Linq 外连接，两边都有默认值

Question

我有一个外部联接的问题。我发现这篇非常有用的 stackoverflow 文章： linq-full-outer-join 但是当检索到错误的数据时，我的加入出现问题：

NotSupportedException: Unsupported overload used for query operator 'DefaultIfEmpty'.

代码如下：

var query =
        from i2 in (from sel in (from i in (from a1 in Activities
                    where a1.ActivityDate >= DateTime.Parse("4/15/2011")
                    && a1.ActivityDate < DateTime.Parse("4/19/2011")
                    select new { 
                        a1.ActivityDate,
                        a1.RecID} )
        group i by new { i.RecID }
        into g
        select new {
            g.Key.RecID,
            MaxDate = g.Max(i => i.ActivityDate)})
        join a in Activities on 1 equals 1 
        where (sel.MaxDate == a.ActivityDate && sel.RecID == a.RecID) 
        select new { 
            a.RecID,  
            a.UserName, 
            ActivityDate = a.ActivityDate.Date, 
            Sum1 = (a.StatusID == 7) ? 1 : 0,
            Sum2 = (a.StatusID == 5) ? 1 : 0,
            Sum3 = (a.StatusID == 4) ? 1 : 0})
        group i2 by new {
                UserName = i2.UserName,
                ActivityDate = i2.ActivityDate
                }
        into g2 
        select new {
            JoinId = (string)(g2.Key.ActivityDate + "_" + g2.Key.UserName),
            ActivityDate = (DateTime)g2.Key.ActivityDate,
            UserName = (string)g2.Key.UserName,
            Sum1 = (int)g2.Sum(i2 => i2.Sum1),
            Sum2 = (int)g2.Sum(i2 => i2.Sum2),
            Sum3 = (int)g2.Sum(i2 => i2.Sum3),
        };
    var query2 = from s in ProdHistories
    where s.CompletedDate >= DateTime.Parse("4/15/2011") 
    && s.CompletedDate <= DateTime.Parse("4/19/2011")
    select new { 
        JoinId = (string)(s.CompletedDate + "_" + s.UserName),
        CompletedDate = (DateTime)s.CompletedDate,
        UserName = (string)s.UserName,
        Type1 = (int)s.Type1,
    Type2 = (int)s.Type2,
            Type3 = (int)s.Type3,
            Type4 = (int)s.Type4,
            Type5 = (int)s.Type5,
            Type6 = (int)s.Type6,
            Type7 = (int)s.Type7,
            Type8 = (int)s.Type8,
            };

    var joinLeft = from ph in query2
        join act in query 
        on ph.JoinId equals act.JoinId
        into temp 
        from act in temp.DefaultIfEmpty(new { 
            JoinId = (string)ph.JoinId,
            ActivityDate = (DateTime)ph.CompletedDate,
            UserName = (string)ph.UserName, 
            Sum1 = default(int),
            Sum2 = default(int),
            Sum3 = default(int),
             })
        select new { ph.UserName, 
            ph.CompletedDate,
            ph.Type1,
            ph.Type2,
            ph.Type3,
            ph.Type4,
            ph.Type5,
            ph.Type6,
            ph.Type7,
            ph.Type8,
            act.Sum1,
            act.Sum2,
            act.Sum3};

    query.Dump(); // successfully dumps (in LinqPad) data - no nulls
    query2.Dump(); // successfully dumps (in LinqPad) data - no nulls
    joinLeft.Dump(); // raises: NotSupportedException: Unsupported overload used for query operator 'DefaultIfEmpty'.

我尝试了相同的外连接子句并使用固定数据并且它有效：

 var query = new[]
 {
      new {     JoinId = "12345", ActivityDate = DateTime.Parse("1/1/2011"), UserName = "UID1", Sum1 = 10, Sum2 = 11, Sum3 = 12 },
      new {     JoinId = "23456", ActivityDate = DateTime.Parse("1/2/2011"), UserName = "UID2", Sum1 = 20, Sum2 = 21, Sum3 = 22 },
      new {     JoinId = "34567", ActivityDate = DateTime.Parse("1/3/2011"), UserName = "UID3", Sum1 = 30, Sum2 = 31, Sum3 = 32 },
 };

 var query2 = new[]
 {
      new {     JoinId = "12345", CompletedDate = DateTime.Parse("1/1/2011"), UserName = "UID1", Type1 = 110, Type2 = 111, Type3 = 112, Type4 = 113, Type5 = 114, Type6 = 115, Type7 = 116, Type8 = 117 },
      new {     JoinId = "23456", CompletedDate = DateTime.Parse("1/2/2011"), UserName = "UID2", Type1 = 210, Type2 = 211, Type3 = 212, Type4 = 213, Type5 = 214, Type6 = 215, Type7 = 216, Type8 = 217 },
      new {     JoinId = "45678", CompletedDate = DateTime.Parse("1/5/2011"), UserName = "UID4", Type1 = 310, Type2 = 311, Type3 = 312, Type4 = 313, Type5 = 314, Type6 = 315, Type7 = 316, Type8 = 317 },
 };

 var joinLeft = from ph in query2
           join act in query 
           on ph.JoinId equals act.JoinId
           into temp 
           from act in temp.DefaultIfEmpty(new { 
                JoinId = (string)ph.JoinId,
                ActivityDate = (DateTime)ph.CompletedDate,
                UserName = (string)ph.UserName, 
                Sum1 = default(int),
                Sum2 = default(int),
                Sum3 = default(int),
                 })
           select new { ph.UserName, 
                ph.CompletedDate,
                ph.Type1,
                ph.Type2,
                ph.Type3,
                ph.Type4,
                ph.Type5,
                ph.Type6,
                ph.Type7,
                ph.Type8,
                act.Sum1,
                act.Sum2,
                act.Sum3};

 joinLeft.Dump();

结果：

UserName CompletedDate    Type1 Type2 Type3 Type4 Type5 Type6 Type7 Type8 Sum1 Sum2 Sum3 
UID1 1/1/2011 12:00:00 AM 110   111   112   113   114   115   116   117   10   11   12
UID2 1/2/2011 12:00:00 AM 210   211   212   213   214   215   216   217   20   21   22
UID4 1/5/2011 12:00:00 AM 310   311   312   313   314   315   316   317   0    0    0

我看到another stackoverflow article Jon Skeet 在另一个上下文中使用 IEnumerable 作为解决此错误的一部分，但我不太确定如何应用它。

感谢您提供任何线索！

Answer 1

我认为post you took the code from 也可以回答您的问题（粗体字是我的）。

因为它是在 LINQ to Objects 中，所以它按书面形式工作。 如果 LINQ to SQL 或其他，采用默认值的DefaultIfEmpty() 的重载可能 不行。然后你必须使用条件运算符 有条件地获取值。

即，

var leftOuterJoin = from first in firstNames
                    join last in lastNames
                    on first.ID equals last.ID
                    into temp
                    from last in temp.DefaultIfEmpty()
                    select new
                    {
                        first.ID,
                        FirstName = first.Name,
                        LastName = last != null ? last.Name : default(string),
                    };

来自 Robb 的更新

这就是答案，我可以通过使用 ToList() 将 IQueryable 转换为 List 来使我的代码正常工作。

我对更新的评论

如果您不希望 joins 在数据库上运行并且您对在对象上执行它们感到满意，那么这绝对是另一种选择。然而，如果有很多对象（有多少——这取决于），这可能会影响性能。遵循通常的优化口头禅：测量、测量和测量。如果这无关紧要，ToList 可以正常工作。