Winforms 禁用toolstrip菜单

我正在尝试对menustrip启用=false。。但代码代码代码正在执行，但无法禁用菜单

        MenuStrip ms = (MenuStrip)frm_MDI.objMdi.Controls["msMainMenu"];
        foreach (ToolStripMenuItem tmm in ms.Items)
        {
            tmm.Enabled = false;
        }

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添加objMdi后=此；加载事件。没问题

    private void frm_MDI_Load_1(object sender, EventArgs e)
    {
        objMdi = this;
    }