C++ / C# Interop

Here is code for basic dllExport in C++  and dllImport in C#
C++
#include <windows.h>
#define EXPORT __declspec(dllexport)
extern "C"
{
 EXPORT int dllStartClient()
 {
  return (WinMain((HINSTANCE)0x500000,(HINSTANCE)0x500000,"",0));
 }
}
int WINAPI WinMain(HINSTANCE hInstance,HINSTANCE hPrevInstance,LPSTR lpCmdLine,int nShowCmd)
{
 MessageBox(NULL,L"Hello",L"Hello",MB_ICONEXCLAMATION | MB_OK);
 return 0;
}
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C#
using System.Runtime.InteropServices;
namespace Test1
{
   public partial class Form1 : Form
   {
       [DllImport("Client.dll")]
       private static extern int dllStartClient();

       public Form1()
       {
           InitializeComponent();
       }

       private void BtnStartClient_Click(object sender, EventArgs e)
       {
           StartClient();
       }
       private void StartClient()
       {
           dllStartClient();
       }
   }
}