Question

How do you display a custom UserControl as a dialog in C#/WPF (.NET 3.5)?

Answer 1

Place it in a Window and call Window.ShowDialog.
(Also, add references to: PresentationCore, WindowsBase and PresentationFramework if you have not already done so.)

private void Button1_Click(object sender, EventArgs e)
{
        Window window = new Window 
        {
            Title = "My User Control Dialog",
            Content = new MyUserControl()
        };

        window.ShowDialog();
}