Here is a C# program that tries `Marshal.SizeOf` on a few different types:

```
using System;
using System.Runtime.InteropServices;

[StructLayout(LayoutKind.Sequential)]
class AClass { }

[StructLayout(LayoutKind.Sequential)] 
struct AStruct { }

[StructLayout(LayoutKind.Sequential)]
class B { AClass value; }

[StructLayout(LayoutKind.Sequential)]
class C<T> { T value; }

class Program
{
    static void M(object o) { Console.WriteLine(Marshal.SizeOf(o)); }

    static void Main()
    {
        M(new AClass());
        M(new AStruct());
        M(new B());
        M(new C<AStruct>());
        M(new C<AClass>());
    }
}

```

The first four calls to M() succeed, but on the last one, SizeOf throws an ArgumentException:

```
"Type 'C`1[AClass]' cannot be marshaled as an unmanaged structure; no meaningful size or offset can be computed."

```

Why? Specifically, why does SizeOf choke on `C<AClass>`, but not on `B` or on `C<AStruct>`?

---

**EDIT:** Because it was asked about in the comments, here's the "real-world" problem that inspired this mostly-academic question:
I'm calling into a C API that is basically one C function that operates on (pointers to) lots of different types of simple C structures. All contain a common header followed by one field, but the type of that field is different in different structures. A flag in the header indicates the type of the field. (Strange, yes, but that's what I have to work with). 

If I could define a single generic type `C<T>` and a single C# extern declaration `M(C<T>)`, and then call `M(C<int>)` on one line, and `M(C<double>)` on another, I'd have a short and sweet interop solution. But given JaredPar's answer, it appears that I have to make a separate C# type for each structure (though inheritance can provide the common header).