@HTML Dropdownlist选定的值

我是Razor的新手，我不明白为什么我选择的值不起作用。

var listItems = (List<Categories>)ViewBag.Categories;

用于

@for (var i = 0; i < Model.TagActivities.Count; i++)
{
    <tr>
        @Html.HiddenFor(modelItem => Model.TagActivities[i].TAGActivityId, new { htmlAttributes = new { @class = "form-control" } })
        @Html.HiddenFor(modelItem => Model.TagActivities[i].CategoryId, new { htmlAttributes = new { @class = "form-control"
         <td>
            @Html.EditorFor(modelItem => Model.TagActivities[i].ActivityName, new { htmlAttributes = new { @class = "form-control" } })
        </td>
        <td>
            @Html.EditorFor(modelItem => Model.TagActivities[i].ActivityDescription, new { htmlAttributes = new { @class = "form-control" } })
        </td>
        <td>
            @Html.DropDownListFor(modelItem => Model.TagActivities[i].Category.CategoryName, new SelectList(listItems, "CategoryId", "CategoryName", Model.TagActivities[i].Category.CategoryName), new { @class = "form-control" })
        </td>
        <td>
            @Html.EditorFor(modelItem => Model.TagActivities[i].AlertMinutes, new { htmlAttributes = new { @class = "form-control" } })
        </td>
        <td>
            @Html.EditorFor(modelItem => Model.TagActivities[i].IsDefault)
        </td>
    </tr>
}

请帮忙。谢谢。