Flickering Shadow on Borderless Form

Question

Is there a way to create a shadow for a borderless form that doesn't flicker when the form is resized? Right now I'm using CreateParams.

protected override CreateParams CreateParams
{
    get
    {
        const int CS_DROPSHADOW = 0x20000;
        CreateParams cp = base.CreateParams;
        cp.ClassStyle |= CS_DROPSHADOW;
        return cp;
    }
}

But when the form is resized, the shadow part turns white, then back to a shadow, flickering. The rest of the form doesn't because I used this.DoubleBuffered = true;
Any help is appreciated, thanks!

Edit:
I resize the form by using SendMessage

private const int WM_NCLBUTTONDOWN = 0xa1;
SendMessage(handle, WM_NCLBUTTONDOWN, dir, 0);

dir is an int that varies depending on the direction I want to resize the form.

Answer 1

I think your problem is caused by your implementation to support custom resizing using SendMessage as posted in your question. I've tried implementing the resizing using WndProc catching the message WM_NCHITTEST and returns according result to the mouse position. It looks like there is not any much flicker when you resize the top and left edge and no flicker at all for other edges. So I think you can try this code, it works for me:

public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();
        DoubleBuffered = true;
        FormBorderStyle = FormBorderStyle.None;
    }
    protected override CreateParams CreateParams
    {
        get
        {
            const int CS_DROPSHADOW = 0x20000;
            CreateParams cp = base.CreateParams;
            cp.ClassStyle |= CS_DROPSHADOW;
            return cp;
        }
    }
    protected override void WndProc(ref Message m)
    {
        if (m.Msg == 0x84) //WM_NCHITTEST = 0x84
        {
            int x = m.LParam.ToInt32() & 0xffff;
            int y = m.LParam.ToInt32() >> 16;
            int codex, codey;
            Point p = PointToClient(new Point(x, y));
            codey = p.Y < 5 ? 2 : p.Y > ClientSize.Height - 5 ? 1 : 0;
            codex = p.X < 5 ? 2 : p.X > ClientSize.Width - 5 ? 1 : 0;
            switch (codex + (codey<<2))
            {
                case 10://Top-Left
                    m.Result = (IntPtr)13;
                    return;
                case 8://Top
                    m.Result = (IntPtr)12;
                    return;
                case 9://Top-Right
                    m.Result = (IntPtr)14;
                    return;
                case 2://Left
                    m.Result = (IntPtr)10;
                    return;
                case 1://Right
                    m.Result = (IntPtr)11;
                    return;
                case 6://Bottom-Left
                    m.Result = (IntPtr)16;
                    return;
                case 4://Bottom
                    m.Result = (IntPtr)15;
                    return;
                case 5://Bottom-Right;
                    m.Result = (IntPtr)17;
                    return;
            }                
        }
        base.WndProc(ref m);
    }
}

Note that don't use your custom resizing with SendMessage, just the code above.