当Form的边框样式设置为FormBorderStyle.None时，GetSystemMenu()始终返回NULL句柄。
因为在将边框样式设置为None时，从HWND中删除了一些重要的窗口样式，所以此函数不会返回HMENU。
它可能会检查窗口是否有标题栏，这就是为什么它返回NULL。
此问题的解决方法是在将FoormBorderStyle设置为None之前获取菜单句柄。

using System.Runtime.InteropServices;

public partial class MainForm : Form
{
    public MainForm() => InitializeComponent();

    [DllImport("user32.dll")]
    private static extern IntPtr GetSystemMenu(IntPtr hWnd, bool bRevert);

    [DllImport("user32.dll")]
    private static extern int TrackPopupMenu(IntPtr hMenu, uint uFlags, int x, int y,
       int nReserved, IntPtr hWnd, IntPtr prcRect);
    [DllImport("user32.dll")]
    private static extern IntPtr SendMessage(IntPtr hWnd, int msg, IntPtr wp, IntPtr lp);
    private IntPtr hMenu;
    private const int WM_SYSCOMMAND = 0x112;
    protected override void OnHandleCreated(EventArgs e)
    {
        // Get the system menu and set the border style after that.
        hMenu = GetSystemMenu(Handle, false);
        FormBorderStyle = FormBorderStyle.None;
        
    }
    protected override void OnMouseUp(MouseEventArgs e)
    {
        int menuIdentifier = TrackPopupMenu(hMenu, 0x102, Control.MousePosition.X, Control.MousePosition.Y, 0, Handle, IntPtr.Zero);
        if(menuIdentifier != 0)
        {
            PostMessage(Handle, WM_SYSCOMMAND, (IntPtr)menuIdentifier, IntPtr.Zero);
        }
    }
}