不能在 DropDownList 中选择多个项

protected void ddlProjectRealName_SelectedIndexChanged(object sender, EventArgs e)
    {
        foreach (ListItem li in ddlProjectName.Items)
        {
            if (ddlProjectRealName.SelectedValue == li.Value)
            {
                ddlProjectName.ClearSelection();  //注意
                li.Selected = true;
            }
        }
    }