MVC 4和延迟加载:“Collection”被修改了删除多条记录时出错

本文关键字：删除 记录 出错 修改 延迟加载 Collection MVC | 更新日期: 2023-09-27 18:12:03

. NET MVC 4项目，我有一个模型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; }
}

在联赛模式中，我有:

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

在League控制器的Edit视图中，我有LeagueMembers的下拉菜单，在其编辑操作中，我试图删除现有的league成员并创建新成员:

[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);
}

执行时，foreach (var leagueMember in leagueMembers)行报错forloop的第二次迭代:

Collection被修改;枚举操作不能执行

在一对多关系中是否有更好的方法来删除批量记录?

在这种情况下，删除所有的leagueMemberViewModel.League.LeagueMembers 可能没有遍历集合?

类似，替换:

context.Entry(league).Collection("LeagueMembers").Load();
var leagueMembers = leagueMemberViewModel.League.LeagueMembers;
foreach (var leagueMember in leagueMembers)
{
    context.LeagueMembers.Remove(leagueMember);
}

: 

 leagueMember.League.LeagueMembers.Remove();

MVC 4和延迟加载:“Collection”被修改了删除多条记录时出错

我认为你的代码有一些错别字。例如，看起来var league = leagueMember.League应该是var league = leagueMemberViewModel.League。从字里行间看，我怀疑leagueMembers与附加的league对象的leagueMembers相同。(通过设置EntityState.Modified来附加)。

如果是，则错误解释为循环遍历已加载的集合并同时从中删除项。补救方法很简单:

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

将集合复制到不更改的本地数组。

至于你的第二个问题:没有，没有内置的方法来做批量删除在实体框架

不能在遍历集合时修改它，最好的方法是使用像这样的反向for循环:

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

这是一个常见的。net错误，您可以通过将foreach循环替换为for循环来解决此问题:

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