Question

我编写了一个可以正常工作的SQL查询，但是在转换为LINQ时遇到了一些麻烦。这是SQL：

SELECT        
  CourseID, 
  CourseName, 
  CreditHours, 
  CPTRequired, 
  COTRequired, 
  CPTElective, 
  COTElective
FROM            
  Courses
WHERE        
  (CPTRequired = 'true') 
  AND 
  (CourseID NOT IN
    (SELECT        
      Courses_1.CourseID
    FROM
      Courses AS Courses_1 INNER JOIN
      Sections ON Sections.CourseID = Courses_1.CourseID INNER JOIN
      Enrollment ON Enrollment.SectionID = Sections.SectionID INNER JOIN
      Students ON Students.StudentID = Enrollment.StudentID
    WHERE        
      (Students.StudentID = '11110004')))

这是我到目前为止用LINQ编写的内容：

Dim maj = (From c In connect.Courses _
           Where c.CPTRequired = "True" _
           Select c.CourseID, c.CourseName, c.CreditHours).Except _
          (From en In connect.Enrollments _
           Join s In connect.Sections On en.SectionID Equals s.SectionID _
           Join cs In connect.Courses On s.CourseID Equals cs.CourseID _
           Join st In connect.Students On en.StudentID Equals st.StudentID _
           Order By cs.CourseName _
           Where st.StudentID = StudentID _
           Select cs.CourseID)

执行LINQ语句时会抛出以下错误。

System.InvalidCastException是未处理的消息=“无法投射对象类型 'System.Data.Linq.DataQuery'1 [System.String]' 输入 “System.Collections.Generic.IEnumerable'1 [VB $ AnonymousType_6'3 [System.String，System.String，System.Nullable`1 [System.Int32]]]”“。来源=“最终项目”

我错过了什么？我是LINQ的新手，所以请温柔!!

Answer 1

我不能肯定地说因为VB语法让我失望了，但我相信这是因为在你之前的语句中你选择了一个新的匿名对象（具有多个属性）并且在查询中被传递除非您只选择一列。因此无法正确比较类型。

有效地，您似乎正在尝试比较：

c.CourseID, c.CourseName, c.CreditHours

与

c.CourseID

并且LINQ不知道如何正确地执行此操作..如果您展开辅助选项，它可能会有效，请选择包括c.CourseName和c.CreditHours

成功：

Dim maj = (From c In connect.Courses _
           Where c.CPTRequired = "True" _
           Select c.CourseID, c.CourseName, c.CreditHours).Except _
          (From en In connect.Enrollments _
           Join s In connect.Sections On en.SectionID Equals s.SectionID _
           Join cs In connect.Courses On s.CourseID Equals cs.CourseID _
           Join st In connect.Students On en.StudentID Equals st.StudentID _
           Order By cs.CourseName _
           Where st.StudentID = StudentID _
           Select cs.CourseID, cs.CourseName, cs.CreditHours)

但是我再也不熟悉VB语法，所以我能说的最好是试一试。

编辑：将上传日志记录代码从评论移至答案..

Dim sb As New StringBuilder
Dim sw As New StringWriter(sb)
connect.Log = sw
...Do Query & Enumerate Results...
sb.ToString() 'will contain the generated SQL

Answer 2

我认为除了工作之外，你的选择中的类型不能有所不同：

选择c.CourseID，c.CourseName，c.CreditHours VS

选择cs.CourseID

我认为LINQ会比较整个选定的结果并进行比较。比较这些不同的结果应该在编译时失败。 ???

Answer 3

我认为您尝试调用Except on的查询尚未运行。尝试添加一个.toList（），或者在其结尾处添加一些IEnumerable的转换：

Dim maj =（来自c in connect.Courses _ 其中c.CPTRequired =“True”_ 选择c.CourseID，c.CourseName，c.CreditHours）

然后在结果上调用Except。

Answer 4

怎么样

Dim maj = (From c In connect.Courses _
       Where c.CPTRequired = "True" &&_
       !(From en In connect.Enrollments _
       Join s In connect.Sections On en.SectionID Equals s.SectionID _
       Join cs In connect.Courses On s.CourseID Equals cs.CourseID _
       Join st In connect.Students On en.StudentID Equals st.StudentID _
       Order By cs.CourseName _
       Where st.StudentID = StudentID _
       Select cs.CourseID).Contains(c.CourseID) _
       Select c.CourseID, c.CourseName, c.CreditHours)

换句话说......

从y在哪里！（来自b中的a选择 a.foo）.Contains（x.foo）select x

Linq to Sql：在我的查询中帮助InvalidCastException

4 个答案: