using System;
using System.Runtime.InteropServices;
using System.Runtime.CompilerServices;

[ComImport]
[Guid("A4E23A7E-C932-4A31-8455-2F137015DCB2")]
public interface INakeTTClass
{
    [DispId(1)]
    int Add(int a, int b, out int c);

    [DispId(2)]
    int GetAge();
} 

[ComImport]
[Guid("73269D6A-0BB4-46B8-8062-7C8DCDEE81D3")]
public class NakeTTClass : INakeTTClass
{
    [DispId(1)]
    [PreserveSig]        //<---------------
    [MethodImpl(MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime)]
    public extern int Add(int a, int b, out int c);

    [DispId(2)]
    [PreserveSig]        //<---------------
    [MethodImpl(MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime)]
    public extern int GetAge();
}

class Program
{
    static void Main()
    {
        NakeTTClass nake = new NakeTTClass();
        int sum;
        int hresult = nake.Add(3, 4, out sum);
        // hresult == S_OK == 0
        // sum == 7

        hresult = nake.GetAge();
        // hresult = 123
    }
}