Question

I am WPF newbie. In a WPF sample code I am looking at, there is a mouse event handler as follows:

namespace Recipe_04_15
{
    public class DragCanvasControl : Canvas
    {
        ...
        static DragCanvasControl()
        {
            DefaultStyleKeyProperty.OverrideMetadata(typeof(DragCanvasControl),
                new FrameworkPropertyMetadata(typeof(DragCanvasControl)));
        }
        protected override void OnPreviewMouseLeftButtonDown(MouseButtonEventArgs e)
        {
            ...
        }
...

What I don’t understand is how the OnPreviewMouseLeftButtonDown method is wired to the mouse button down event. The XAML code does not have events specified?

Answer 1

The function is already wired in the UIElement class.
http://msdn.microsoft.com/en-us/library/system.windows.uielement.previewmouseleftbuttondown.aspx

Since you inherit from the canvas you also inherit also from UIElement somewhere deeper.
DragCanvasControl => Canvas => Panel => FrameworkElement => UIElement

By overiding the OnPreviewMouseLeftButtonDown from UIElement you get access to this event.