HMODULE HXlsDll = LoadLibrary(_J("TestLibrary.dll"));
if (HXlsDll)
{
  f_NetUse_TEST = (f_NetUseFUNC) GetProcAddress(HXlsDll, _J("f_TESTFUNCTION"));
}
if( f_NetUse_TEST )
{
  f_NetUse_TEST(&dsNetUse);
}

typedef struct tagDSNETUSE
{
  long	            nParam1;  
} DSNETUSE, *lpdsNetUse;

typedef LRESULT (*f_NetUseFUNC)(lpdsNetUse);
f_NetUseFUNC  f_NetUse_TEST;

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using System.Runtime.InteropServices;

namespace TestC
{
    [ComVisible(true)]
    [InterfaceType(ComInterfaceType.InterfaceIsDual)]
    public interface ITestInterface
    {
        void f_Message(string strMessage);
    }

    [ComVisible(true)]
    [ClassInterface(ClassInterfaceType.None)]
    public class TestC: ITestInterface
    {                
        [ComVisible(true)]
        public void f_Message(string strMessage)
        {
            strMessage = "Welcome to C#";
            MessageBox.Show(strMessage, "УРА-УРА");
        }
    }
}

#define DLLImport extern "C" __declspec(dllimport) 

DLLImport void f_Message(wchar_t strMessage);


f_Message(L'wdwd');