我在代码中 使用 using System.Timers;添加Timers类，但是编译却出现错误，
F:\Program_Files\Microsoft Visual Studio .NET 2003\mywork\ConApp5\Class1.cs(4): 类型或命名空间名称“Timers”在类或命名空间“System”中不存在(是否缺少程序集引用？)如附件所示

不知道除了在代码中这样编辑之外，还要在哪里设置才能正确添加呢？

//-----------------------------------------------------------------------
//  This file is part of the Microsoft .NET Framework SDK Code Samples.
//
//  Copyright (C) Microsoft Corporation.  All rights reserved.
//
//This source code is intended only as a supplement to Microsoft
//Development Tools and/or on-line documentation.  See these other
//materials for detailed information regarding Microsoft code samples.
//
//THIS CODE AND INFORMATION ARE PROVIDED AS IS WITHOUT WARRANTY OF ANY
//KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE
//IMPLIED WARRANTIES OF MERCHANTABILITY AND/OR FITNESS FOR A
//PARTICULAR PURPOSE.
//
/*=====================================================================
  File:      TimedCallback.cs

  Summary:   Demonstrates the use of the Timer class to generate a
             periodic callback to a method.

=====================================================================*/

using System;
using System.Threading;


class App
{
    public static void Main()
    {
        Console.WriteLine("Checking for status updates every 2 seconds.");
        Console.WriteLine("   (Hit Enter to terminate the sample)");
        Timer timer = new Timer(new TimerCallback(CheckStatus), null, 0, 2000);
        Console.ReadLine();
        timer.Dispose();
    }


    // The callback method's signature MUST match that of a System.Threading.TimerCallback
    // delegate (it takes an Object parameter and returns void)
    static void CheckStatus(Object state)
    {
        Console.WriteLine("Checking Status.");
        // ...
    }
}

System.Windows.Forms.Timer是应用于WinForm中的，它是通过Windows消息机制实现的，类似于VB或Delphi中的Timer控件，内部使用API     SetTimer实现的。它的主要缺点是计时不精确，而且必须有消息循环，Console     Application无法使用。      
     
  System.Timers.Timer和System.Threading.Timer非常类似，它们是通过.NET     Thread     Pool实现的，轻量，计时精确，对应用程序、消息没有特别的要求。System.Timers.Timer还可以应用于WinForm，完全取代上面的 Timer控件。它们的缺点是不支持直接的拖放，需要手工编码。      
   
  推荐使用System.Timers.Timer,代码这个地方有,自己读读就好.  
  ms-help://MS.VSCC/MS.MSDNVS.2052/cpref/html/frlrfsystemtimerstimerclasstopic.htm