Question

我有一张用于用户屏幕的表。

用户ID-屏幕ID-烫发

当使用UserID和ScreenID打开表单时，我需要隐藏控件，并且无法使用带有条件的循环。

这是我的代码：

Try
    Dim da As New SqlDataAdapter
    Dim ds As New DataSet
    If dbconnect.State = ConnectionState.Closed Then dbconnect.Open()
    da = New SqlDataAdapter("SELECT * FROM UserScreens WHERE UserID='" & UserID & "'", dbconnect)
    da.Fill(ds, "Table")
    If ds.Tables(0).Rows.Count > 0 Then
        Dim M As DataRow = ds.Tables(0).Rows(0)
        For Each ctrl As Control In Ribbon1.Controls
            ctrl.Visible = M.Item("Perm")
        Next
        'DocIDtxt.Text = M.Item("DOCID")
    End If
Catch ex As Exception
    If dbconnect.State = ConnectionState.Open Then dbconnect.Close()
    MsgBox(ex.ToString)
End Try

Answer 1

我总是首先担心连接问题。来自MS文档

“与select命令关联的IDbConnection对象必须有效，但不必打开。如果IDbConnection在调用Fill之前关闭，则先打开它以检索数据，然后关闭。如果连接是打开的在调用Fill之前，它保持打开状态。”

这意味着您在.Fill方法之前打开了连接，因此它将保持打开状态。坏！代码关闭连接的唯一方法是出现错误。在Try ... End Try中使用.Close的Final部分可以解决问题，但最好使用Using ... End Using。即使出现错误，此块也可以确保关闭并正确放置对象。当对象具有Dispose方法时，它可能具有必须清除的非托管资源。

接下来，打开“严格选项”。指出潜在的运行时错误将有很大帮助。

第三，请始终使用参数。

请参阅内嵌说明和评论。

Private Sub OpCode()
        Try
            'A DataTable does not have a Dispose() method so it will fall
            'out of scope and be garbage collected.
            Dim dt As New DataTable
            'SqlConnection has a Dispose() method so use Using
            Using cn As New SqlConnection("Your connection string")
                'SqlCommand has a Dispose() method so use Using
                Using cmd As New SqlCommand("SELECT * FROM UserScreens WHERE UserID= @UserID", cn)
                    'Always use Parameters. Never concatenate strings in SQL statements.
                    cmd.Parameters.Add("@UserID", SqlDbType.VarChar).Value = UserID
                    cn.Open()
                    'SqlDataReader has a Dispose() method so use Using
                    Using dr As SqlDataReader = cmd.ExecuteReader
                        dt.Load(dr)
                    End Using
                End Using
            End Using
            If dt.Rows.Count > 0 Then
                Dim M As DataRow = dt.Rows(0)
                For Each ctrl As Control In Ribbon1.Controls
                    'If Perm is not a boolean this line of code can't work
                    ctrl.Visible = CBool(M.Item("Perm"))
                Next
                'DocIDtxt.Text = M.Item("DOCID")
            End If
        Catch ex As Exception
            MsgBox(ex.ToString)
        End Try
    End Sub

使用循环隐藏功能区控件vb.net

1 个答案: