How do you detect a mouse double left click in WPF?

MouseDoubleClick passes MouseButtonEventArgs as the event arguments. This exposes the ChangedButton property, which tells you which button was double clicked.

void OnMouseDoubleClick(Object sender, MouseButtonEventArgs e)
{
    if (e.ChangedButton == MouseButton.Left)
    {
        // Left button was double clicked
    }
}

Are you looking for MouseButtonEventArgs.ChangedButton? API here.

private void MouseButtonDownHandler(object sender, MouseButtonEventArgs e)
{
    Control src = e.Source as Control;

    if (src != null)
    {
        switch (e.ChangedButton)
        {
            case MouseButton.Left:
                src.Background = Brushes.Green;
                break;
            case MouseButton.Middle:
                src.Background = Brushes.Red;
                break;
            case MouseButton.Right:
                src.Background = Brushes.Yellow;
                break;
            case MouseButton.XButton1:
                src.Background = Brushes.Brown;
                break;
            case MouseButton.XButton2:
                src.Background = Brushes.Purple;
                break;
            default:
                break;
        }
    }
}