Question

System.ValueType is a class,
yet all value types are structs.

If I create an instance of ValueType like so:

System.ValueType foo = 5;

…is it saved on the heap or on the stack?

Answer 1

System.ValueType is not a value-type. Value-types are things that inherit from ValueType, but not ValueType itself. So:

System.ValueType foo = 5; 

here, 5 is loaded on the stack. This is then boxed (with a box of type int) onto the heap. The object reference is then assigned to the reference foo. We can see this by looking at the IL for that:

ldc.i4.5 // push int32 value 5 onto the stack
box int32 // box the value at the top of the stack, type int32
stloc.0 // assign to "foo"

In general, though: structs can be either on the heap or on the stack, depending on the context. Frankly, it doesn’t matter which – because both are implementation details. It is the behaviour that matters… and (at least in their unboxed form), the key point about structs is their copy-on-assign semantic.