managed side:

$ cat ofs.cs
using System;
using System.Runtime.InteropServices;

[StructLayout (LayoutKind.Explicit)]
struct Embedded {
	[FieldOffset (0)] IntPtr b;
	[FieldOffset (0)] int c;
}

struct X {
	int A;
	Embedded B;
}

class Test {
	public static void Main ()
	{
		Console.WriteLine ("offsetof(struct X, b)={0}", Marshal.OffsetOf (typeof (X), "B"));
	}
}

$ csc /t:exe /platform:x86 /out:ofs-32.exe ofs.cs
$ csc /t:exe /platform:x64 /out:ofs-64.exe ofs.cs
$ ofs-32.exe
offsetof(struct X, b)=4
$ ofs-32.exe
offsetof(struct X, b)=8

native side

$ cat ofs.c
/* offsets */
#include <stdio.h>
#include <stddef.h>

struct X {
	int a;
	union {
		void *b;
		int c;
	};
};

int main ()
{
	printf ("offset(X::b)=%i\n", (int) offsetof (struct X, b));
	return 0;
}

$ gcc -m32 ofs.c -o ofs-32
$ gcc -m64 ofs.c -o ofs-64
$ ./ofs-32
offset(X::b)=4
$ ./ofs-64
offset(X::b)=8