CODEWITHSUNDEEP
c#.netboxingunboxing
smwikipedia
smwikipedia

Reputation: 64221

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

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?

Upvotes: 8

Views: 830

Answers (5)

Rune FS
Rune FS

Reputation: 21742

declarations of variable is compile time only information whereas method execution is runtime. In other words there's no way GetType() can know what type the object was declared as it can only know the actual type of the object at runtime.

similar if you had

class a
{
}

class b : a

a bInstance = new b();
bInstance.GetType();

the call to bInstance.GetType() have no way of knowing that the variable was declared as type 'a' and I don't think you expect it to return 'a' in this case either. However in the example above a is my abbreviation of object and b is for System.Int32

Upvotes: 0

Gishu
Gishu

Reputation: 136633

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.

Upvotes: 4

Merlyn Morgan-Graham
Merlyn Morgan-Graham

Reputation: 59111

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

Upvotes: 3

AakashM
AakashM

Reputation: 63358

This isn't really about boxing; this is about the behaviour of GetType. It returns the type of the value of the variable, not the type the variable was declared with:

    object var4 = new List<string>();
    Console.WriteLine(var4.GetType().ToString());

won't return System.Object either.

Upvotes: 1

SLaks
SLaks

Reputation: 887547

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.

Upvotes: 7

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