Question

所以，我目前正在开发一个带有MSSQL数据库后端的小型客户端应用程序。

我一直在使用Linq to SQL，其中CompiledQuery。但是，从数据库中检索对象列表时，我收到一条InvalidCastException，其中包含以下消息：

Unable to cast object of type 'OneTimeEnumerable`1[DiallerBlacklistManager.Data.Entities.BlacklistDataContext.BlacklistEntry]' to type 'System.Collections.Generic.List`1

这是我第一次遇到这个错误，而且我对它的含义感到有些不知所措。

我的编译查询Func看起来像这样：

    public static Func<BlacklistDataContext, DateTime, DateTime, List<BlacklistEntry>>
        GetBlacklistEntriesByBlacklistDateRangeFunc =
            CompiledQuery.Compile<BlacklistDataContext, DateTime, DateTime, List<BlacklistEntry>>(
                (db, startTime, endTime) =>
                    (from BlacklistEntry e in db.BlacklistEntries
                        where e.BlacklistDate >= startTime && e.BlacklistDate <= endTime
                        select e).ToList());

调用它的方法：

    public List<BlacklistEntryDisplayItem> DoSearch(string searchText, DateTime startTime, DateTime endTime,
        bool includeDeleted)
    {
        List<BlacklistEntryDisplayItem> output = new List<BlacklistEntryDisplayItem>();
        if (!string.IsNullOrEmpty(searchText))
        {
            output.AddRange(GetBlacklistEntriesBySearchAndDateRange(searchText, startTime, endTime).Select(be => (BlacklistEntryDisplayItem) be));
            if (includeDeleted)
            {
                output.AddRange(
                    GetDeletedBlacklistEntriesBySearchAndDateRange(searchText, startTime, endTime)
                        .Select(be => (BlacklistEntryDisplayItem) be));
            }
        }
        else
        {
            output.AddRange(GetBlacklistEntriesByDateRange(startTime, endTime).Select(be => (BlacklistEntryDisplayItem)be));
            if (includeDeleted)
            {
                output.AddRange(
                    GetDeletedBlacklistEntriesByBlacklistDateRange(startTime, endTime)
                        .Select(be => (BlacklistEntryDisplayItem) be));
            }
        }
        return output;
    }

请注意，BlacklistEntryDisplayItem是一个用于在单一类型中同时显示BlacklistEntry和DeletedBlacklistEntry的类，用于整洁。（已指定删除的条目必须存储在单独的表中）

如果它被认为是相关的，BlacklistEntryDisplayItem类是：

public class BlacklistEntryDisplayItem
{
    public BlacklistEntryDisplayItem(int id, string phoneNumber, DateTime blacklistDate, string comments, int userId, bool deleted)
    {
        Id = id;
        PhoneNumber = phoneNumber;
        BlacklistDate = blacklistDate;
        Comments = comments;
        UserId = userId;
        Deleted = deleted;
    }

    public int Id { get; private set; }

    public string PhoneNumber { get; private set; }

    public DateTime BlacklistDate { get; private set; }

    public string Comments { get; set; }

    public int UserId { get; private set; }

    public bool Deleted { get; set; }

    public static implicit operator BlacklistEntryDisplayItem(BlacklistEntry entry)
    {
        return new BlacklistEntryDisplayItem(
            entry.Id, 
            entry.PhoneNumber, 
            entry.BlacklistDate, 
            entry.Comments, 
            entry.UserId, 
            false);
    }

    public static implicit operator BlacklistEntryDisplayItem(DeletedBlacklistEntry entry)
    {
        return new BlacklistEntryDisplayItem(
            entry.Id,
            entry.PhoneNumber,
            entry.BlacklistDate,
            entry.Comments,
            entry.UserId,
            true);
    }
}

是否有人能够告知OneTimeEnumerable是什么，以及为什么结果集不是Func类型参数中定义的类型？

提前致谢。

Answer 1

之前发生在我身上。问题的原因是编译的查询块中的ToList。编译的查询显然只返回OneTimeEnumerable<T>而不是List<T>。删除ToList并让编译的查询返回IEnumerable<BlacklistEntry>，然后在调用编译的查询后调用ToList。

public static Func<BlacklistDataContext, DateTime, DateTime, IEnumerable<BlacklistEntry>>
    GetBlacklistEntriesByBlacklistDateRangeFunc =
        CompiledQuery.Compile<BlacklistDataContext, DateTime, DateTime, List<BlacklistEntry>>(
            (db, startTime, endTime) =>
                from BlacklistEntry e in db.BlacklistEntries
                where e.BlacklistDate >= startTime && e.BlacklistDate <= endTime
                select e);

然后像这样使用它：

GetBlacklistEntriesByBlacklistDateRangeFunc(dataContext, startTime, endTime).ToList()

无法将OneTimeEnumerable转换为System.Collections.Generic.List

1 个答案: