FormClosed事件是否在窗体关闭后触发？自定义Tooltip显示异常排查

问题核心 #

你遇到的是WinForm中FormClosed事件的触发时机问题：该事件在窗体开始关闭流程时就会触发，而非完全消失后。此时NewForm仍处于视觉可见状态，导致你的自定义ToolTip无法正常显示（因存在上层窗体遮挡或层级问题）。

现有代码梳理 #

OldForm（UserControl）代码 #

public partial class OldForm: UserControl
{
    public OldForm(FocusMoreMenu myFocusMore)
    {
        InitializeComponent();
    }
    private void createNewForm(object sender, EventArgs e)
    {
        NewForm newForm = new NewForm (this);
        newForm.Show();
    }
    public void handleCloseEvent(object sender, FormClosedEventArgs e) 
    {
         CustomToolTip notifyError = new CustomToolTip();
         notifyError.Show("Some notification ", this, Xposition, Yposition, 2000);
    }
}

NewForm代码 #

public partial class NewForm : Form
{
    OldForm oldFormObject;

    public NewForm(OldForm oldFormObject)
    {
        this.oldFormObject = oldFormObject;
        InitializeComponent();
        this.FormClosed += new System.Windows.Forms.FormClosedEventHandler(oldFormObject.handleCloseEvent);
    }
    
    private void closeButton(object sender, EventArgs e)
    {
        oldFormObject.passedVariable = someTextBox.Text;
        this.Close();
    }
}

解决办法 #

方法1：延迟ToolTip显示 #

利用异步延迟，等待窗体完全关闭后再显示ToolTip，修改handleCloseEvent方法即可：

public async void handleCloseEvent(object sender, FormClosedEventArgs e) 
{
     await Task.Delay(100); // 等待窗体关闭的短暂间隙（可根据实际调整时长）
     CustomToolTip notifyError = new CustomToolTip();
     notifyError.Show("Some notification ", this, Xposition, Yposition, 2000);
}

方法2：让ToolTip置顶显示 #

如果你的CustomToolTip支持设置TopMost属性，直接将其设为true，强制ToolTip显示在所有窗体上层：

public void handleCloseEvent(object sender, FormClosedEventArgs e) 
{
     CustomToolTip notifyError = new CustomToolTip();
     notifyError.TopMost = true; // 假设CustomToolTip具备该属性
     notifyError.Show("Some notification ", this, Xposition, Yposition, 2000);
}

方法3：改用主窗体作为ToolTip载体 #

将ToolTip的显示目标从UserControl改为主窗体，避免UserControl的层级问题：

// 在OldForm中添加获取主窗体的属性
private Form MainForm => this.FindForm();

public void handleCloseEvent(object sender, FormClosedEventArgs e) 
{
     CustomToolTip notifyError = new CustomToolTip();
     // 计算相对于主窗体的位置
     Point screenPos = this.PointToScreen(new Point(Xposition, Yposition));
     Point formPos = MainForm.PointToClient(screenPos);
     notifyError.Show("Some notification ", MainForm, formPos.X, formPos.Y, 2000);
}

内容的提问来源于stack exchange，提问作者Meric Ozcan