主题：求一段时间内，星期（星期一，二，三......）的天数

 你本来的代码就没问题啊。

For Each tt As DataRow In DataTables("时间表").DataRows
    Dim d1 As Date = tt("开始时间")
    Dim d2 As Date = tt("结束时间")
    Dim d3 As Date = d1
    Dim w1 As Integer = 0
    Dim w2 As Integer = 0
    Dim w3 As Integer = 0
    Dim w4 As Integer = 0
    Dim w5 As Integer = 0
   
    Do While d3<= d2
        Select Case d3.DayOfWeek
            Case 1
                w1 = w1 +1
            Case 2
                w2 = w2 +1
            Case 3
                w3 = w3 +1
            Case 4
                w4 = w4 +1
            Case 5
                w5 = w5 +1
        End Select
        d3 =d3.AddDays(1)
    Loop
   
    Select Case tt("星期")
        Case "星期一"
            tt("天数") = w1
        Case "星期二"
            tt("天数") = w2
        Case "星期三"
            tt("天数") = w3
        Case "星期四"
            tt("天数") = w4
        Case "星期五"
            tt("天数") = w5
    End Select
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