How do I make the form partially transparent when moving?

Question

How can I set the opacity of the winform to something like 50% while moving the form by draging the title bar and reset its opacity to 100% once left mouse button is up.

Answer 1

Set Form.Opacity to 0.5 in response to WM_NCLBUTTONDOWN in the WndProc of your form.

Then set Opacity to 1.0 when WM_NCLBUTTONUP is received.

Answer 2

Here's a code example:

    public partial class Form1 : System.Windows.Forms.Form
{
    private const long BUTTON_DOWN_CODE = 0xa1;
    private const long BUTTON_UP_CODE = 0xa0;
    private const long WM_MOVING = 0x216;

    static bool left_button_down = false;

    protected override void DefWndProc(ref System.Windows.Forms.Message m)
    {
        //Check the state of the Left Mouse Button
        if ((long)m.Msg == BUTTON_DOWN_CODE)
            left_button_down = true;
        else if ((long)m.Msg == BUTTON_UP_CODE)
            left_button_down = false;

        if (left_button_down)
        {
            if ((long)m.Msg == WM_MOVING)
            {
                //Set the forms opacity to 50% if user is moving
                if (this.Opacity != 0.5)
                    this.Opacity = 0.5;
            }
        }

        else if (!left_button_down)
            if (this.Opacity != 1.0)
                this.Opacity = 1.0;

        base.DefWndProc(ref m);
    }
}

Answer 3

Interestingly, you can also do it in the OnResizeBegin and OnResizeEnd overrides -- this will apply to both moving and resizing the form.

If you want to change the opacity only when moving, and not when resizing, then alex's answer is better.