Jtdylktuy

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?

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#?

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.

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()?

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

A lot is made clear when you leave out the type interference:

GenericMethod(1) is actually GenericMethod<int>(1).

GenericMethod((object) 1) is inferred as GenericMethod<object>((object) 1).

When you ask typeof(T), it returns the T you specified in the method call. You could also do GenericMethod<object>("a"), which will return object on typeof(T).

GetType returns the actual runtime type of the instance provided.