Moving a borderless form results in weird side effects

Question

I am fairly new to C#. In order to get a modern design application, I decided to make my form borderless. I then found a code snippet for making it possible to move my borderless form, which works perfectly fine.

private const int WM_NCHITTEST = 0x84;
private const int HT_CAPTION = 0x2;

protected override void WndProc(ref Message m)
{
    base.WndProc(ref m);
    switch (m.Msg) {
        case WM_NCHITTEST:
            m.Result = (IntPtr)(HT_CAPTION);
            break;
    }
}

I also need to fetch the form maximize event, and found another code snippet, which again, works perfectly. At least if I use them independently.

                case WM_SYSCOMMAND:
                if (IsMaximized == false)
                {
                    IsMaximized = true;
                    Btn_Ribbon_MaximizeMinimize.Image = Properties.Resources.Img_MinimizeForm;
                    this.WindowState = FormWindowState.Maximized;
                }
                else if (IsMaximized == true)
                {
                    IsMaximized = false;
                    Btn_Ribbon_MaximizeMinimize.Image = Properties.Resources.Img_MaximizeForm;
                    this.WindowState = FormWindowState.Normal;
                }
                break;

Now here comes the weird part. If I use them both together...

        #region Move borderless Form
    private const int WM_NCHITTEST = 0x84;
    private const int HT_CLIENT = 0x1;
    private const int HT_CAPTION = 0x2;
    private const int WM_SYSCOMMAND = 0x0112;

    protected override void WndProc(ref Message m)
    {
        base.WndProc(ref m);
        switch (m.Msg) {
            case WM_NCHITTEST:
                m.Result = (IntPtr)(HT_CAPTION);
                break;
            case WM_SYSCOMMAND:
                if (IsMaximized == false)
                {
                    IsMaximized = true;
                    Btn_Ribbon_MaximizeMinimize.Image = Properties.Resources.Img_MinimizeForm;
                    this.WindowState = FormWindowState.Maximized;
                }
                else if (IsMaximized == true)
                {
                    IsMaximized = false;
                    Btn_Ribbon_MaximizeMinimize.Image = Properties.Resources.Img_MaximizeForm;
                    this.WindowState = FormWindowState.Normal;
                }
                break;
        }
    }
    #endregion

...I get all kind of weird side effects:

A single click on my form is enought for it to maximize itself
After minimizing my application, clicking its icon in the windows taskbar again, does not result in it normalizing, but maximizing

How can I get rid of those side effects, or even better than a workaround, is there a better way to make this work?

@duDE no, it certainly isn't. I don't want to make my borderless form movable, since it is already working as intended. I wanna get rid of the bugs, caused by it. — arvenyon

C.Evenhuis C.Evenhuis · Accepted Answer · 2019-10-09T13:39:14

The WM_SYSCOMMAND message can contain information about more than just the maximize event. When you handle the WM_NCHITTEST and tell the OS that the caption bar was clicked, it also causes a WM_SYSCOMMAND when you release the mouse button.

You should inspect the m.WParam value to determine what action was performed.

The documentation mentions SC_MAXIMIZE (0xF030) but on my machine the value is actually SC_MAXIMIZE2 (0xF032). I can't find any documentation about that value, but this answer also mentions it.

Moving a borderless form results in weird side effects

1 Answers