Question

我有这个代码来比较两个控制标记值的值：

Protected Function GetLabelTextForTag(tagVal As String) As String
    Dim CoName As String = ""
    For Each cntrl As Control In Me.Controls
        If TypeOf cntrl Is Label Then
            If DirectCast(cntrl, Label).Tag Is tagVal Then
                CoName = DirectCast(cntrl, Label).Text
                Exit For
            End If
        End If
    Next
    Return CoName
End Function

即使我到达这条线，但是：

If DirectCast(cntrl, Label).Tag Is tagVal Then

...和Tag与tagVal相同（＆＃34; 1＆＃34;），未到达下一行 - ＆＃34; 1＆＃34;并不像＆＃34; 1＆＃34;

那样被视为同一件事

不熟悉VB，我想也许是＆＃34; Is＆＃34;是问题。但我之所以使用＆＃34; IS＆＃34;但首先，是因为当我第一次尝试这个时：

If DirectCast(cntrl, Label).Tag = tagVal Then

...我得到了＆＃34; 错误1选项Strict On禁止运算符类型为Object的操作数＆＃39; =＆＃39;。使用＆＃39; Is＆＃39;运算符来测试对象标识。＆＃34;

该函数由按钮单击的事件处理程序调用：

Private Sub Button1_Click( sender As Object,  e As EventArgs) Handles Button1.Click
        Dim connStr As String = "SERVER=PLATYPUS42;DATABASE=duckbilldata;UID=durante;PWD=pondscum"
        Dim upd8DML As String = "UPDATE CustomerCategoryLog SET Category = 'Exploding' WHERE Unit = @Unit And MemberNo = @MemberNo AND Custno = @CustNo"

        Dim coName As String
        Dim argVals(2) As String
        Dim _Unit As String
        Dim _MemberNo As String
        Dim _CustNo As String
        Dim curTagVal As String

        For Each cntrl As Control In Me.Controls
            If TypeOf cntrl Is CheckBox Then
                If DirectCast(cntrl, CheckBox).Checked = True Then
                    curTagVal = CStr(DirectCast(cntrl, CheckBox).Tag)
                    coName = GetLabelTextForTag(curTagVal)
                    argVals = GetArgValsForCompanyName(coName)
                    _Unit = argVals(0)
                    _MemberNo = argVals(1)
                    _CustNo = argVals(2)
                    Using conn As New SqlConnection(connStr), _
                        cmd As New SqlCommand(upd8DML, conn)
                        cmd.Parameters.Add("@CoName", SqlDbType.VarChar, 50).Value = coName
                        conn.Open
                        cmd.ExecuteScalar()
                    End Using
                End If
            End If
        Next
End Sub

我正在查找具有相同标记值的标签和复选框，因为我正在动态创建这些控件的一些对，并且需要知道检查哪些标签的相应复选框。我创建它们（一些，这只是一个测试）就像这样：

Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
        Dim sqlConnection1 As New SqlConnection("SERVER=PLATYPUS42;DATABASE=duckbilldata;UID=durante;PWD=pondscum")
        Dim cmd As New SqlCommand
        Dim reader As SqlDataReader

        cmd.CommandText = "select C.CompanyName from CustomerCategoryLog CCL join Customers C on C.CustNo = CCL.Custno where CCL.Category = 'New' and C.Active <> 0 order by C.CompanyName"
        cmd.CommandType = CommandType.Text
        cmd.Connection = sqlConnection1

        sqlConnection1.Open()

        reader = cmd.ExecuteReader()
        ' Data is accessible through the DataReader object here.
        If reader.HasRows Then
            Dim i As Integer = 0

            While reader.Read()
                i = i+1
                If i > 12 'There are thousands...just grab the first dozen for this test
                    Exit While
                End If

                Dim lblCompanyName = New Label()
                lblCompanyName.Tag = i.ToString() 'ID not available...?!?
                lblCompanyName.Text = reader.Item(0).ToString()
                Me.Controls.Add(lblCompanyName)

