public class AutoClosingMessageBox
{
    System.Threading.Timer _timeoutTimer;
    string _caption;
    DialogResult _result;
    DialogResult _timerResult;
    AutoClosingMessageBox(string text, string caption, int timeout, MessageBoxButtons buttons = MessageBoxButtons.OK, DialogResult timerResult = DialogResult.None)
    {
        _caption = caption;
        _timeoutTimer = new System.Threading.Timer(OnTimerElapsed,
            null, timeout, System.Threading.Timeout.Infinite);
        _timerResult = timerResult;
        using (_timeoutTimer)
            _result = MessageBox.Show(text, caption, buttons);
    }
    public static DialogResult Show(string text, string caption, int timeout, MessageBoxButtons buttons = MessageBoxButtons.OK, DialogResult timerResult = DialogResult.None)
    {
        return new AutoClosingMessageBox(text, caption, timeout, buttons, timerResult)._result;
    }
    void OnTimerElapsed(object state)
    {
        IntPtr mbWnd = FindWindow("#32770", _caption); // lpClassName is #32770 for MessageBox
        if (mbWnd != IntPtr.Zero)
            SendMessage(mbWnd, WM_CLOSE, IntPtr.Zero, IntPtr.Zero);
        _timeoutTimer.Dispose();
        _result = _timerResult;
    }
    const int WM_CLOSE = 0x0010;
    [System.Runtime.InteropServices.DllImport("user32.dll", SetLastError = true)]
    static extern IntPtr FindWindow(string lpClassName, string lpWindowName);
    [System.Runtime.InteropServices.DllImport("user32.dll", CharSet = System.Runtime.InteropServices.CharSet.Auto)]
    static extern IntPtr SendMessage(IntPtr hWnd, UInt32 Msg, IntPtr wParam, IntPtr lParam);
}

用法：

AutoClosingMessageBox.Show("Text", "Caption", 1000);

返回值 

var userResult = AutoClosingMessageBox.Show("Yes or No?", "Caption", 1000, MessageBoxButtons.YesNo);if(userResult == System.Windows.Forms.DialogResult.Yes) {
// do something}

也可以通过Nuget安装这个包

Install-Package AutoClosingMessageBox