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typeof generic and casted type

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typeof generic and casted type

When and where to use GetType() or typeof()?When is the generic type resolved in c#?Run-time type vs compile-time type in C#Cast int to enum in C#Create Generic method constraining T to an EnumHow do I use reflection to call a generic method?Collections.emptyList() returns a List<Object>?How do I make the method return type generic?Type Checking: typeof, GetType, or is?Using Mockito to mock classes with generic parametersHow do I generate a random int number in C#?Google Gson - deserialize list<class> object? (generic type)Unable to cast object of type System.Int32 to System.Object[] when calling generic method with parameters via reflection

7

Let's say we have generic method:

public void GenericMethod<T>(T item)

{

    var typeOf = typeof(T);

    var getType = item.GetType();

}

And we are invoking it with the following parameters:

GenericMethod(1)

GenericMethod((object) 1)

The results are:

typeOf = System.Int32

getType = System.Int32

and

typeOf = System.Object

getType = System.Int32

Can someone explain me why typeof integer casted to object returns System.Object, but .GetType() returns System.Int32?

c# generics

share|improve this question

edited 53 mins ago

Uwe Keim

27.5k32131212

asked 1 hour ago

user3450929user3450929

654

• 
  
  
  

  
  3
  

  
  
  
  
  
  

  
  
  typeof takes a type name (which you specify at compile time), GetType gets the runtime type of an instance.
  
  – João Paulo Amorim
  1 hour ago
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Also, if you find yourself doing any kind of type test inside a generic, ask yourself whether you've picked the right tool for the job. Because it may mean that you fail to work properly at runtime something that you "promised" at compile time you could do (by saying you could work for any type, subject to any generic type constraints on that type parameter)
  
  – Damien_The_Unbeliever
  1 hour ago
  

  
  
  
  

add a comment | 

7

Let's say we have generic method:

public void GenericMethod<T>(T item)

{

    var typeOf = typeof(T);

    var getType = item.GetType();

}

And we are invoking it with the following parameters:

GenericMethod(1)

GenericMethod((object) 1)

The results are:

typeOf = System.Int32

getType = System.Int32

and

typeOf = System.Object

getType = System.Int32

Can someone explain me why typeof integer casted to object returns System.Object, but .GetType() returns System.Int32?

c# generics

share|improve this question

edited 53 mins ago

Uwe Keim

27.5k32131212

asked 1 hour ago

user3450929user3450929

654

• 
  
  
  

  
  3
  

  
  
  
  
  
  

  
  
  typeof takes a type name (which you specify at compile time), GetType gets the runtime type of an instance.
  
  – João Paulo Amorim
  1 hour ago
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Also, if you find yourself doing any kind of type test inside a generic, ask yourself whether you've picked the right tool for the job. Because it may mean that you fail to work properly at runtime something that you "promised" at compile time you could do (by saying you could work for any type, subject to any generic type constraints on that type parameter)
  
  – Damien_The_Unbeliever
  1 hour ago
  

  
  
  
  

add a comment | 

Let's say we have generic method:

public void GenericMethod<T>(T item)

{

    var typeOf = typeof(T);

    var getType = item.GetType();

}

And we are invoking it with the following parameters:

GenericMethod(1)

GenericMethod((object) 1)

The results are:

typeOf = System.Int32

getType = System.Int32

and

typeOf = System.Object

getType = System.Int32

Can someone explain me why typeof integer casted to object returns System.Object, but .GetType() returns System.Int32?

c# generics

share|improve this question

edited 53 mins ago

Uwe Keim

27.5k32131212

asked 1 hour ago

user3450929user3450929

654

public void GenericMethod<T>(T item)

{

    var typeOf = typeof(T);

    var getType = item.GetType();

}

GenericMethod(1)

GenericMethod((object) 1)

typeOf = System.Int32

getType = System.Int32

typeOf = System.Object

getType = System.Int32

share|improve this question

edited 53 mins ago

Uwe Keim

27.5k32131212

asked 1 hour ago

user3450929user3450929

654

share|improve this question

edited 53 mins ago

Uwe Keim

27.5k32131212

asked 1 hour ago

user3450929user3450929

654

edited 53 mins ago

Uwe Keim

27.5k32131212

edited 53 mins ago

Uwe Keim

27.5k32131212

asked 1 hour ago

user3450929user3450929

654

asked 1 hour ago

user3450929user3450929

654

4 Answers
4

active

oldest

votes

16

typeof returns the static (compile-time) type of the generic parameter T.

GetType returns the dynamic (run-time) type of the value contained in variable item.

