Question

我在将LINQ语句输出到Visual Studio 2010中编写的ASP.NET列表时遇到问题。下面的函数尝试在其数据源中搜索相关的DVD并输出一条记录：

 Public Function GetDVD(ByVal id As Integer) As DVDRaritiesDVDs _
    Implements IDVDDAO.GetDVD
    Dim DVD = From DVDList In _context.DVDRaritiesDVDs _
              Where DVDList.DVDID = id Select DVDList
    Return DVD.ToList().First()
End Function

但是当我尝试加入它时：

 Public Function GetDVD(ByVal id As Integer) As DVDRaritiesDVDs _
    Implements IDVDDAO.GetDVD
    Dim DVD = From DVDList In _context.DVDRaritiesDVDs _
              Join Category In _context.DVDRaritiesCategories _
              On Category.CategoryID Equals DVDList.CategoryID _
              Where DVDList.DVDID = id Select DVDList, Category
    Return DVD.ToList().First()
End Function

在构建代码时，它给出了以下错误：

错误1类型'（第23行）'的值无法转换为'DVDRarities.Data.DVDRaritiesDVDs'。 C：\ Users \ a9010799 \ Projects \ DVDRarities \ DVDRarities.Data \ DAO \ DVDDAO.vb 24 16 DVDRarities.Data

请有人指出我出错的地方，所以我可以在一个声明中做到这一点吗？

Answer 1

在您的查询中，您将针对DVDList实例返回{DVDList，Category}。你应该使用

Public Function GetDVD(ByVal id As Integer) As DVDRaritiesDVDs _
    Implements IDVDDAO.GetDVD
    Dim DVD = From DVDList In _context.DVDRaritiesDVDs _
              Join Category In _context.DVDRaritiesCategories _
              On Category.CategoryID Equals DVDList.CategoryID _
              Where DVDList.DVDID = id Select DVDList
    Return DVD.First()
End Function

或者您应该更改此方法的签名。

Public Function GetDVD(ByVal id As Integer) As Tuple(Of DVDRaritiesDVDs, DVDRaritiesCategories) _
        Implements IDVDDAO.GetDVD
        Dim DVD = From DVDList In _context.DVDRaritiesDVDs _
                  Join Category In _context.DVDRaritiesCategories _
                  On Category.CategoryID Equals DVDList.CategoryID _
                  Where DVDList.DVDID = id Select  New Tuple(Of DVDRaritiesDVDs, DVDRaritiesCategories)(DVDList, Category)
        Return DVD.First()
    End Function

Answer 2

您的函数返回DVDRaritiesDVDs对象。显然你的Linq查询返回一个包含对的列表（DVDRaritiesDVDs，DVDRaritiesCategories）

您应该按以下方式更新代码：

 Public Class Pair
     Public First as DVDRaritiesDVDs
     Public Second as DVDRaritiesCategories
 End Class

 Public Function GetDVD(ByVal id As Integer) As DVDRaritiesDVDs _
    Implements IDVDDAO.GetDVD
    Dim DVD = (From DVDList In _context.DVDRaritiesDVDs _
              Join Category In _context.DVDRaritiesCategories _
              On Category.CategoryID Equals DVDList.CategoryID _
              Where DVDList.DVDID = id
              Select New Pair() With {First = DVDList, Second = Category})
    Return DVD.ToList().First()
End Function

当然，你可以使用泛型来概括你的代码，但我决定用最简单易懂的解决方案来帮助你。

输出具有连接的LINQ查询

2 个答案: