Asp.net mvc 3 根据ASP.NET MVC 3中的选定ID在SelectList中为集合中的每个对象设置选定属性

后续问题：

我遇到的问题是编辑视图中的下拉框没有根据BugAssignment模型中的AssignedUserID属性保留/选择适当的项

型号：

public class BugAssignment
{
    public int BugAssignmentID { get; set; }
    public int BugNumber { get; set; }
    public int? AssignedUserID { get; set; }
    public virtual User AssignedUser { get; set; }
    public virtual Status Status { get; set; }
    public IEnumerable<SelectListItem> Users { get; set; }
}

public class BugAssignmentList
{
    public BugAssignmentList()
    {
        BugAssignments = new List<BugAssignment>();
    }
    public int BugAssignmentListID { get; set; }
    public string Name { get; set; }
    public List<BugAssignment> BugAssignments { get; set; }
}

编辑/详细信息视图模型

public class BugAssignmentListDetailsViewModel
{
    public int BugAssignmentListID { get; set; }
    public string Name { get; set;}
    public List<BugAssignment> BugAssignments { get; set; }
}

我的控制器中有以下两种方法：

    public ActionResult Edit(int id)
    {
        var balData = db.BugAssignmentLists.Include(bn => bn.BugAssignments).ToList();
        //this is a reference to the bug assignment list object whose ID was passed in
        BugAssignmentList bugAssignmentList = balData.Where(b => b.BugAssignmentListID == id).FirstOrDefault();
        //loop through the bug assignment list and for each bug assignment
        foreach (BugAssignment b in bugAssignmentList.BugAssignments)
        {
            //create the select list for each BugAssignment object
            b.Users = users.Select(u => new SelectListItem
            {
                Value = u.UserID.ToString(),
                Text = u.FullName,
                Selected = u.UserID == b.AssignedUserID //<<THIS IS WHERE IT IS MESSING UP
            });

        }
        BugAssignmentListDetailsViewModel detailsVM = new BugAssignmentListDetailsViewModel
        {
            BugAssignmentListID = id
        };
        if (bugAssignmentList != null)
        {
            detailsVM.BugAssignments = bugAssignmentList.BugAssignments;
            detailsVM.Name = bugAssignmentList.Name;
        }
        return View(detailsVM);
    }

    // POST
    [HttpPost]
    public ActionResult Edit(BugAssignmentListDetailsViewModel viewModel)
    {
        var balData = db.BugAssignmentLists.Include(bn => bn.BugAssignments).ToList();
        var users = db.Users.ToList();
        BugAssignmentList bugAssignmentList = balData.Where(b => b.BugAssignmentListID == viewModel.BugAssignmentListID).FirstOrDefault();
        bugAssignmentList.Name = viewModel.Name;
        bugAssignmentList.BugAssignments = viewModel.BugAssignments;

        if (ModelState.IsValid)
        {
            UpdateModel(bugAssignmentList, "BugAssignmentList");
            db.Entry(bugAssignmentList).State = EntityState.Modified;
            db.SaveChanges();
            return RedirectToAction("Index");
        }
        return View(bugAssignmentList);
    }

视图：

这个职位运作良好。在数据库中查看，BugAssignmentList中BugAssignments集合中的每个BugAssignment都在使用我的编辑视图中下拉框中的适当AssignedUserID进行更新，因此它不是POST方法

问题在于Editint id。发生的事情非常奇怪。我已经在foreach循环前面放置了一个断点，并查看了bugList.BugAssignments的内部。所有AssignedUserID都是正确的，但是循环完成后，我再次检查列表，BugAssignment中的所有SelectListItems。用户选择了相同的用户

以下代码有什么问题会导致这种情况

    foreach (BugAssignment b in bugAssignmentList.BugAssignments)
    {
        //create the select list for each BugAssignment object
        b.Users = users.Select(u => new SelectListItem
        {
            Value = u.UserID.ToString(),
            Text = u.FullName,
            Selected = u.UserID == b.AssignedUserID //<<THIS IS WHERE IT IS MESSING UP
        });

    }

另外，作为旁注：我希望界面看起来像这样，这就是为什么我需要将SelectList放在BugAssignment模型上，除非有更简单的方法。我不想编写BugAssignment控制器/视图，我希望有人能够快速将用户分配到bug编号，如下所示：

下面是另一个屏幕截图，可以更好地显示正在发生的事情：

---

我猜这是闭包的问题。由于您在lambda中生成SelectListItem，因此它只获取第一个b.AssignedUserID的副本，并在任何地方使用它。关于这方面的一些阅读材料：

我认为这一修改可能会解决问题：

foreach (BugAssignment b in bugAssignmentList.BugAssignments)
    {
        var bugAssignedUser = b.AssignedUserID;
        //create the select list for each BugAssignment object
        b.Users = users.Select(u => new SelectListItem
        {
            Value = u.UserID.ToString(),
            Text = u.FullName,
            Selected = u.UserID == bugAssignedUser  //<<THIS IS WHERE IT IS MESSING UP
        });

    }

---

我猜这是闭包的问题。由于您在lambda中生成SelectListItem，因此它只获取第一个b.AssignedUserID的副本，并在任何地方使用它。关于这方面的一些阅读材料：

我认为这一修改可能会解决问题：

foreach (BugAssignment b in bugAssignmentList.BugAssignments)
    {
        var bugAssignedUser = b.AssignedUserID;
        //create the select list for each BugAssignment object
        b.Users = users.Select(u => new SelectListItem
        {
            Value = u.UserID.ToString(),
            Text = u.FullName,
            Selected = u.UserID == bugAssignedUser  //<<THIS IS WHERE IT IS MESSING UP
        });

    }

---

我认为这是一个结束问题。尝试创建b.assignedUserID的本地副本：

---

我认为这是一个结束问题。尝试创建b.assignedUserID的本地副本：

---

谢谢你的链接，我正要向上面的海报询问更多信息，了解为什么这样做不起作用。谢谢你的链接，我正要向上面的海报询问更多信息，了解为什么这样做不起作用。我读了一些关于关闭问题的文章……我对C和ASP.NET还是新手，这是你必须意识到的陷阱之一，我想是吧。再次感谢。我读了一些关于闭包问题的书……我对C和ASP.NET还是新手，我想这是你必须注意的陷阱之一。再次感谢。

foreach (BugAssignment b in bugAssignmentList.BugAssignments)
{
    var assignedUserID = b.AssignedUserID;

    //create the select list for each BugAssignment object
    b.Users = users.Select(u => new SelectListItem
    {
        Value = u.UserID.ToString(),
        Text = u.FullName,
        Selected = (u.UserID == assignedUserID),
    });

}