---

The difference is easier to see if you make your method non-generic. Let's assume that B is a subtype of A:

public void NonGenericMethod(A item)

{

    var typeOf = typeof(A);

    var getType = item.GetType();

}

In that case, calling NonGenericMethod(new B()) would yield

A

B

Recommended further reading:

• Run-time type vs compile-time type in C#

---

Now, you might ask: Why did you use NonGenericMethod(A item) in your example instead of NonGenericMethod(B item)? That's a very good question! Consider the following (non-generic) example code:

public static void NonGenericMethod(A item)

{

    Console.WriteLine("Method A");

    var typeOf = typeof(A);

    var getType = item.GetType();

}

public static void NonGenericMethod(B item)

{

    Console.WriteLine("Method B");

    var typeOf = typeof(B);

    var getType = item.GetType();

}

What do you get when you call NonGenericMethod((A) new B()) (which is analogous to the argument (object) 1 in your example)?

Method A

A

B

Why? Because overload resolution is done at compile-time, not at run-time. At compile-time, the type of the expression (A) new B() is A, just like the compile-time type of (object) 1 is object.

Recommended further reading:

• When is the generic type resolved in c#?

share|improve this answer

edited 29 mins ago

answered 1 hour ago

HeinziHeinzi

123k38269408

add a comment | 

2

In GenericMethod((object) 1), T will be object. typeof reflects that.

But item.GetType(); is a virtual method and will execute at runtime on Int32.

share|improve this answer

answered 1 hour ago

Henk HoltermanHenk Holterman

210k22232404

add a comment | 

1

The call to GetType gets resolved at runtime, while typeof is resolved at compile time.
That is why it is giving different results.
you can check here - When and where to use GetType() or typeof()?

share|improve this answer

answered 50 mins ago

Deepankshee JainDeepankshee Jain

112

add a comment | 

0

This Tells me Typeof gives you compile time type whereas GetType gives you Exact Run time type.

share|improve this answer

answered 1 hour ago

Gagan DeepGagan Deep

76616

add a comment | 

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16

typeof returns the static (compile-time) type of the generic parameter T.

GetType returns the dynamic (run-time) type of the value contained in variable item.

---

The difference is easier to see if you make your method non-generic. Let's assume that B is a subtype of A:

public void NonGenericMethod(A item)

{

    var typeOf = typeof(A);

    var getType = item.GetType();

}

In that case, calling NonGenericMethod(new B()) would yield

A

B

Recommended further reading:

• Run-time type vs compile-time type in C#

---

Now, you might ask: Why did you use NonGenericMethod(A item) in your example instead of NonGenericMethod(B item)? That's a very good question! Consider the following (non-generic) example code:

public static void NonGenericMethod(A item)

{

    Console.WriteLine("Method A");

    var typeOf = typeof(A);

    var getType = item.GetType();

}

public static void NonGenericMethod(B item)

{

    Console.WriteLine("Method B");

    var typeOf = typeof(B);

    var getType = item.GetType();

}

What do you get when you call NonGenericMethod((A) new B()) (which is analogous to the argument (object) 1 in your example)?

Method A

A

B

Why? Because overload resolution is done at compile-time, not at run-time. At compile-time, the type of the expression (A) new B() is A, just like the compile-time type of (object) 1 is object.

Recommended further reading:

• When is the generic type resolved in c#?

share|improve this answer

edited 29 mins ago

answered 1 hour ago

HeinziHeinzi

123k38269408

add a comment | 

16

typeof returns the static (compile-time) type of the generic parameter T.

GetType returns the dynamic (run-time) type of the value contained in variable item.

---

The difference is easier to see if you make your method non-generic. Let's assume that B is a subtype of A:

public void NonGenericMethod(A item)

{

    var typeOf = typeof(A);

    var getType = item.GetType();

}

In that case, calling NonGenericMethod(new B()) would yield

A

B

Recommended further reading:

• Run-time type vs compile-time type in C#

---

Now, you might ask: Why did you use NonGenericMethod(A item) in your example instead of NonGenericMethod(B item)? That's a very good question! Consider the following (non-generic) example code:

public static void NonGenericMethod(A item)

{

    Console.WriteLine("Method A");

    var typeOf = typeof(A);

    var getType = item.GetType();

}

public static void NonGenericMethod(B item)

{

    Console.WriteLine("Method B");

    var typeOf = typeof(B);

    var getType = item.GetType();

}

What do you get when you call NonGenericMethod((A) new B()) (which is analogous to the argument (object) 1 in your example)?

Method A

A

B

Why? Because overload resolution is done at compile-time, not at run-time. At compile-time, the type of the expression (A) new B() is A, just like the compile-time type of (object) 1 is object.

Recommended further reading:

• When is the generic type resolved in c#?

share|improve this answer

edited 29 mins ago

answered 1 hour ago

HeinziHeinzi

123k38269408

add a comment | 

typeof returns the static (compile-time) type of the generic parameter T.

GetType returns the dynamic (run-time) type of the value contained in variable item.

