IComparer problem

目的:使用我的类的属性有多个排序选项。我设法使用samAccountName使用Comparable排序，但未能正确实现IComparer。但是我知道我得到了下面指定的错误。

错误:没有实现接口成员System.Collections.Icomparer

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Collections;
namespace ActiveDirectory
{
    public class sortLastName : IComparer
    {
        int IComparer.CompareTo(Employee oEmployee, Employee oEmployee2)
        {
            return String.Compare(oEmployee.lastName, oEmployee2.lastName);
        }
        public static IComparer sortYearAscending()
        {
            return (IComparer)new sortLastName();
        }
    }
//The code works great when I make a call like 
// List<Employee> x = new List<Employee>();
// x.sort();// sorts by samAccountName
// Now I would like to figure out how to sort by lastName and still be able to sort by 
//samAccountName if necceaary. 
    public class Employee :IComparable
    {
        //default sort order
        public int CompareTo(object oEmployee)
        {
            Employee emp1 = (Employee)oEmployee;
            return String.Compare(this.samAccountName, emp1.samAccountName);
        }

        public string lastName
        {
            get;
            set;
        }
          public string samAccountName
        {
            get;
            set;
        }
    }
}