Ajax.BeginForm UpdateTargetId不适用于DropDownList

Code:

<% using (Ajax.BeginForm("GetResourcesByProject", "CreateRequest", new AjaxOptions { UpdateTargetId = "ResourceListDiv"}))
{
 Response.Write(Html.DropDownList("SelectProject", Model.ProjectList, "Select Project", new { onchange = "this.form.submit();" }));
} %>
<% using (Ajax.BeginForm("GetResourcesB