ecall methods must be packaged into a system module

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C#

Hey Everyone
I am getting following exception
ecall methods must be packaged into a system module

My code is

using System;
using GHIElectronics.NETMF.Hardware;
using Microsoft.SPOT.Hardware;
namespace DateTime_RTC_
{
    class Program
    {

        static void Main(string[] args)
        {
            Console.WriteLine(RealTimeClock.GetTime());
            Console.ReadLine();
        }

    }
}