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Question

Getting the "Collection was modified" exception when attempting to add to a collection

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);
    }
}

The problem is specifically with the "Add" method. If I comment that part out, no exception is thrown. It's important to note that I've already tried re-writing the foreach as a for loop and adding "ToList()" to form.Links. The same exception is thrown in all cases. I use this exact pattern on other parts of the site without issue which is why this is so frustrating. This also works on "Create". The problem only affects the "Edit" action.

Other relevant code:

[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; }
}

UPDATE

Some more info: Adding to the collection at all... even outside of a loop throws the same error. Does that give anyone an idea?

Answer 1

Try this instead:

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);
}

and Use lsEvents according to the requirement. This will fix your issue.

Answer 2

You can use LINQ to filter out entries before adding them.

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);
    }
}

In both methods, we are filtering resources before enumerating and adding. You can not enumerate and add at the same time.

Answer 3

You can't modify a collection you are enumerating (eg, for each).

You need to loop once to get the items you want to add and/or remove, then add or remove all of them in a second loop outside the first.

Eg:

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

It can be done a better way, but this shows the gist of the error. You can't change the collection you are looping over.

Answer 4

Not sure exactly what was responsible for solving the issue, but after upgrading to Visual Studio 2013 Express (from 2010 Professional), and installing ASP.Net 4.5/IIS8 with it, even though this application continues to target ASP.Net 4.0, I am no longer experiencing the issue and the code used in the original post works as is.

Perhaps this was caused by a certain build of the ASP.Net framework or an older version of IIS?