Type Checking typeof GetType or is

Question

I ve seen many people use the following code   Type t   typeof obj1   if  t    typeof int          Some code here   But I know you could also do this   if  obj1 GetType      typeof int          Some code here   Or this   if  obj1 is int         Some code here   Personally  I feel the last one is the cleanest  but is there something I m missing  Which one is the best to use  or is it personal preference

User · Answer

Use typeof when you want to get the type at compilation time   Use GetType when you want to get the type at execution time   There are rarely any cases to use is as it does a cast and  in most cases  you end up casting the variable anyway   There is a fourth option that you haven t considered  especially if you are going to cast an object to the type you find as well   that is to use as   Foo foo   obj as Foo   if  foo    null         your code here   This only uses one cast whereas this approach   if  obj is Foo      Foo foo    Foo obj    requires two   Update  Jan 2020     As of C  7   you can now cast inline  so the  is  approach can now be done in one cast as well    Example   if obj is Foo newLocalFoo           For example  you can now reference  newLocalFoo  in this local scope     Console WriteLine newLocalFoo

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The last one is cleaner  more obvious  and also checks for subtypes  The others do not check for polymorphism

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All are different    typeof takes a type name  which you specify at compile time   GetType gets the runtime type of an instance  is returns true if an instance is in the inheritance tree    Example  class Animal      class Dog   Animal      void PrintTypes Animal a         Console WriteLine a GetType      typeof Animal       false      Console WriteLine a is Animal                        true      Console WriteLine a GetType      typeof Dog          true     Console WriteLine a is Dog                           true     Dog spot   new Dog     PrintTypes spot          What about typeof T   Is it also resolved at compile time    Yes  T is always what the type of the expression is  Remember  a generic method is basically a whole bunch of methods with the appropriate type  Example   string Foo lt T gt  T parameter    return typeof T  Name     Animal probably a dog   new Dog    Dog    definitely a dog   new Dog     Foo probably a dog      this calls Foo lt Animal gt  and returns  Animal  Foo lt Animal gt  probably a dog      this is exactly the same as above Foo lt Dog gt  probably a dog          This will not compile  The parameter expects a Dog  you cannot pass in an Animal   Foo definitely a dog      this calls Foo lt Dog gt  and returns  Dog  Foo lt Dog gt  definitely a dog      this is exactly the same as above  Foo lt Animal gt  definitely a dog      this calls Foo lt Animal gt  and returns  Animal    Foo  Animal definitely a dog      this does the same as above  returns  Animal

User · Answer

1   Type t   typeof obj1   if  t    typeof int     This is illegal  because typeof only works on types  not on variables  I assume obj1 is a variable  So  in this way typeof is static  and does its work at compile time instead of runtime   2   if  obj1 GetType      typeof int     This is true if obj1 is exactly of type int  If obj1 derives from int  the if condition will be false   3   if  obj1 is int    This is true if obj1 is an int  or if it derives from a class called int  or if it implements an interface called int

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It depends on what I m doing   If I need a bool value  say  to determine if I ll cast to an int   I ll use is   If I actually need the type for some reason  say  to pass to some other method  I ll use GetType

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Performance test typeof   vs GetType     using System  namespace ConsoleApplication1           class Program               enum TestEnum   E1  E2  E3           static void Main string   args                                          var start   DateTime UtcNow                  for  var i   0  i  lt  1000000000  i                        Test1 TestEnum E2                   Console WriteLine DateTime UtcNow - start                                               var start   DateTime UtcNow                  for  var i   0  i  lt  1000000000  i                        Test2 TestEnum E2                   Console WriteLine DateTime UtcNow - start                             Console ReadLine                      static Type Test1 lt T gt  T value    gt  typeof T           static Type Test2 object value    gt  value GetType              Results in debug mode   00 00 08 4096636 00 00 10 8570657   Results in release mode   00 00 02 3799048 00 00 07 1797128

User · Answer

You can use  typeof    operator in C  but you need to call the namespace using System IO  You must use  is  keyword if you wish to check for a type

