mc.c

struct B {
	int a;
};

struct D {
	struct B base;
	int d;
};

int
mc_check (struct D *value, int a, int d)
{
	if (value->base.a != a)
		return 1;
	if (value->d != d)
		return 2;
	return 0;
}

mc.cs

// Marshal Classes
using System;
using System.Runtime.InteropServices;

[StructLayout(LayoutKind.Sequential)]
class B {
  public int a;
}

// Sequential Layout "mirrors" the mc.c `struct D` declaration:
// The members of B are present before the members of D:
[StructLayout(LayoutKind.Sequential)]
class D : B {
  public int d;
}

// Explicit layout allows controlling how B is marshaled
[StructLayout(LayoutKind.Explicit)]
class ExplicitD : B {
  [FieldOffset(0)] public int a2;
  [FieldOffset(4)] public int d2;
}

class App {
  [DllImport ("mc")]
  public static extern int mc_check (D value, int a, int d);

  [DllImport ("mc")]
  public static extern int mc_check (ExplicitD value, int a, int d);
  
  public static void Main ()
  {
    var d = new D {
      a = 1,
      d = 2,
    };
    int r = mc_check (d, 1, 2);
    Console.WriteLine ("r={0}", r);  // r=0
    var ed = new ExplicitD {
      a  = 0,
      a2 = 1,
      d2 = 2,
    };
    r = mc_check (ed, 1, 2);
    Console.WriteLine ("r={0}", r);  // r=0
  }
}