下拉大小设置不采用

使用以下代码.....

ToolStripControlHost treeViewHost;
    ToolStripDropDown dropDown;
    public MyTreeViewCombo()
    {
        TreeView treeView = new TreeView();
        treeView.BorderStyle = BorderStyle.None;
        treeViewHost = new ToolStripControlHost(treeView);
        // create drop down and add it
        dropDown = new ToolStripDropDown();
        dropDown.Items.Add(treeViewHost);
    }
    public TreeView TreeView
    {
        get { return treeViewHost.Control as TreeView; }
    }
    private void ShowDropDown()
    {
        if (dropDown != null)
        {
            dropDown.Height = DropDownHeight;
            dropDown.Width = DropDownWidth;
            treeViewHost.Width = DropDownWidth;
            treeViewHost.Height = DropDownHeight;
            dropDown.Show(this, 0, this.Height);
        }
    }
    [...]

无论 dropDownHeight 设置为什么，它只会显示一个小的下拉列表（高度不会改变）。

知道为什么会这样吗？