extern "C" {
    __declspec(dllexport) int __cdecl SimulateGameDLL (int a, int b);
}

#include "Li.h"
extern int __cdecl SimulateGameDLL (int num_games, int rand_in) {
	return 121;
} 

[ServiceContract] public interface IService1{
[OperationContract][WebGet] string GetData();
}

public class Service1 : IService1{

    public string GetData(){

        ClassLibrary1.Class1 cl = new ClassLibrary1.Class1();
        var rt = cl.M(); 
        return string.Format("Value = : {0}", rt);  
    }
}

public class Class1{

        [DllImport("Li.dll", CharSet = CharSet.Unicode, CallingConvention = CallingConvention.Cdecl)]
        public static extern int SimulateGameDLL(int a, int b);

        public int M() {

            var r = SimulateGameDLL(10, 20);            
            return r;
        }
    }