添加到集合时修改了集合

Question

尝试添加到集合时出现“Collection was modified”异常

public void UpdateLinks(EventViewModel form)
{
    var selectedIds = form.Links.Select(r => r.ResourceTypeID).ToList();
    var assignedIds = form.Event.Links.Select(r => r.ResourceTypeID).ToList();
    foreach (var resource in form.Links)
    {
        resource.EventID = form.Event.ID;
        if (!assignedIds.Contains(resource.ResourceTypeID))
            form.Event.Links.Add(resource);
    }
    foreach (var resource in form.Event.Links.ToList())
    {
        if (!selectedIds.Contains(resource.ResourceTypeID))
            form.Event.Links.Remove(resource);
    }
}

问题出在“添加”方法上。如果我注释掉该部分，则不会引发异常。重要的是要注意，我已经尝试将foreach重写为for循环并添加“ToList（）”以形成。链接。在所有情况下都会引发相同的异常。我在站点的其他部分使用这种完全相同的模式没有问题，这就是为什么这如此令人沮丧。这也适用于“创建”。问题只影响“编辑”操作。

其他相关代码：

[HttpPost]
public ActionResult Edit(EventViewModel form, HttpPostedFileBase[] eventFiles)
{
    if (ModelState.IsValid)
    {
        eventsService.UpdateEvent(form.Event);
        eventsService.UpdateManufacturerTags(form);
        eventsService.UpdateFiles(form, eventFiles);
        eventsService.UpdateLinks(form);
        eventsService.Save();
        return RedirectToAction("Details", new { id = form.Event.ID });
    }
    return View(form);
}

public class EventViewModel : ContentLeftViewModel
{
    public Event Event { get; set; }
    public string[] SelectedManufacturers { get; set; }
    public MultiSelectList Manufacturers { get; set; }
    public IList<EventResource> Files { get; set; }
    public IList<EventResource> Links { get; set; }

    public EventViewModel()
    {
        SelectedManufacturers = new string[0];
        Files = new List<EventResource>();
        Links = new List<EventResource>();
    }
}

public class Event
{
    [Key]
    public int ID { get; set; }

    [Required]
    public string Title { get; set; }

    [Required]
    [DisplayName("Start Time")]
    [DisplayFormat(ApplyFormatInEditMode = true, DataFormatString = "{0:M/d/yyyy h:mm tt}")]
    public DateTime? StartTime { get; set; }

    [Required]
    [DisplayName("End Time")]
    [DisplayFormat(ApplyFormatInEditMode = true, DataFormatString = "{0:M/d/yyyy h:mm tt}")]
    public DateTime? EndTime { get; set; }

    public string Venue { get; set; }

    public string Address { get; set; }

    public string City { get; set; }

    public string State { get; set; }

    public string Zip { get; set; }

    [AllowHtml]
    [DataType(DataType.MultilineText)]
    public string Description { get; set; }

    [DisplayName("Registration Link")]
    public string RegistrationUrl { get; set; }

    public virtual IList<Manufacturer> Manufacturers { get; set; }

    public virtual IList<EventResource> Files { get; set; }

    public virtual IList<EventResource> Links { get; set; }

    //public IEnumerable<EventResource> Resources
    //{
    //    get { return Files.Concat(Links); }
    //}

    public string StartDate
    {
        get { return StartTime.Value.ToShortDateString(); }
    }

    public string StartTimeOnly
    {
        get { return StartTime.Value.ToShortTimeString(); }
    }

    public string EndDate
    {
        get { return EndTime.Value.ToShortDateString(); }
    }

    public string EndTimeOnly
    {
        get { return EndTime.Value.ToShortTimeString(); }
    }

    public Event()
    {
        Manufacturers = new List<Manufacturer>();
        Files = new List<EventResource>();
        Links = new List<EventResource>();
    }
}

public class EventResource
{
    [Key, Column(Order = 0)]
    public int EventID { get; set; }

    [Key, Column(Order = 1)]
    public int ResourceTypeID { get; set; }

    [Key, Column(Order = 2)]
    public string Path { get; set; }

    public virtual Event Event { get; set; }

    public virtual ResourceType Type { get; set; }
}

更新

更多信息：添加到集合中...即使在循环之外也会抛出同样的错误。这给任何人一个想法吗？

Answer 1

您不能修改正在枚举的集合（例如，每个集合）。

您需要循环一次以获取要添加和/或删除的项目，然后在第一个循环之外的第二个循环中添加或删除所有项目。

例如：

Dim coll = New List(of String)({"1", "2", "3", "4", "6"})
dim coll2 = New List(of String)({"5", "8", "9", "2"})

Dim removeItems as new list(of String)()

For Each item in coll
 For Each item2 in coll2
  If item2 = item
    removeItems.Add(item)
  end if
 Next item2
Next item

' remove the items gathered
For each itemToRemove in removeItems
 coll.Remove(itemToRemove)
Next itemToRemove

可以用更好的方法来完成，但这说明了错误的要点。无法更改正在循环的集合。

Answer 2

在添加条目之前，可以使用LINQ过滤条目。

public void UpdateLinks(EventViewModel form)
{
    var selectedIds = form.Links.Select(r => r.ResourceTypeID).ToArray();
    var assignedIds = form.Event.Links.Select(r => r.ResourceTypeID).ToArray();
    foreach (var resource in form.Links
        .Where(r=> !assignedIds.Contain(r.ResourceTypeID)).ToArray())
    {
        resource.EventID = form.Event.ID;
        form.Event.Links.Add(resource);
    }
    foreach (var resource in form.Event.Links
        .Where(r=> !selectedIds.Contain(r.ResourceTypeID)).ToArray())
    {
        form.Event.Links.Remove(resource);
    }
}

在这两种方法中，我们都在枚举和添加之前过滤资源。您不能同时枚举和添加。

Answer 3

试试这个：

var lsEvents = form.Event.Links.ToList();
foreach (var resource in form.Links)
{
    resource.EventID = form.Event.ID;
    if (!assignedIds.Contains(resource.ResourceTypeID))
        lsEvents.Add(resource);
}
foreach (var resource in form.Event.Links)
{
    if (!selectedIds.Contains(resource.ResourceTypeID))
        lsEvents.Remove(resource);
}

并根据要求使用lsEvents。这将解决您的问题。