Borderless and Resizable Form (C#)

Try this:

public Form1()
{
    InitializeComponent();
    this.FormBorderStyle = FormBorderStyle.None;
}

protected override void WndProc(ref Message m)
{
    const int RESIZE_HANDLE_SIZE = 10;

    switch (m.Msg)
    {
        case 0x0084/*NCHITTEST*/ :
            base.WndProc(ref m);

            if ((int)m.Result == 0x01/*HTCLIENT*/)
            {
                Point screenPoint = new Point(m.LParam.ToInt32());
                Point clientPoint = this.PointToClient(screenPoint);                        
                if (clientPoint.Y <= RESIZE_HANDLE_SIZE)
                {
                    if (clientPoint.X <= RESIZE_HANDLE_SIZE)
                        m.Result = (IntPtr) 13/*HTTOPLEFT*/ ;
                    else if (clientPoint.X < (Size.Width - RESIZE_HANDLE_SIZE))
                        m.Result = (IntPtr) 12/*HTTOP*/ ;
                    else
                        m.Result = (IntPtr) 14/*HTTOPRIGHT*/ ;
                }
                else if (clientPoint.Y <= (Size.Height - RESIZE_HANDLE_SIZE))
                {
                    if (clientPoint.X <= RESIZE_HANDLE_SIZE)
                        m.Result = (IntPtr) 10/*HTLEFT*/ ;
                    else if (clientPoint.X < (Size.Width - RESIZE_HANDLE_SIZE))
                        m.Result = (IntPtr) 2/*HTCAPTION*/ ;
                    else
                        m.Result = (IntPtr) 11/*HTRIGHT*/ ;
                }
                else
                {
                    if (clientPoint.X <= RESIZE_HANDLE_SIZE)
                        m.Result = (IntPtr) 16/*HTBOTTOMLEFT*/ ;
                    else if (clientPoint.X < (Size.Width - RESIZE_HANDLE_SIZE))
                        m.Result = (IntPtr) 15/*HTBOTTOM*/ ;
                    else
                        m.Result = (IntPtr)17/*HTBOTTOMRIGHT*/ ;
                }
            }
            return;
    }
    base.WndProc(ref m);
}

protected override CreateParams CreateParams
{
    get
    {
        CreateParams cp = base.CreateParams;
        cp.Style |= 0x20000; // <--- use 0x20000
        return cp;
    }
}

informational sources:

• https://stackoverflow.com/a/19670447/2352507
• Custom Resize Handle in Border-less Form C#
• http://www.csharp411.com/add-drop-shadow-to-borderless-form/