Question

您好我能够成功删除我的列表框中尚未签出的项目，但是当我使用listview中的其余项目更新我的数据库时，它会产生无限循环。问题出在我更新列表视图中剩余部分的行，它只更新第一行而不在下一行循环。

Dim ListOfExistingItem As List(Of Integer) = New List(Of Integer)
    Try
        For x = 0 To ListView2.Items.Count - 1
            Dim a As String = ListView2.Items.Item(x).Text
            a = ListView2.Items.Item(x).Text

            '-------------------------Max TRansaction----------------------------------
            Dim SQLMAX As String = ("Select Max(transaction_no) from tbl_list1 where barcode = '" + a + "' and [Check-In] IS NULL and [User] = '" + rsuser + "'")
            Dim STRMAX As String = ""

            Dim cmdmax As New SqlCommand(SQLMAX, locconn)
            Dim MAXREAD As SqlDataReader

            locconn.Open()
            MAXREAD = cmdmax.ExecuteReader
            MAXREAD.Read()
            If IsDBNull(MAXREAD(0)) = False Then
                STRMAX = MAXREAD(0)
            End If
            MAXREAD.Close()
            locconn.Close()
            '-----Remove items to listview that are not yet checked out-------------------------------------

            Dim sqlvalreader As SqlDataReader
            Dim TMP_SQL_VAL2 = "select [Check-In] from tbl_list1 where barcode = '" + a + "' and transaction_no = '" & STRMAX & "'"
            Dim cmdval = New SqlCommand(TMP_SQL_VAL2, locconn)
            locconn.Open()
            sqlvalreader = cmdval.ExecuteReader
            sqlvalreader.Read()



            If sqlvalreader.HasRows Then
                If IsDBNull(sqlvalreader(0)) = False Then
                    MsgBox("Barcode: " & a & "is still Inside", MsgBoxStyle.Exclamation)
                    clear_text()
                    ListOfExistingItem.Add(x)
                End If
            Else
                ListOfExistingItem.Add(x)
            End If
            sqlvalreader.Close()
            locconn.Close()
        Next
        Call ListOfExistingItem.Reverse()
        If Not IsNothing(ListOfExistingItem) Then
            For Each x As Integer In ListOfExistingItem
                If Not x > ListView2.Items.Count - 1 Then
                    ListView2.Items.RemoveAt(x)
                End If
            Next
        End If
    Catch ex As Exception
        MsgBox(ex.Message)
    End Try
    '------------------------------------------------------------------------------------------




    '-------------------------update the remaining items in the listview-----------------------

    Try

        Dim i As Integer
        Dim a, b, c, d As String
        For i = 0 To ListView2.Items.Count - 1
            a = ListView2.Items.Item(i).Text
            b = ListView2.Items.Item(i).SubItems.Item(1).Text
            c = ListView2.Items.Item(i).SubItems.Item(2).Text
            d = ListView2.Items.Item(i).SubItems.Item(3).Text
            i = i - 1


            Dim SQLMAX As String = ("Select Max(transaction_no) from tbl_list1 where barcode = '" + a + "' and [Check-In] IS NULL and [User] = '" + rsuser + "'")
            Dim STRMAX As String = ""

            Dim cmdmax As New SqlCommand(SQLMAX, locconn)
            Dim MAXREADER As SqlDataReader

            locconn.Open()
            MAXREADER = cmdmax.ExecuteReader
            MAXREADER.Read()
            If IsDBNull(MAXREADER(0)) = False Then
                STRMAX = MAXREADER(0)
            End If

            locconn.Close()

            Dim command As New SqlCommand
            TMP_SQL = "update tbl_list1 " & _
                      "set [Check-In] = GETDATE() " & _
                      "where ID_ = (select top (1) ID_ " & _
                      "from tbl_list1 " & _
                      "where barcode = '" + a + "' and transaction_no = '" + STRMAX + "' and [Check-In] IS NULL) "

            locconn.Open()
            command = New SqlCommand(TMP_SQL, locconn)
            command.ExecuteNonQuery()
            locconn.Close()
        Next

Answer 1

此行可疑，只是尝试将其删除：

i = i - 1

i是For循环中的计数器变量。它期望在每次迭代时递增，以便在某个点i值将达到循环的上限，即ListView2.Items.Count - 1。由于上面的代码递减了i的值，它永远不会达到上限，从而导致无限循环。

如果您不相信上述说明，只需在新的控制台应用程序中尝试：

For i As Integer = 0 To 100
    i = i - 1
Next
Console.WriteLine("Finish loop")

应用程序永远不会打印＆＃34;完成循环＆＃34;，但只要删除i = i - 1行，您就可以立即看到输出。因此，如果你的循环在某种情况下以某种方式被终止，那么正确的问题是＆＃34; 为什么它应该是无限的？必须有一些奇怪的行为导致它起作用＆＃34;而不是相反的＆＃34; 为什么这个循环是无限的？＆＃34;

这就是我的想法:)。

在列表视图中成功删除的项目，但在更新列表视图中的其余项目时会导致无限循环

1 个答案: