使用此类

class CustomContextMenu : ContextMenuStrip
{
    [DllImport("dwmapi.dll", CharSet = CharSet.Unicode, SetLastError = true)]
    private static extern long DwmSetWindowAttribute(IntPtr hwnd,
                                                        DWMWINDOWATTRIBUTE attribute,
                                                        ref DWM_WINDOW_CORNER_PREFERENCE pvAttribute,
                                                        uint cbAttribute);
    
    public CustomContextMenu()
    {
        var preference = DWM_WINDOW_CORNER_PREFERENCE.DWMWCP_ROUND;     //change as you want
        DwmSetWindowAttribute(Handle,
                              DWMWINDOWATTRIBUTE.DWMWA_WINDOW_CORNER_PREFERENCE,
                              ref preference,
                              sizeof(uint));
    }

    public enum DWMWINDOWATTRIBUTE
    {
        DWMWA_WINDOW_CORNER_PREFERENCE = 33
    }
    public enum DWM_WINDOW_CORNER_PREFERENCE
    {
        DWMWA_DEFAULT = 0,
        DWMWCP_DONOTROUND = 1,
        DWMWCP_ROUND = 2,
        DWMWCP_ROUNDSMALL = 3,
    }
}