对表进行排序/搜索不起作用

>排序和搜索机制在我们的.NET MVC项目中不起作用。我完成了控制器和视图的编码，除了排序和搜索之外，一切都很好。

控制器：

  public ActionResult Index(string sortOrder, string searchString)
    {
        ViewBag.NameSortParm = String.IsNullOrEmpty(sortOrder) ? "name_desc" : "";
        ViewBag.CustomerNameSortParm = String.IsNullOrEmpty(sortOrder) ? "cust_name_desc" : "";
        ViewBag.LastSucceedOnSortParm = sortOrder == "Date" ? "succeed_date_desc" : "Date";
        var farms = from f in db.Farms
                    select f;
        if (!String.IsNullOrEmpty(searchString))
        {
            farms = farms.Where(s => s.LicenseKey.ToUpper().Contains(searchString.ToUpper()) || s.Name.ToUpper().Contains(searchString.ToUpper()));
        }
        switch (sortOrder)
        {
            case "name_desc":
                farms = farms.OrderByDescending(f => f.Name);
                break;
            case "cust_name_desc":
                farms = farms.OrderByDescending(f => f.CustomerName);
                break;
            case "succeed_date_desc":
                farms = farms.OrderBy(f => f.LastSucceedOn);
                break;
        }
        return View(db.Farms.ToList());
    }

视图：

@using (Html.BeginForm())
{
    <p>
        Search: @Html.TextBox("SearchString")
        <input type="submit" value="Search" />
    </p>
}
<table class="table">
    <tr>
        <th>
            @Html.DisplayNameFor(model => model.LicenseKey)
        </th>
        <th>
            @*@Html.DisplayNameFor(model => model.Name)*@
        @Html.ActionLink("Name", "Index", new { sortOrder = ViewBag.NameSortParm })
        </th>
        <th>
            @*@Html.DisplayNameFor(model => model.CustomerName)*@
            @Html.ActionLink("CustomerName", "Index", new { sortOrder = ViewBag.CustomerNameSortParm })
        </th>
        <th>
            @Html.DisplayNameFor(model => model.FarmDate)
        </th>
        <th>
            @*@Html.DisplayNameFor(model => model.LastSucceedOn)*@
            @Html.ActionLink("LastSucceedOn", "Index", new { sortOrder = ViewBag.LastSucceedOnSortParm })
        </th>
        <th></th>
        <th></th>
    </tr>
@foreach (var item in Model) {
    <tr>
        <td>
            @Html.ActionLink(item.LicenseKey, "FarmDetails", "Farm", new { id = item.Id }, null)
        </td>
        <td>
            @Html.DisplayFor(modelItem => item.Name)
        </td>
        <td>
            @Html.DisplayFor(modelItem => item.CustomerName)
        </td>
        <td>
            @Html.DisplayFor(modelItem => item.FarmDate)
        </td>
        <td>
            @Html.DisplayFor(modelItem => item.LastSucceedOn)
        </td>
        <td>
            @Html.ActionLink("Edit", "Edit", new { id=item.Id })
        </td>
        <td>
            @Html.ActionLink("Settings", "Index", "Settings")
        </td>
    </tr>
}
</table>

我的代码的哪一部分是错误的？一定在某处有一个简单的错误，但我看不到它。谢谢。

return View(farms.ToList());