Question

我写了下面的sql查询

SELECT       *

FROM          dbo.PR_ADDRESS LEFT OUTER JOIN
              dbo.Book_MarkText ON dbo.PR_ADDRESS.GroupID = dbo.Book_MarkText.BMData
where 
        dbo.PR_ADDRESS.GroupID = dbo.Book_MarkText.BMData 

OR  
        PR_ADDRESS.GroupID in ('001','002','003')

到LINQ as

string[] srGroupIDs =  new string[] { "001", "002", "003" };
 var objResult =
      (from PR_ADDRESS in dtPR_ADDRESS.AsEnumerable()
       join
          Book_MarkText in dtBookmarkText.AsEnumerable() on
         (string)PR_ADDRESS["GroupID"] equals (string)Book_MarkText["BMData"] 
                                                               into Book_MarkText_join

          from Book_MarkText_Temp in Book_MarkText_join.DefaultIfEmpty()
          where srGroupIDs.Contains(PR_ADDRESS["Groupid"])
             || ((string)PR_ADDRESS["Groupid"] == (string)Book_MarkText_Temp["BMData"])

            select new
            {
                ID = PR_ADDRESS["ID"],
                Name1 = PR_ADDRESS["Name1"]
            }).ToList();

但是在行

上给我空引用异常

 where  srGroupIDs.Contains(PR_ADDRESS["Groupid"]) 
              ||  ((string)PR_ADDRESS ["Groupid"] == (string)Book_MarkText_Temp["BMData"])

为什么？

如果我删除.DefaultIfEmpty()，则无异常，但我需要'LEFT OUTER JOIN'，因此无法删除.DefaultIfEmpty()。我该如何解决？

修改： -

我也添加了OR案例，希望现在很清楚

Answer 1

通过处理Book_MarkText_Temp null解决Jon Skeet的评论。感谢您抽出时间来帮助我。

 string[] srGroupIDs =  new string[] { "001", "002", "003" };

var objResult =
        (from PR_ADDRESS in dtPR_ADDRESS.AsEnumerable() 
                join
             Book_MarkText in dtBookmarkText.AsEnumerable() on
            (string)PR_ADDRESS ["GroupID"] equals (string)Book_MarkText["BMData"] into Book_MarkText_join

    from Book_MarkText_Temp in Book_MarkText_join.DefaultIfEmpty()                      
    where Book_MarkText_Temp == null ? srGroupIDs.Contains(PR_ADDRESS["Groupid"]) :

    srGroupIDs.Contains(PR_ADDRESS["Groupid"])
    || ((string)PR_ADDRESS["Groupid"] == (string)Book_MarkText_Temp["BMData"])

    select new {
                  ID = PR_ADDRESS ["ID"],
                  Name1 = PR_ADDRESS ["Name1"]
                }).ToList();

使用DefaultIfEmpty（）时出现null引用异常

1 个答案: