boxing and unboxing, why aren't the outputs both "System.Object"?

Question

I got the following code:

object var3 = 3;
Console.WriteLine(var3.GetType().ToString());
Console.WriteLine(typeof(object).ToString());

The output is:

System.Int32
System.Object

Why aren't they both System.Object?

Answer 1

The GetType() function returns the actual type of the instance in the variable.

Even though your variable is declared as object, it's actually holding a boxed Int32 instance.

Answer 2

If you're asking why the boxedObject.GetType() does not return Object.. check out the image under the Section 'Boxing Conversion' on the MSDN Boxing and Unboxing page. Good question btw.. atleast my understanding of your question.

Although I may not be technically correct, it looks like

• When moved to the heap, a new object is created - its Type pointer set to the original value type's Type object (here System.Int32). This explains GetType() (and also the error if you try to unbox it to a different type).
• The actual value is then copied over into this object.

Answer 3

Ignoring the topic of boxing, all classes inherit from type object. This is true for both reference types and value types. GetType shows the most derived type, which in this case is System.Int32.

One of the few times GetType is going to return System.Object is if you do this:

object var = new Object();
Console.WriteLine(var.GetType().ToString());

Boxing refers to when a value type is pointed to by a reference type. Generally this is done as a System.Object reference. TypeOf will return the most derived actual type, not the reference type.

class A
{
}

class B : A
{
}

class C : B
{
}

object obj1 = new ClassA();
ClassB obj2 = new ClassB();
ClassB obj3 = new ClassC();

GetType will do similar things for these types.

System.Console.WriteLine(obj1.GetType().ToString());
System.Console.WriteLine(obj2.GetType().ToString());
System.Console.WriteLine(obj3.GetType().ToString());

ClassA
ClassB
ClassC