修改了 EF6 集合；枚举操作可能无法执行

Question

我需要将 adin 用户添加到他们不存在的表（和子表）中。

我有以下代码引发错误：“集合已修改；枚举操作可能无法执行。”在“_userPrefsContext.UserRecs.Add(userRec);”行上

var allUserRecs = _userPrefsContext.UserRecs.ToList();

        foreach (string adminUser in adminUsers.Where(x => x.Length > 0))
        {
            domain = adminUser.Split('\\')[0];
            login = adminUser.Split('\\')[1];

            var userRec = new Domain.UserPrefs.BamUser()
            {
                AdLogonDomain = domain,
                AdLogonId = login,
                UserViews = colUserView
            };

            if (allUserRecs.Where(x => x.AdLogonDomain == domain
                                    && x.AdLogonId == login).Count() == 0)
            {
                //don't currently have a user rec for this login so create one
                _userPrefsContext.UserRecs.Add(userRec);
            }
        }
        _userPrefsContext.SaveChanges();

我在 SO 上看到过类似的帖子，但在每种情况下，他们似乎都在迭代他们正在添加的集合。就我而言，我只是想遍历一个字符串列表。

我哪里做错了？

现在编辑成如下（还是一样的错误）：

            List<string> adminUsers = WebConfigurationManager.AppSettings["AdminUsers"].Split(';').ToList();
        adminUsers = adminUsers.Where(x => x.Length > 0).ToList();

        string domain = "", login = "";
        string userViewString = Helpers.ViewNames.UserView.ToString();

        int viewId = _userPrefsContext.ViewRecs.Where(x => x.Name == userViewString).Select(x => x.BamViewId).Single();

        Domain.UserPrefs.UserView userView = new Domain.UserPrefs.UserView()
        {
            BamViewId = viewId
        };

        List<Domain.UserPrefs.UserView> colUserView = new List<Domain.UserPrefs.UserView>();
        colUserView.Add(userView);

        var allUserRecs = _userPrefsContext.UserRecs.ToList();
        string[] users = new string[adminUsers.Count()];
        int y = 0;
        foreach(string user in adminUsers)
        {
            users[y] = user;
            y++;
        }

        int total = adminUsers.Count;
        for (var i = 0; i < total; i++)
        {

            domain = users[i].Split('\\')[0];
            login = users[i].Split('\\')[1];

            if (allUserRecs.Where(x => x.AdLogonDomain == domain
                                    && x.AdLogonId == login).Count() == 0)
            {
                //don't currently have a user rec for this login so create one
                _userPrefsContext.UserRecs.Add(new Domain.UserPrefs.BamUser()
                {
                    AdLogonDomain = domain,
                    AdLogonId = login,
                    UserViews = colUserView
                });
            }
        }
        _userPrefsContext.SaveChanges();

Answer 1

UserViews 定义为域实体如下：

public virtual ICollection<UserView> UserViews { get; set; }

为了完成这项工作，我改变了

var userRec = new Domain.UserPrefs.BamUser()
            {
                AdLogonDomain = domain,
                AdLogonId = login,
                UserViews = colUserView
            };

到var userRec = new Domain.UserPrefs.BamUser() { AdLogonDomain = domain, AdLogonId = login, UserViews = colUserView.ToList() };

说实话，我不知道为什么需要 .ToList() - 如果有人这样做，那么我认为解释可能对许多人有帮助！

Answer 2

仅供参考 最好使用 any 替换 count。 any 工作速度比计数快得多。 使用此代码

 if (!allUserRecs.any(x => x.AdLogonDomain == domain                                   && x.AdLogonId == login))
            {
                //don't currently have a user rec for this login so create one
                _userPrefsContext.UserRecs.Add(new Domain.UserPrefs.BamUser()
                {
                    AdLogonDomain = domain,
                    AdLogonId = login,
                    UserViews = colUserView
                });
            }

替换

if (allUserRecs.Where(x => x.AdLogonDomain == domain
                                    && x.AdLogonId == login).Count() == 0)
            {
                //don't currently have a user rec for this login so create one
                _userPrefsContext.UserRecs.Add(new Domain.UserPrefs.BamUser()
                {
                    AdLogonDomain = domain,
                    AdLogonId = login,
                    UserViews = colUserView
                });
            }

Answer 3

问题是您有一个列表，然后您在 foreach 中运行 where 子句。这意味着它必须在每次运行时重新查询列表，但如果您在运行期间修改了列表，现在查询可能是错误的。

这将是一个更好的解决方案：

var allUserRecs = _userPrefsContext.UserRecs.Where(x => x.Length > 0).ToList();

    foreach (string adminUser in adminUsers)
    {
        domain = adminUser.Split('\\')[0];
        login = adminUser.Split('\\')[1];

        var userRec = new Domain.UserPrefs.BamUser()
        {
            AdLogonDomain = domain,
            AdLogonId = login,
            UserViews = colUserView
        };

        if (allUserRecs.Where(x => x.AdLogonDomain == domain
                                && x.AdLogonId == login).Count() == 0)
        {
            //don't currently have a user rec for this login so create one
            _userPrefsContext.UserRecs.Add(userRec);
        }
    }
    _userPrefsContext.SaveChanges();