ComboBox.SelectedItem无法正常工作C＃asp.net

Question

我有一个组合框，我在页面加载方法中添加了项目，在按钮单击中，我试图设置键入的文本或从下拉列表中选择的项目。我正在使用ComboBox.SelectedItem.ToString（），当输入内容时可以正常工作，但是一旦用户从下拉列表中选择了某些内容，字符串为空即可通过。

  protected void Page_Load(object sender, EventArgs e)
        {
            if (ScriptManager.GetCurrent(Page) == null)
            {
                Page.Form.Controls.AddAt(0, new ScriptManager());
            }

            DiffComboBoxUrls.Items.Add(XMLConstants.PolicyStoreApplications.Value1);
            DiffComboBoxUrls.Items.Add(XMLConstants.PolicyStoreApplications.Value2);
            DiffComboBoxUrls.Items.Add(XMLConstants.PolicyStoreApplications.Value3);
            DiffComboBoxUrls.Items.Add(XMLConstants.PolicyStoreApplications.Value4);
            DiffComboBoxUrls.Items.Add(XMLConstants.PolicyStoreApplications.Value5);
        }

 protected void ChangeAppSettings_Click(object sender, EventArgs e)
        {
            string FOPath = FODirectory.Text;
            string PolicyStoreName = DiffComboBoxUrls.SelectedItem.ToString(); //this comes back as "" when an item is selected and works when something is typed in
         .....
}