如何获取WPF MessageBox的PresentationSource？避免关联异常

问题根源 #

关闭控件时触发LostFocus事件，此时控件的Visual已经脱离了PresentationSource（控件正在卸载/已卸载），调用MessageBox.Show时找不到有效的视觉上下文，从而抛出异常。

可行解决方法 #

1. 直接用用户控件自身作为Visual对象检查 #

用户控件本身就是Visual的子类，直接用PresentationSource.FromVisual(this)判断是否仍处于有效上下文：

private void UserControl_LostFocus(object sender, RoutedEventArgs e)
{
    // 检查当前控件是否仍关联到PresentationSource
    if (PresentationSource.FromVisual(this) != null)
    {
        MessageBox.Show("提示内容");
    }
}

2. 结合控件加载状态与Dispatcher延迟执行 #

通过IsLoaded判断控件是否仍处于加载状态，并用Dispatcher延迟执行，避免在卸载过程中触发：

private void UserControl_LostFocus(object sender, RoutedEventArgs e)
{
    Dispatcher.InvokeAsync(() =>
    {
        if (this.IsLoaded && PresentationSource.FromVisual(this) != null)
        {
            MessageBox.Show("提示内容");
        }
    }, DispatcherPriority.Background);
}

3. 标记控件卸载状态 #

在Unloaded事件中标记控件已卸载，LostFocus事件先判断该状态：

private bool _hasUnloaded = false;

private void UserControl_Unloaded(object sender, RoutedEventArgs e)
{
    _hasUnloaded = true;
}

private void UserControl_LostFocus(object sender, RoutedEventArgs e)
{
    if (!_hasUnloaded && PresentationSource.FromVisual(this) != null)
    {
        MessageBox.Show("提示内容");
    }
}

4. 使用自定义窗口替代MessageBox #

自定义窗口可以明确指定Owner为当前窗口，确保有有效的视觉上下文：

private void UserControl_LostFocus(object sender, RoutedEventArgs e)
{
    var parentWindow = Window.GetWindow(this);
    if (parentWindow != null && PresentationSource.FromVisual(parentWindow) != null)
    {
        var dialog = new Window
        {
            Title = "提示",
            Width = 300,
            Height = 150,
            WindowStyle = WindowStyle.ToolWindow,
            Owner = parentWindow,
            WindowStartupLocation = WindowStartupLocation.CenterOwner
        };
        dialog.Content = new TextBlock 
        { 
            Text = "提示内容", 
            HorizontalAlignment = HorizontalAlignment.Center, 
            VerticalAlignment = VerticalAlignment.Center 
        };
        dialog.ShowDialog();
    }
}

内容的提问来源于stack exchange，提问作者VivekDev