怎样禁用form右上角的关闭按钮？

using System.Runtime.InteropServices;

[DllImport("User32.dll")]
public static extern int GetSystemMenu(int hWnd, int bRevert);

[DllImport("User32.dll")]
public static extern int RemoveMenu(int hMenu, int nPosition, int wFlags);

private void yourform_Load(object sender, System.EventArgs e)
{
    int lSysMenu;
    const int  MF_BYPOSITION = 1024;
    lSysMenu = GetSystemMenu(this.Handle.ToInt32(), 0);
    RemoveMenu(lSysMenu, 6, MF_BYPOSITION);
}