添加到集合时修改了集合

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);
    }
}

问题特别在于"Add"方法。如果我把那个部分注释掉，就不会抛出任何异常。需要注意的是，我已经尝试过将foreach重写为for循环，并将"ToList（）"添加到form.Links中。在所有情况下都会抛出相同的异常。我在网站的其他部分使用这个确切的模式，没有问题，这就是为什么这是如此令人沮丧。这也适用于"创建"。该问题只影响"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：正在添加到集合中。。。即使在循环之外也会抛出相同的错误。这能给任何人一个想法吗？