Error using singleton with linq to sql execting stored procedure

'System.Data.Linq.DataContext' does not contain a constructor that takes 0 arguments

using System;
using System.Data;
using System.Configuration;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Xml.Linq;
using System.Data.Linq.Mapping;
using System.Data.Linq;
using System.Reflection;
namespace WcfService5
{
   
    public class Singleton : DataContext
    {
        private static Singleton _instance = null;

        private static Object _mutex = new Object();
        static String connection;
        private Singleton(String connectionString)
        {
            // whatever
        }

        public static Singleton GetInstance(String ConnectionString)
        {
            if (_instance == null)
            {
                lock (_mutex) // now I can claim some form of thread safety...
                {
                    if (_instance == null)
                    {
                        _instance = new Singleton(ConnectionString);
                    }
                }
            }

            return _instance;
        }
        [Function(Name = @"GetEmpByNames", IsComposable = false)]
        public ISingleResult<myems> getCustomerAll([Parameter(DbType = "Nvarchar(50)")] string Ename, [Parameter(DbType = "Int")] Int32 Eno)
        {

            IExecuteResult objResult = this.ExecuteMethodCall(this, (MethodInfo)MethodInfo.GetCurrentMethod(), Ename, Eno);
            ISingleResult<myems> objresults = (ISingleResult<myems>)objResult.ReturnValue;
            return objresults;
        }
    }  
   
}