使用Linq with lists需要帮助

Question

我正在创建一个打开SQL阅读器并加载"Strapping Chart“的例程(这将查找与特定容器体积相关的预定义数字)。在表单加载时，我想填充列表，这些数字将不会改变。(VolCnt，音量)。我想根据卷计数来检索相关的卷。

这就是我所拥有的：

Public HltVolVals As List(Of HLTVolValChart)

Public Class HLTVolValChart
    Public Property VCnt As Double
    Public Property Volume As Double
    Public Sub New(ByVal n_Vcnt As Double, ByVal n_Volume As Double)
        Vcnt = n_Vcnt
        Volume = n_Volume
    End Sub
End Class


Public Sub PopulateHLTVolVals()
    Dim sqlstr As String = "SELECT VCnt, Volume FROM Vol_Strap WHERE Vessel = 'HLT' ORDER BY Volume"
    Dim CN As New SqlConnection
    Dim Reader As SqlClient.SqlDataReader
    Dim SQL As New SqlCommand

    CN.ConnectionString = Connectionstr
    CN.Open()
    SQL.Connection = CN
    SQL.CommandText = sqlstr
    Reader = SQL.ExecuteReader
    While Reader.Read
        Dim Vol As Double = CDbl(Reader.GetValue(1))
        Dim VCnt As Double = CDbl(Reader.GetValue(0))
        Dim NewHLTVolValChartItem As List(Of HLTVolValChart) = New List(Of HLTVolValChart)
        NewHLTVolValChartItem.Add(New HLTVolValChart(VCnt, Vol))
    End While
    Reader.Close()

    CN.Close()
    CN.Dispose()

End Sub

Public Function ListHLTVals(ByVal CurCnt As Double) As Double

    Dim QVol = From _HLTVol In HltVolVals
           Where _HLTVol.VCnt >= CurCnt
           Select _HLTVol.Volume Take 1

    Return QVol.First
End Function

我遇到的问题(我认为)是，当我遍历记录时，我没有在HLTVolValChart中创建多个值。然而，我不确定实现这一点的正确方法。任何帮助都将不胜感激。

Answer 1

将NewHLTVolValChartItem设置为类变量。在您的循环中，不要重新初始化NewHLTVolValChartItem变量。每次迭代都会生成一个新列表--只需删除该部分即可。

Private NewHLTVolValChartItem As New List(Of HLTVolValChart)

Public Sub PopulateHLTVolVals()
 Dim sqlstr As String = "SELECT VCnt, Volume FROM Vol_Strap WHERE Vessel = 'HLT' ORDER BY Volume"
 Dim CN As New SqlConnection
 Dim Reader As SqlClient.SqlDataReader
 Dim SQL As New SqlCommand

 CN.ConnectionString = Connectionstr
 CN.Open()
 SQL.Connection = CN
 SQL.CommandText = sqlstr
 Reader = SQL.ExecuteReader
 While Reader.Read
   Dim Vol As Double = CDbl(Reader.GetValue(1))
   Dim VCnt As Double = CDbl(Reader.GetValue(0))
   NewHLTVolValChartItem.Add(New HLTVolValChart(VCnt, Vol))
 End While
 Reader.Close()

 CN.Close()
 CN.Dispose()

End Sub