perpetuum | Generating event-triggered code | Runtime Evironment library

The difference is: the sizeof operator takes a type name and tells you how many bytes of managed memory need to be allocated for an instance of that struct.This is not necessarily stack memory; structs are allocated off the heap when they are array elements, fields of a class, and so on. By contrast, Marshal.SizeOf takes either a type object or an instance of the type, and tells you how many bytes of unmanaged memory need to be allocated. These can be different for a variety of reasons. The name of the type gives you a clue: Marshal.SizeOf is intended to be used when marshaling a structure to unmanaged memory.

Another difference between the two is that the sizeof operator can only take the name of an unmanaged type; that is, a struct type whose fields are only integral types, Booleans, pointers and so on. (See the specification for an exact definition.) Marshal.SizeOf by contrast can take any class or struct type.