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Type t   typeof obj1   if  t    typeof int          Some code here   This is an error   The typeof operator in C  can only take type names  not objects   if  obj1 GetType      typeof int          Some code here   This will work  but maybe not as you would expect   For value types  as you ve shown here  it s acceptable  but for reference types  it would only return true if the type was the exact same type  not something else in the inheritance hierarchy   For instance   class Animal   class Dog   Animal    static void Foo        object o   new Dog         if o GetType      typeof Animal           Console WriteLine  o is an animal        Console WriteLine  o is something else        This would print  o is something else   because the type of o is Dog  not Animal   You can make this work  however  if you use the IsAssignableFrom method of the Type class   if typeof Animal  IsAssignableFrom o GetType        note use of tested type     Console WriteLine  o is an animal      This technique still leaves a major problem  though   If your variable is null  the call to GetType   will throw a NullReferenceException   So to make it work correctly  you d do   if o    null  amp  amp  typeof Animal  IsAssignableFrom o GetType         Console WriteLine  o is an animal      With this  you have equivalent behavior of the is keyword   Hence  if this is the behavior you want  you should use the is keyword  which is more readable and more efficient   if o is Animal      Console WriteLine  o is an animal      In most cases  though  the is keyword still isn t what you really want  because it s usually not enough just to know that an object is of a certain type   Usually  you want to actually use that object as an instance of that type  which requires casting it too   And so you may find yourself writing code like this   if o is Animal        Animal o  Speak      But that makes the CLR check the object s type up to two times   It will check it once to satisfy the is operator  and if o is indeed an Animal  we make it check again to validate the cast   It s more efficient to do this instead   Animal a   o as Animal  if a    null      a Speak      The as operator is a cast that won t throw an exception if it fails  instead returning null   This way  the CLR checks the object s type just once  and after that  we just need to do a null check  which is more efficient   But beware  many people fall into a trap with as   Because it doesn t throw exceptions  some people think of it as a  safe  cast  and they use it exclusively  shunning regular casts   This leads to errors like this    o as Animal  Speak      In this case  the developer is clearly assuming that o will always be an Animal  and as long as their assumption is correct  everything works fine   But if they re wrong  then what they end up with here is a NullReferenceException   With a regular cast  they would have gotten an InvalidCastException instead  which would have more correctly identified the problem   Sometimes  this bug can be hard to find   class Foo      readonly Animal animal       public Foo object o           animal   o as Animal             public void Interact            animal Speak              This is another case where the developer is clearly expecting o to be an Animal every time  but this isn t obvious in the constructor  where the as cast is used   It s not obvious until you get to the Interact method  where the animal field is expected to be positively assigned   In this case  not only do you end up with a misleading exception  but it isn t thrown until potentially much later than when the actual error occurred   In summary    If you only need to know whether or not an object is of some type  use is  If you need to treat an object as an instance of a certain type  but you don t know for sure that the object will be of that type  use as and check for null  If you need to treat an object as an instance of a certain type  and the object is supposed to be of that type  use a regular cast

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Used to obtain the System Type object for a type  A typeof expression takes the following form   System Type type   typeof int    Example       public class ExampleClass              public int sampleMember         public void SampleMethod              static void Main                      Type t   typeof ExampleClass                Alternatively  you could use              ExampleClass obj   new ExampleClass                 Type t   obj GetType               Console WriteLine  Methods               System Reflection MethodInfo   methodInfo   t GetMethods               foreach  System Reflection MethodInfo mInfo in methodInfo               Console WriteLine mInfo ToString                Console WriteLine  Members               System Reflection MemberInfo   memberInfo   t GetMembers               foreach  System Reflection MemberInfo mInfo in memberInfo               Console WriteLine mInfo ToString                                Output          Methods          Void SampleMethod           System String ToString           Boolean Equals System Object          Int32 GetHashCode           System Type GetType           Members          Void SampleMethod           System String ToString           Boolean Equals System Object          Int32 GetHashCode           System Type GetType           Void  ctor           Int32 sampleMember          This sample uses the GetType method to determine the type that is used to contain the result of a numeric calculation  This depends on the storage requirements of the resulting number       class GetTypeTest               static void Main                         int radius   3              Console WriteLine  Area    0    radius   radius   Math PI               Console WriteLine  The type is  0                                   radius   radius   Math PI  GetType                                             Output      Area   28 2743338823081     The type is System Double

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I believe the last one also looks at inheritance  e g  Dog is Animal    true   which is better in most cases

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If you re using C  7  then it is time for an update to Andrew Hare s great answer  Pattern matching has introduced a nice shortcut that gives us a typed variable within the context of the if statement  without requiring a separate declaration cast and check   if  obj1 is int integerValue        integerValue        This looks pretty underwhelming for a single cast like this  but really shines when you have many possible types coming into your routine  The below is the old way to avoid casting twice   Button button   obj1 as Button  if  button    null           do stuff        return    TextBox text   obj1 as TextBox  if  text    null           do stuff        return    Label label   obj1 as Label  if  label    null           do stuff        return           and so on   Working around shrinking this code as much as possible  as well as avoiding duplicate casts of the same object has always bothered me  The above is nicely compressed with pattern matching to the following   switch  obj1        case Button button             do stuff            break      case TextBox text             do stuff            break      case Label label             do stuff            break         and so on        EDIT  Updated the longer new method to use a switch as per Palec s comment

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if  c is UserControl  c Enabled   enable

User · Answer

I prefer is  That said  if you re using is  you re likely not using inheritance properly   Assume that Person   Entity  and that Animal   Entity   Feed is a virtual method in Entity  to make Neil happy   class Person        A Person should be able to Feed      another Entity  but they way he feeds      each is different   public override void Feed  Entity e           if  e is Person                  feed me           else if  e is Animal                  ruff               Rather  class Person     public override void Feed  Person p              feed the person       public override void Feed  Animal a              feed the animal

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I had a Type-property to compare to and could not use is  like my type is  BaseTypetoLookFor   but I could use these   base type IsInstanceOfType derived object   base type IsAssignableFrom derived type   derived type IsSubClassOf base type     Notice that IsInstanceOfType and IsAssignableFrom return true when comparing the same types  where IsSubClassOf will return false   And IsSubclassOf does not work on interfaces  where the other two do    See also this question and answer    public class Animal    public interface ITrainable    public class Dog   Animal  ITrainable    Animal dog   new Dog     typeof Animal  IsInstanceOfType dog          true typeof Dog  IsInstanceOfType dog             true typeof ITrainable  IsInstanceOfType dog      true  typeof Animal  IsAssignableFrom dog GetType             true typeof Dog  IsAssignableFrom dog GetType                true typeof ITrainable  IsAssignableFrom dog GetType        true  dog GetType   IsSubclassOf typeof Animal                  true dog GetType   IsSubclassOf typeof Dog                     false dog GetType   IsSubclassOf typeof ITrainable       false

[c#] Type Checking: typeof, GetType, or is?

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