Resizable Form with BorderStyle None and Child with Dock Fill

Question

My goal is to create window (using WindowsForms) that is resizable like normal Win10 windows but has no titlebar, so I can draw it myself (like UWP apps).

Answer 1

I finally found a really good working anwser by modifing this code: https://stackoverflow.com/a/29788300/15213858

What I have now:

public class MyForm : Form
{
    //Window Messages
    public const uint WM_NCPAINT = 0x85;
    public const uint WM_NCCALCSIZE = 0x83;
    public const uint WM_NCHITTEST = 0x84;

    //RECT Structure
    [StructLayout(LayoutKind.Sequential)]
    public struct RECT
    {
        public int left, top, right, bottom;
    }

    //WINDOWPOS Structure
    [StructLayout(LayoutKind.Sequential)]
    public struct WINDOWPOS
    {
        public IntPtr hwnd;
        public IntPtr hwndinsertafter;
        public int x, y, cx, cy;
        public int flags;
    }

    //NCCALCSIZE_PARAMS Structure
    [StructLayout(LayoutKind.Sequential)]
    public struct NCCALCSIZE_PARAMS
    {
        public RECT rgrc0, rgrc1, rgrc2;
        public WINDOWPOS lppos;
    }

    //Window Procedure Hook
    protected override void WndProc(ref Message m)
    {
        //Don't style window in designer...
        if (DesignMode)
            base.WndProc(ref m);

        //Handle Message
        switch ((uint)m.Msg)
        {
            case WM_NCCALCSIZE: WmNCCalcSize(ref m); break;
            default: base.WndProc(ref m); break;
        }
    }

    //WM_NCCALCSIZE
    private void WmNCCalcSize(ref Message m)
    {

        //Check WPARAM
        if (m.WParam != IntPtr.Zero)    //TRUE
        {
            //When TRUE, LPARAM Points to a NCCALCSIZE_PARAMS structure
            var nccsp = (NCCALCSIZE_PARAMS)Marshal.PtrToStructure(m.LParam, typeof(NCCALCSIZE_PARAMS));

            //We're adjusting the size of the client area here. Right now, the client area is the whole form.
            //Adding to the Top, Bottom, Left, and Right will size the client area.
            nccsp.rgrc0.top += 0;      //30-pixel top border
            nccsp.rgrc0.bottom -= 8;    //4-pixel bottom (resize) border
            nccsp.rgrc0.left += 8;      //4-pixel left (resize) border
            nccsp.rgrc0.right -= 8;     //4-pixel right (resize) border

            //Set the structure back into memory
            Marshal.StructureToPtr(nccsp, m.LParam, true);
        }
        else    //FALSE
        {
            //When FALSE, LPARAM Points to a RECT structure
            var clnRect = (RECT)Marshal.PtrToStructure(m.LParam, typeof(RECT));

            //Like before, we're adjusting the rectangle...
            //Adding to the Top, Bottom, Left, and Right will size the client area.
            clnRect.top += 0;      //30-pixel top border
            clnRect.bottom -= 8;    //4-pixel bottom (resize) border
            clnRect.left += 8;      //4-pixel left (resize) border
            clnRect.right -= 8;     //4-pixel right (resize) border

            //Set the structure back into memory
            Marshal.StructureToPtr(clnRect, m.LParam, true);
        }

        //Return Zero
        m.Result = IntPtr.Zero;
    }
}