Why can't a Type be used as a constant value?

Question

Quoting MSDN - const (C# reference):

A constant expression is an expression that can be fully evaluated at compile time. Therefore, the only possible values for constants of reference types are string and a null reference.

According to: typeof(T) vs. Object.GetType() performance, typeof(T) is a compile time expression.

So why can't a Type be a constant value?

The following code will not compile:

public const Type INT_TYPE = typeof(int);

Answer 1

Constants are substituted by the compiler with literal values in the resulting IL code. But typeof is a method call:

typeof(int);

// Becomes:
L_0000: ldtoken int32
L_0005: call class [mscorlib]System.Type [mscorlib]System.Type::GetTypeFromHandle(valuetype [mscorlib]System.RuntimeTypeHandle)

Answer 2

The C# compiler and IL certainly support types as constant expressions, at least in certain situations. Look at attributes, they use this a lot:

[DebuggerTypeProxy(typeof(Mscorlib_CollectionDebugView<>))]

The type is embedded as a string in the compiler generated code, above line compiles to the following IL code:

.custom instance void System.Diagnostics.DebuggerTypeProxyAttribute::.ctor(class System.Type) = (
    01 00 39 53 79 73 74 65 6d 2e 43 6f 6c 6c 65 63
    74 69 6f 6e 73 2e 47 65 6e 65 72 69 63 2e 4d 73
    63 6f 72 6c 69 62 5f 43 6f 6c 6c 65 63 74 69 6f
    6e 44 65 62 75 67 56 69 65 77 60 31 00 00
)

If you inspect the binary data, you will notice this is the fully qualified class name without any assembly identification (System.Collections.Generic.Mscorlib_CollectionDebugView`1).

To answer your question: I do not see any technical reason why this should not be possible, nor can I imagine compatibility considerations that prevent it as there is no assembly reference serialized and therefore the DLL declaring this type still can be updated without affecting the previously compiled type referencing it.