extern "C" __declspec(dllexport) __cdecl int sum(int a,int b); ---->

using System.Runtime.InteropServices;
namespace ImportDLL
{
public class importdll
{
public importdll()
{
}


DllImport("mysum.dll",
          EntryPoint="sum",
          ExactSpelling=false,
          CallingConvention = CallingConvention.Cdecl)]
public extern int myfun(int a, int b);
}
}

using ImportDLL;
namespace TEST
{
 public int my_result;
 protected importdll imp = new importdll();
 my_result = imp.myfun(1,1);
}