exception  That s why dereferencing both a null pointer and a null reference produces the same exception  What about the second point  Dereferencing any invalid pointer that falls in the lowest page of virtual memory causes the same operating system error  and thereby the same exception  Why does this make sense   Well  suppose we have a struct containing two ints  and an unmanaged pointer equal to null  If we attempt to dereference the second int in the struct  the CLR will not attempt to access the storage at location zero  it will access the storage at location four  But logically this is a null dereference because we are getting to that address via the null  If you are working with unsafe code and you get a null dereference exception  just be aware that the offending pointer need not be null  It can be any location in the lowest page  and this exception will be produced

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Update C 8 0  2019  Nullable reference types  C 8 0 introduces nullable reference types and non-nullable reference types  So only nullable reference types must be checked to avoid a NullReferenceException     If you have not initialized a reference type  and you want to set or read one of its properties  it will throw a NullReferenceException   Example   Person p   null  p Name    Harry      NullReferenceException occurs here    You can simply avoid this by checking if the variable is not null   Person p   null  if  p  null        p Name    Harry      Not going to run to this point     To fully understand why a NullReferenceException is thrown  it is important to know the difference between value types and  reference types  3    So  if you re dealing with value types  NullReferenceExceptions can not occur  Though you need to keep alert when dealing with reference types   Only reference types  as the name is suggesting  can hold references or point literally to nothing  or  null    Whereas value types always contain a value   Reference types  these ones must be checked     dynamic object string   Value types  you can simply ignore these ones     Numeric types Integral types Floating-point types decimal bool User defined structs

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Another general case where one might receive this exception involves mocking classes during unit testing  Regardless of the mocking framework being used  you must ensure that all appropriate levels of the class hierarchy are properly mocked  In particular  all properties of HttpContext which are referenced by the code under test must be mocked   See  NullReferenceException thrown when testing custom AuthorizationAttribute  for a somewhat verbose example

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Another scenario is when you cast a null object into a value type  For example  the code below   object o   null  DateTime d    DateTime o    It will throw a NullReferenceException on the cast  It seems quite obvious in the above sample  but this can happen in more  late-binding  intricate scenarios where the null object has been returned from some code you don t own  and the cast is for example generated by some automatic system   One example of this is this simple ASP NET binding fragment with the Calendar control    lt asp Calendar runat  server  SelectedDate   lt   Bind  Something    gt     gt    Here  SelectedDate is in fact a property - of DateTime type - of the Calendar Web Control type  and the binding could perfectly return something null  The implicit ASP NET Generator will create a piece of code that will be equivalent to the cast code above  And this will raise a NullReferenceException that is quite difficult to spot  because it lies in ASP NET generated code which compiles fine

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Adding a case when the class name for entity used in entity framework is same as class name for a web form code-behind file   Suppose you have a web form Contact aspx whose codebehind class is Contact and you have an entity name Contact   Then following code will throw a NullReferenceException when you call context SaveChanges    Contact contact   new Contact   Name    Abhinav    var context   new DataContext    context Contacts Add contact   context SaveChanges       NullReferenceException at this line   For the sake of completeness DataContext class  public class DataContext   DbContext        public DbSet lt Contact gt  Contacts  get  set       and Contact entity class  Sometimes entity classes are partial classes so that you can extend them in other files too   public partial class Contact        public string Name  get  set       The error occurs when both the entity and codebehind class are in same namespace  To fix this  rename the entity class or the codebehind class for Contact aspx   Reason I am still not sure about the reason  But whenever any of the entity class will extend System Web UI Page this error occurs   For discussion have a look at NullReferenceException in DbContext saveChanges

[c#] What is a NullReferenceException, and how do I fix it?

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