I’m making a borderless form based on WinForms with rounded corners on Windows 7. To make its edges are visible, I used the following code to drop a shadow on it. This does work, but I’m finding that there’s an almost transparent thing around the form that makes the rounded corners less noticeable.

private const int WM_NCPAINT = 0x0085;
private const int CS_DROPSHADOW = 0x00020000;

private void MainForm_SizeChanged(object sender, EventArgs e)
{
    Region = Region.FromHrgn(CreateRoundRectRgn(0, 0, Width, Height, 30, 30));
}

protected override CreateParams CreateParams
{
    get
    {
        CreateParams cp = base.CreateParams;
        if (!AeroEnabled)
        {
            cp.ClassStyle |= CS_DROPSHADOW;
        }
        return cp;
    }
}

protected override void WndProc(ref Message m)
{
    if (m.Msg == WM_NCPAINT)
    {
        var v = 2;
        DwmSetWindowAttribute(Handle, 2, ref v, 4);
        MARGINS margins = new()
        {
            bottomHeight = 1,
            leftWidth = 0,
            rightWidth = 0,
            topHeight = 0
        };

        DwmExtendFrameIntoClientArea(Handle, ref margins);
    }
    base.WndProc(ref m);
}

[DllImport("dwmapi.dll")]
public static extern int DwmExtendFrameIntoClientArea(IntPtr hWnd, ref MARGINS pMarInset);

[DllImport("dwmapi.dll")]
public static extern int DwmSetWindowAttribute(IntPtr hwnd, int attr, ref int attrValue, int attrSize);

[DllImport("gdi32.dll", EntryPoint = "CreateRoundRectRgn")]
public static extern IntPtr CreateRoundRectRgn(int nLeftRect, int nTopRect, int nRightRect, int nBottomRect, int nWidthEllipse, int nHeightEllipse);

public struct MARGINS
{
    public int leftWidth;
    public int rightWidth;
    public int topHeight;
    public int bottomHeight;
}

The one on the left is my form. The one on the right is the system window, which has a more rounded shadow. On the yellow background it’s obvious that there’s something transparent around my form, and on the white background it looks more like a right angle. So how can I create a rounded shadow? By the way, how to add a border to this borderless, rounded-corner form? I’ve tried most methods found online, but none of them work. Thanks in advance.