ASP.NET绑定部分视图下拉列表

Question

我的代码类似于：

class StudentsViewModel：

public class StudentsViewModel
{
    public List<Student> Students { get; set; }
    public Student SelectedStudent { get; set; }
    public string DisplayMode { get; set; }
}

StudentsController：

[HttpPost]
public ActionResult New(int? page, int? SelectedGroup)
{
    // some code

    StudentsViewModel model = new StudentsViewModel();
    model.Students = db.Students.ToList().ToPagedList(pageNumber, pageSize);
    model.SelectedStudent = null;
    model.DisplayMode = "WriteOnly";
    ViewBag.IDGroup = new SelectList(db.Groups, "IDGroup", "Name");
    return View("Index", model);
}

查看：索引

<form method="post">
    <input type="submit" value="Add Student" formaction="/Students/new" class="btn btn-default" />


//some code

@{
    if (Model.SelectedStudent != null)
    {
        if (Model.DisplayMode == "ReadWrite")
        {
            Html.RenderPartial("_EditStudent", Model.SelectedStudent);
        }
    }
    if (Model.DisplayMode == "WriteOnly")
    {
        Html.RenderPartial("_InsertStudent", new StudentList.Models.Student());
    }
}</form>

部分视图： _InsertStudent.cshtml

<div class="form-group">
    @Html.DropDownList("IDGroup", String.Empty)
    @Html.ValidationMessageFor(model => model.IDGroup)
</div>

我有一个非常大的问题，因为我的DropDownList不起作用...现在我点击按钮时显示_InsertStudent但它不起作用...如果我有

Html.RenderPartial("_InsertStudent", new StudentList.Models.Student());

直接（没有按钮）它有效......