using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Runtime.InteropServices;

namespace WindowsFormsApplication1
{
public partial class Form1 : Form
{
System.Windows.Forms.Timer timer = null;
string WM_Caption = string.Empty;

public Form1()
{
InitializeComponent();
}

[DllImport("user32.dll", EntryPoint = "FindWindow", CharSet = CharSet.Auto)]
private extern static IntPtr FindWindow(string lpClassName, string lpWindowName);

[DllImport("user32.dll", CharSet = CharSet.Auto)]
public static extern int PostMessage(IntPtr hWnd, int msg, IntPtr wParam, IntPtr lParam);
public const int WM_CLOSE = 0x10;

private void button1_Click(object sender, EventArgs e)
{
ShowMsg("test", "Title");
}

void ShowMsg(string msg, string caption)
{
WM_Caption = caption;

if (timer != null && timer.Enabled == false)
{
timer.Enabled = true;
}
else
{
timer = new Timer(); // 宣告一個Timer物件，名稱為 SetTimer
timer.Interval = (3 * 1000); //x秒后启动
timer.Tick += new EventHandler(SetTimer_Tick); // 建立一个触发Tick事件
timer.Start(); // 启动Timer (即SetTimer 开始启动 )
}

MessageBox.Show(msg, caption,MessageBoxButtons.OKCancel);
}

void SetTimer_Tick(object sender, EventArgs e)
{
KillMessageBox(); // 執行把 MessageBox 刪除 (即让MessageBox消失)
((Timer)sender).Enabled = false; //停止時間。記得要转型为 Timer 型別
}

/// <summary>
/// 杀窗口
/// </summary>
private void KillMessageBox()
{
// 按照MessageBox的标题，找出Messagebox这个视窗
IntPtr ptr = FindWindow(null, WM_Caption);
if (ptr != IntPtr.Zero) // 此表示只要 ptr 不为空值的内容时
{
//找到则关闭 MessageBox 視窗
PostMessage(ptr, WM_CLOSE, IntPtr.Zero, IntPtr.Zero);
}
}
}
}

C# MessageBox.Show每隔3秒自动关闭