c# - LinQ 获取具有平均数字误差的不同字段

Question

我遇到了“预期类型”错误，我不知道为什么。

我的查询只是将 3 个表链接在一起，同时尝试获取不同的包和平均评分数。

结果应该是这样的

| PackageName | Average Rating |
|    SG       |        4       |
|    USA      |        4       |    

<小时/>

        IQueryable<Recommendation> recommendationQuery = db.Recommendations;
        IQueryable<Booking> bookingQuery = db.Bookings;
        IQueryable<Package> packageQuery = db.Packages;

        recommendationQuery =  (from recommendationItem in recommendationQuery
                               join bookingItem in bookingQuery
                               on recommendationItem.BookingId equals bookingItem.BookingId
                               join packageItem in packageQuery
                               on recommendationItem.Booking.PackageId equals packageItem.PackageId
                               select recommendationItem).GroupBy(c => c.Booking.Package.PackageTitle)
                               .Select(c => new ( c.Key, c.Average(d=>d.Rating)));

预期的类型出现在 .Select(c => new (.....

请问我是否查询错误？

因为

1)我内部将所有 3 个表连接在一起

2)假设我已连接所有表，我尝试按 PackageName 将它们分组，以将名称区分为一个名称

3)我尝试选择同一包的评分的平均总和。

知道是否有更好的解决方案吗？

database class diagram

Solution error

Answer 1

您需要将结果存储回新变量以匹配您的新类型:

var results = from recommendationItem in recommendationQuery
                join bookingItem in bookingQuery
                on recommendationItem.BookingId equals bookingItem.BookingId
                join packageItem in packageQuery
                on recommendationItem.Booking.PackageId equals packageItem.PackageId
                group recommendationItem
                by recommendationItem.Booking.Package.PackageTitle
                into grp
                select new
                {
                    PackageName = grp.Key,
                    AverageRating = grp.Average(d => d.Rating)
                };