A colleague at DevelopMentor recently asked me about creating popup windows in XAML browser applications (XBAP). Normally this is not allowed - if you try to create a top-level window you will get a SecurityException because WPF asks for UIPermission which is strictly prohibited when hosted in the browser.

It turns out, however, that you can get a popup window - there’s a hidden little gem in the System.Windows.Controls.Primitive namespace that is your friend: Popup.

It’s the same underlying class that ToolTip, Menu, and ComboBox use to display drop-down menus and overlays and it is browser-hosting aware! It’s pretty limited in functionality - I’m not sure you can get it to move around with the mouse for example, but for simple cases it works great. Here’s a code snippet - wire this up to a button in an XBAP:

void OnClick(object sender, EventArgs e)
    Popup window = new Popup();

    StackPanel sp = new StackPanel { Margin = new Thickness(5) };
    sp.Children.Add(new TextBlock { Text = "Hi from a popup" });

    Button newButton = new Button { Content = "Another button" };
    newButton.Click += delegate { window.IsOpen = false; };

    sp.Children.Add(new Slider { Minimum = 0, Maximum = 50, Value = 25, Width = 100 });

    window.Child = new Border { Background = Brushes.White, BorderBrush = Brushes.Black, BorderThickness = new Thickness(2), Child = sp };
    window.PlacementTarget = this;
    window.Placement = PlacementMode.Center;
    window.IsOpen = true;

The key thing you need to do is set the PlacementTarget property. That associates a “parent” window and without it, the Popup class asserts UIPermission which will fail in the browser environment.