---

The difference is easier to see if you make your method non-generic. Let's assume that B is a subtype of A:

public void NonGenericMethod(A item)

{

    var typeOf = typeof(A);

    var getType = item.GetType();

}

In that case, calling NonGenericMethod(new B()) would yield

A

B

Recommended further reading:

• Run-time type vs compile-time type in C#

---

Now, you might ask: Why did you use NonGenericMethod(A item) in your example instead of NonGenericMethod(B item)? That's a very good question! Consider the following (non-generic) example code:

public static void NonGenericMethod(A item)

{

    Console.WriteLine("Method A");

    var typeOf = typeof(A);

    var getType = item.GetType();

}

public static void NonGenericMethod(B item)

{

    Console.WriteLine("Method B");

    var typeOf = typeof(B);

    var getType = item.GetType();

}

What do you get when you call NonGenericMethod((A) new B()) (which is analogous to the argument (object) 1 in your example)?

Method A

A

B

Why? Because overload resolution is done at compile-time, not at run-time. At compile-time, the type of the expression (A) new B() is A, just like the compile-time type of (object) 1 is object.

Recommended further reading:

• When is the generic type resolved in c#?

share|improve this answer

edited 29 mins ago

answered 1 hour ago

HeinziHeinzi

123k38269408

public void NonGenericMethod(A item)

{

    var typeOf = typeof(A);

    var getType = item.GetType();

}

A

B

public static void NonGenericMethod(A item)

{

    Console.WriteLine("Method A");

    var typeOf = typeof(A);

    var getType = item.GetType();

}

public static void NonGenericMethod(B item)

{

    Console.WriteLine("Method B");

    var typeOf = typeof(B);

    var getType = item.GetType();

}

Method A

A

B

share|improve this answer

edited 29 mins ago

answered 1 hour ago

HeinziHeinzi

123k38269408

answered 1 hour ago

HeinziHeinzi

123k38269408

answered 1 hour ago

HeinziHeinzi

123k38269408

2

In GenericMethod((object) 1), T will be object. typeof reflects that.

But item.GetType(); is a virtual method and will execute at runtime on Int32.

share|improve this answer

answered 1 hour ago

Henk HoltermanHenk Holterman

210k22232404

add a comment | 

2

In GenericMethod((object) 1), T will be object. typeof reflects that.

But item.GetType(); is a virtual method and will execute at runtime on Int32.

share|improve this answer

answered 1 hour ago

Henk HoltermanHenk Holterman

210k22232404

add a comment | 

In GenericMethod((object) 1), T will be object. typeof reflects that.

But item.GetType(); is a virtual method and will execute at runtime on Int32.

share|improve this answer

answered 1 hour ago

Henk HoltermanHenk Holterman

210k22232404

share|improve this answer

answered 1 hour ago

Henk HoltermanHenk Holterman

210k22232404

answered 1 hour ago

Henk HoltermanHenk Holterman

210k22232404

answered 1 hour ago

Henk HoltermanHenk Holterman

210k22232404

1

The call to GetType gets resolved at runtime, while typeof is resolved at compile time.
That is why it is giving different results.
you can check here - When and where to use GetType() or typeof()?

share|improve this answer

answered 50 mins ago

Deepankshee JainDeepankshee Jain

112

add a comment | 

1

The call to GetType gets resolved at runtime, while typeof is resolved at compile time.
That is why it is giving different results.
you can check here - When and where to use GetType() or typeof()?

share|improve this answer

answered 50 mins ago

Deepankshee JainDeepankshee Jain

112

add a comment | 

The call to GetType gets resolved at runtime, while typeof is resolved at compile time.
That is why it is giving different results.
you can check here - When and where to use GetType() or typeof()?

share|improve this answer

answered 50 mins ago

Deepankshee JainDeepankshee Jain

112

share|improve this answer

answered 50 mins ago

Deepankshee JainDeepankshee Jain

112

answered 50 mins ago

Deepankshee JainDeepankshee Jain

112

answered 50 mins ago

Deepankshee JainDeepankshee Jain

112

0

This Tells me Typeof gives you compile time type whereas GetType gives you Exact Run time type.

share|improve this answer

answered 1 hour ago

Gagan DeepGagan Deep

76616

add a comment | 

0

This Tells me Typeof gives you compile time type whereas GetType gives you Exact Run time type.

share|improve this answer

answered 1 hour ago

Gagan DeepGagan Deep

76616

add a comment | 

This Tells me Typeof gives you compile time type whereas GetType gives you Exact Run time type.

share|improve this answer

answered 1 hour ago

Gagan DeepGagan Deep

76616

share|improve this answer

answered 1 hour ago

Gagan DeepGagan Deep

76616

answered 1 hour ago

Gagan DeepGagan Deep

76616

answered 1 hour ago

Gagan DeepGagan Deep

76616