LINQ查询InvalidCastException [英] LINQ Query InvalidCastException

问题描述

有人可以告诉我为什么我的LINQ查询功能失败了吗？

Can someone tell me why my LINQ query function is failing?

Public Function PC_Archived(ByVal strPC As String, _
                                ByVal dtName As DataTable) _
                                As Long
        'Purpose:       Check as see if this PC is registered  
        '               for archiving
        'Parameters:    strPC - PC's Name we are looking for
        '               dtName - The DataTable where the PC 
        '                        names are stored
        'Returns:
        Dim lngPCID As Long
        Debug.Print("Table: " & dtName.TableName.ToString & vbCrLf & _
                    vbTab & "Numder of Rows: " & dtName.Rows.Count.ToString)
        Dim Query = _
            From PC In dtName.AsEnumerable() _
            Where PC.Field(Of String)("PC_Name") = strPC _
                Select New With _
                    { _
                        .PCID = PC.Field(Of Long)("PCID"), _
                        .PCName = PC.Field(Of String)("PC_Name") _
                    }
        For Each PC In Query
            Debug.Print("PCID: " & PC.PCID)
            Debug.Print("PC Name: " & PC.PCName)
            lngPCID = PC.PCID
        Next
        Return lngPCID
    End Function

当它执行For Each语句时，我得到InvalidCastException is Unhandled。错误。



谢谢，

When it executes the For Each statement I get "InvalidCastException was Unhandled." error.

Thanks,

推荐答案

此查询返回一个匿名对象 -

This query returns an anonymous object -

Dim Query = _
          From PC In dtName.AsEnumerable() _
          Where PC.Field(Of String)("PC_Name") = strPC _
              Select New With _
                  { _
                      .PCID = PC.Field(Of Long)("PCID"), _
                      .PCName = PC.Field(Of String)("PC_Name") _
                  }

因此你不能直接将它转换为 PC in For Each PC In Query

Thus you cannot directly convert it to PC in For Each PC In Query

For Each PC In Query
      Debug.Print("PCID: " & PC.PCID)
      Debug.Print("PC Name: " & PC.PCName)
      lngPCID = PC.PCID
  Next

想出来。

Figured it out.

Dim Query = (From PC In dtName _
             Where PC.Field(Of String)("PC_Name") = strPC _
             Select PC.Field(Of Int32)("PCID")).FirstOrDefault()

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