MVC 4和延迟加载：删除多个记录时出现“集合已修改”错误

Question

In ASP.NET MVC 4 project, I have a model LeagueMember :

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

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

    public bool? IsActive { get; set; }

    public virtual League League { get; set; }

    public virtual Member Member { get; set; }

}

In League model, I have:

public virtual ICollection<LeagueMember> LeagueMembers { get; set; }

In the Edit view of League controller, I have a dropdown for LeagueMembers and in its Edit action I am trying to delete existing LeagueMembers and creating new ones:

[HttpPost]
public ActionResult Edit(LeagueMembersViewModel leagueMemberViewModel)
{
    if (ModelState.IsValid)
    {
        var league = leagueMemberViewModel.League;
        context.Entry(league).State = EntityState.Modified;

        /* without this line the next foreach statement would complain that 
           League.LeagueMembers is null due to lazy loading..*/
        context.Entry(league).Collection("LeagueMembers").Load();

        var leagueMembers = leagueMemberViewModel.League.LeagueMembers;

        foreach (var leagueMember in leagueMembers)
        {
            context.LeagueMembers.Remove(leagueMember);
        }

        foreach (var memberId in leagueMemberViewModel.SelectedMembers)
        {
            var leagueMember = new LeagueMember { MemberId = memberId, League = league };
            context.LeagueMembers.Add(leagueMember);
        }
        context.SaveChanges();

        return RedirectToAction("Index");
   }

    leagueMemberViewModel.MemberList = new MultiSelectList(context.Members, "Id", "Name", leagueMemberViewModel.SelectedMembers);
    return View(leagueMemberViewModel);
}

When executed, the line foreach (var leagueMember in leagueMembers) complains on second iteration of forloop:

Collection was modified; enumeration operation may not execute

Is there a better way to delete bulk records in a one-to-many relationship?

In this case, to Remove All leagueMemberViewModel.League.LeagueMembers perhaps without iterating through the collection?

Something like, replacing:

context.Entry(league).Collection("LeagueMembers").Load();

var leagueMembers = leagueMemberViewModel.League.LeagueMembers;
foreach (var leagueMember in leagueMembers)
{
    context.LeagueMembers.Remove(leagueMember);
}

with:

 leagueMember.League.LeagueMembers.Remove();

Answer 1

I think your code has some typos. For example, it looks like var league = leagueMember.League should be var league = leagueMemberViewModel.League . Reading between the lines I suspect that leagueMembers is the same as the leagueMembers of the attached league object. (That you attach by setting EntityState.Modified ).

If so, the error is explained by the fact that you loop through a loaded collection and at the same time remove items from it. The remedy is simple:

foreach (var leagueMember in leagueMembers.ToArray()) // <= ToArray

which copies the collection to a local array that does not change.

As for your second question: no there is no built-in way to do bulk deletes in Entity Framework.

Answer 2

You cannot modify a collection while iterating through it, the best approach is to use a reverse for loop like this:

        for (int i = leagueMembers.Count - 1; i >= 0; i--)
        {
            context.LeagueMembers.Remove(leagueMembers[i]);
        }

Answer 3

This is a common .NET error, you can get around this by replacing your foreach loop with a for loop:

for (var i = 0; i < leagueMembers.Count; i++)
{
    context.LeagueMembers.Remove(leagueMembers[i]);
}