Question

I have code that lets be drag around a borderless form in winforms that I’ve been using for several months now, which works extremely well.

But when I first was given the code, they used this.Invalidate(); in the MouseMove event of the Form, and the Form flickered a little and was slow when dragging around. So, I replaced Invalidate() with Update() in the MouseMove event and, to my surprise, the Form can now be dragged very smoothly and has no flickering whatsoever.

Can somebody explain to me why Update makes the code work better than Invalidate, even when Invalidate sounds like it’s the right one to be using?

Thanks 🙂

P.S. Maybe it would help more if I added the code… Adding it now.

Edit – Here’s the code:

---

private void titlebar_MouseDown(object sender, MouseEventArgs e)
{
    this.IsMouseDown = true;

    this.LastCursorPosition = new Point(e.X, e.Y);

    if (this.BackColor == Color.White)
    {
        this.BackColor = Color.GhostWhite;
        tbox.BackColor = Color.GhostWhite;
        tbox.ForeColor = Color.Black;
    }
    else
    {
        this.BackColor = Color.FromArgb(20, 20, 20);
        tbox.BackColor = Color.FromArgb(20, 20, 20);
        tbox.ForeColor = Color.White;
    }
}

private void titlebar_MouseMove(object sender, MouseEventArgs e)
{
    if (this.IsMouseDown == true)
    {
        //Move the form
        this.Location = new Point(this.Left - (this.LastCursorPosition.X - e.X), this.Top - (this.LastCursorPosition.Y - e.Y));

        // Update works better than Invalidate();.
        Update();
    }
}

private void titlebar_MouseUp(object sender, MouseEventArgs e)
{
    this.IsMouseDown = false;
    this.BackColor = fc;
    tbox.BackColor = fc;
}

Answer 1

Invalidate marks the window as needing to be refreshed (at some point). Update does it there and then if I remember correctly

Here is a link to explain the difference better than I’d be able to