Boxing on assignment to ValueType?

If I assign a simple value type (e.g. an int) to an attribute of type ValueT开发者_运维知识库ype, is the value then boxed?

For example if I do this:

int i = 4;
ValueType test = i;

Will the value be boxed?

Yes, it will. This is because each type occupies a constant amount of memory at runtime (int takes 4 bytes, for example). A struct will take as much space is required to lay out all of the fields in memory.

Since you can store any value type in ValueType, and since ValueType would have to be exactly the same size as the type you're assigning to test, the ValueType type is actually a reference type.

Consider:

int a = 0;
long b = 1;

ValueType test;

test = a;
test = b;

This is perfectly valid code. test must occupy a fixed size on the stack, and a and b are different sizes. Hopefully this clarifies why exactly ValueType cannot itself be a value type. (It's related to the reason why you can't derive value types.)

Yes, that will box it - ValueType is a reference type (a class), confusingly enough :) It's simply the type from which every value type "inherits" (directly in the case of structs, and indirectly in the case of enums).

Boxing occurs any time you assign a value type value to a variable of a reference type, including object, ValueType, Enum and any interfaces. (It also occurs when use a value type value as an argument where the parameter is one of those types, etc.)

ValueType is a reference type. The IL generated on this assignment actually boxes the struct.

Here is an example of a box instruction when assigning to object:

void Main()
{
    object o = 1;

    Console.Write(o);
}

IL_0000:  ldc.i4.1    
IL_0001:  box         System.Int32
IL_0006:  stloc.0     
IL_0007:  ldloc.0     
IL_0008:  call        System.Console.Write

And when assigning to ValueType:

void Main()
{
    ValueType o = 1;

    Console.Write(o);
}

IL_0000:  ldc.i4.1    
IL_0001:  box         System.Int32
IL_0006:  stloc.0     
IL_0007:  ldloc.0     
IL_0008:  call        System.Console.Write