Question

我正在尝试比较两个数据表，我正在两个相同的数据表上使用DataTable.Select进行一些测试：

Using DT_NewData As DataTable = DT_DBData.Copy
    For x As Short = 0 To DT_NewData.Rows.Count - 1
        Dim SelRows As DataRow() = DT_DBData.Select( _
            "Type='" & DT_NewData.Rows(x)("Type") & "'" & _
            " AND In_Date='" & DT_NewData.Rows(x)("In_Date") & "'" & _
            " AND Out_Date='" & DT_NewData.Rows(x)("Out_Date") & "'")
    Next

但SelRows.Length总是0.我的代码出了什么问题？

Answer 1

虽然表的副本有点奇怪，但试试这个：

假设Type为String

Imports Microsoft.VisualBasic
Imports System.Linq

Module StartupModule

Sub Main()
    ' Declare the table.
    Dim originalDataTable As New DataTable

    ' Declare the table columns.
    With originalDataTable.Columns
        .Add("Type", GetType(String))
        .Add("InDate", GetType(DateTime))
        .Add("OutDate", GetType(DateTime))
    End With

    ' Delegate to add rows.
    Dim addRow As Action(Of String, DateTime?, DateTime?) = Sub(text, inDate, outDate)
    Dim newRow As DataRow = originalDataTable.NewRow()

    With newRow
        If (Not String.IsNullOrEmpty(text)) Then
            .SetField(Of String)("Type", text)
        End If

        If (inDate.HasValue) Then
            .SetField(Of DateTime?)("InDate", inDate.Value)
        End If

        If (outDate.HasValue) Then
            .SetField(Of DateTime?)("OutDate", outDate.Value)
        End If
    End With

    originalDataTable.Rows.Add (newRow)
End Sub

' Adding rows to the table.
addRow("type1", #2/2/2017#, Nothing)
addRow(Nothing, #1/25/2016#, Nothing)
addRow(Nothing, Nothing, #1/30/2016#)

' Copy the table
Dim copiedDataTable As DataTable = originalDataTable.Copy

' Loop through copied table rows.
For i As Integer = 0 To copiedDataTable.Rows.Count - 1
    Dim type As String = copiedDataTable.Rows(i).Field(Of String)("Type")
    Dim inDate As DateTime? = copiedDataTable.Rows(i).Field(Of DateTime?)("InDate")
    Dim outDate As DateTime? = copiedDataTable.Rows(i).Field(Of DateTime?)("OutDate")

    ' Using DataTable Select.
    Dim filter As String = String.Format("{0} {1} {2}",
        If(type Is Nothing, "( Type Is Null )", String.Format("( Type = '{0}' )", type)),
        If(Not inDate.HasValue, "And ( InDate Is Null )", String.Format("And ( InDate = '{0}' )", inDate.Value.ToString())),
        If(Not outDate.HasValue, "And ( OutDate Is Null )", String.Format("And ( OutDate = '{0}' )", outDate.Value.ToString())))

    Dim usingSelectRows As DataRow() = originalDataTable.Select(filter)
    Console.WriteLine("usingSelectRows.Count = {0}", usingSelectRows.Count)

    ' Using Linq.
    Dim typeSelector As Func(Of DataRow, Boolean) = Function(r)
    If (IsNothing(type)) Then
        Return IsNothing(r.Field(Of String)("Type"))
    Else
        Return r.Field(Of String)("Type") = type
    End If
End Function

    Dim inDateSelector As Func(Of DataRow, Boolean) = Function(r)
    If (Not inDate.HasValue) Then
        Return Not r.Field(Of DateTime?)("InDate").HasValue
    Else
        Return r.Field(Of DateTime?)("InDate").GetValueOrDefault.CompareTo(inDate.GetValueOrDefault) = 0
    End If
End Function

    Dim outDateSelector As Func(Of DataRow, Boolean) = Function(r)
    If (Not outDate.HasValue) Then
        Return Not r.Field(Of DateTime?)("OutDate").HasValue
    Else
        Return r.Field(Of DateTime?)("OutDate").GetValueOrDefault.CompareTo(outDate.GetValueOrDefault) = 0
    End If
End Function

    Dim usingLinqRows = From r In originalDataTable.AsEnumerable
                Where
                    (typeSelector(r)) AndAlso
                    (inDateSelector(r)) AndAlso
                    (outDateSelector(r))
                Select r

    Console.WriteLine("usingLinqRows.Count = {0}", usingLinqRows.Count)

    Console.WriteLine()
Next

    Console.ReadLine()
End Sub

End Module

始终使用DataRow的{{3}}扩展名来检索数据。

带有AND条件的DataTable.Select不会给出预期的结果

1 个答案: