How are typeof and sizeof Implemented

Question

I want to know how the typeof and sizeof keywords are implemented.

What if I want to implement my own 'x'of() expression like timeof(object) where object contains a DateTime as one of it's properties or something?

Answer 1

The short answer is no. typeof and sizeof are part of the C# language, and you cannot (directly) add expressions like them. The normal way of doing what you're asking would be to simply access the property:

DateTime time = myObject.SomeDateTimeProperty;

You could make a method to do this (although it wouldn't make sense for such a trivial thing). E.g. if you used an interface:

public interface ICreateTime
{
    DateTime CreateTime { get; }
}

public DateTime TimeOf(ICreateTime myObject)
{
    return myObject.CreateTime;
}

To be more specific: typeof and sizeof aren't implemented in the same way you implement methods. Instead, typeof translates to ldtoken [type] and call System.Type.GetTypeFromHandle IL instructions, and sizeof becomes a constant. E.g.

Type t = typeof(int);
int s = sizeof(int);

Compiles to:

IL_0001:  ldtoken     System.Int32
IL_0006:  call        System.Type.GetTypeFromHandle
IL_000B:  stloc.0     // t
IL_000C:  ldc.i4.4    // the constant 4, typed as a 4-byte integer
IL_000D:  stloc.1     // s