private void button3_Click(object sender, EventArgs e)
{
   AutoCloseDlg.ShowMessageBoxTimeout("hello,www.vjsdn.com!", "hello window",
   MessageBoxButtons.YesNoCancel, 2000);
}

using System;
using System.Collections.Generic;
using System.Text;
using System.Runtime.InteropServices;
using System.Windows.Forms;
using System.Threading;

namespace TestProject
{
   public class AutoCloseDlg
   {
      [DllImport("user32.dll", SetLastError = true)]
      static extern IntPtr FindWindow(string lpClassName, string lpWindowName);
      
      [DllImport("user32.dll")]
      static extern bool EndDialog(IntPtr hDlg, out IntPtr nResult);
      
      //外部调用的方法
      //参数:timeout定义多少毫秒关闭对话框
      public static void ShowMessageBoxTimeout(string text, string caption,
      MessageBoxButtons buttons, int timeout)
      {
         ThreadPool.QueueUserWorkItem(new WaitCallback(CloseMessageBox),
         new CloseState(caption, timeout));
         
         MessageBox.Show(text, caption, buttons);
      }
      
      /// <summary>
      /// 由WaitCallback调用的关闭窗体方法
      /// </summary>
      private static void CloseMessageBox(object state)
      {
         CloseState closeState = state as CloseState;
         
         Thread.Sleep(closeState.Timeout);
         
         IntPtr dlg = FindWindow(null, closeState.Caption);
         
         if (dlg != IntPtr.Zero)
         {
            IntPtr result;
            EndDialog(dlg, out result);
         }
      }
   }
   
   /// <summary>
   /// 作为ThreadPool.QueueUserWorkItem方法的State参数
   /// </summary>
   public class CloseState
   {
      private int _Timeout;
      
      /// <summary>
      /// In millisecond
      /// </summary>
      public int Timeout
      {
         get
         {
            return _Timeout;
         }
      }
      
      private string _Caption;
      
      /// <summary>
      /// Caption of dialog
      /// </summary>
      public string Caption
      {
         get
         {
            return _Caption;
         }
      }
      
      public CloseState(string caption, int timeout)
      {
         _Timeout = timeout;
         _Caption = caption;
      }
   }  
}