                Dim ckbx = New CheckBox()
                ckbx.Tag = i.ToString()
                ckbx.Checked = True
                Me.Controls.Add(ckbx)
            End While
        End If
        reader.Close()

        sqlConnection1.Close()
    End Sub

我也可以在表单上添加唯一的其他代码，以便进行更好的衡量：

Protected Function GetArgValsForCompanyName(coName As String) As String()
        Dim args(2) As String
        Dim sqlConnection1 As New SqlConnection("SERVER=PLATYPUS42;DATABASE=duckbilldata;UID=durante;PWD=pondscum")
        Dim cmd As New SqlCommand
        Dim reader As SqlDataReader

        cmd.CommandText = "select Unit, MemberNo, CustNo from Customers WHERE CompanyName = @CoName"
        cmd.CommandType = CommandType.Text
        cmd.Parameters.Add("@CoName", SqlDbType.VarChar, 50).Value = coName
        cmd.Connection = sqlConnection1
        sqlConnection1.Open()

        reader = cmd.ExecuteReader()
        ' Data is accessible through the DataReader object here.
        If reader.HasRows Then
                args(0) = reader.Item(0).ToString()
                args(1) = reader.Item(1).ToString()
                args(2) = reader.Item(2).ToString()
        End If
        reader.Close()
        sqlConnection1.Close()

        Return args
    End Function

＆＃34; 1＆＃34;不是＆＃34; 1＆＃34;？

Answer 1

好的，Tag是一个Object而不是一个String因此有严格的选项就可以解决问题，试试

If DirectCast(cntrl, Label).Tag.ToString() = tagVal Then

使用您是在比较参考而不是值。

也为你解决了这个问题

    Private Sub Button1_Click( sender As Object,  e As EventArgs) Handles Button1.Click
            Dim connStr As String = "SERVER=PLATYPUS42;DATABASE=duckbilldata;UID=durante;PWD=pondscum"
            Dim upd8DML As String = "UPDATE CustomerCategoryLog SET Category = 'Exploding' WHERE Unit = @Unit And MemberNo = @MemberNo AND Custno = @CustNo"

            Dim coName As String
            Dim argVals(2) As String
            Dim _Unit As String
            Dim _MemberNo As String
            Dim _CustNo As String
            Dim curTagVal As String

            For Each cntrl As Control In Me.Controls
                If TypeOf cntrl Is CheckBox Then
                    If DirectCast(cntrl, CheckBox).Checked = True Then
                        curTagVal = CStr(DirectCast(cntrl, CheckBox).Tag)
                        coName = GetLabelTextForTag(curTagVal)
                        argVals = GetArgValsForCompanyName(coName)
                        _Unit = argVals(0)
                        _MemberNo = argVals(1)
                        _CustNo = argVals(2)
                        Using conn As New SqlConnection(connStr), _
                            cmd As New SqlCommand(upd8DML, conn)
                            cmd.Parameters.Add("@Unit", SqlDbType.VarChar, 50).Value = _Unit
                            cmd.Parameters.Add("@MemberNo", SqlDbType.VarChar, 50).Value = _MemberNo
                            cmd.Parameters.Add("@CoName", SqlDbType.VarChar, 50).Value = coName
                            conn.Open
                            cmd.ExecuteScalar()
                        End Using
                    End If
                End If
            Next

End Sub

Answer 2

我仍然不知道为什么原始代码不起作用，但将其分为以下内容：

Protected Function GetLabelTextForTag(tagVal As String) As String
    Dim CoName As String = ""
    Dim lblTag As String
    For Each cntrl As Control In Me.Controls
        If TypeOf cntrl Is Label Then
            lblTag = CStr(DirectCast(cntrl, Label).Tag)
            If lblTag = tagVal Then
                CoName = DirectCast(cntrl, Label).Text
                Exit For
            End If
        End If
    Next
    Return CoName
End Function

为什么这＆＃34; Is＆＃34;测试失败了这个比较？

2 个答案: