VBA无效使用null

Question

我继承了一个VBA项目，并且正在尝试调试我收到的错误：Invalid use of Null。一段时间后，我找到了发生错误的位置，但尚未找到具体的罪魁祸首和/或问题的解决方案。请参阅以下snippit（抛出异常的行注释注释）：

Dim db As Database, wsp As Workspace
    Dim retVal As Variant
    Dim tableType As Long

    'Check to see if the table name exists in the zLinked_Tables table; if it does, that means its a SQL table
    If DCount("*", "[zLinked_Tables]", "[LocalName] = '" & TableName & "' AND [ObjectType] = 'Table'") > 0 Then
        'SQL Table
        retVal = ExecuteSPT("TRUNCATE TABLE [" & TableName & "]", 0) 'truncate the table via passthrough query on server
        If KeyColumn <> "" Then
            retVal = ExecuteSPT("DBCC CHECKIDENT([" & TableName & "],RESEED,1)", 0)  'reset the identity column value via passthrough query on server
        End If
    Else
        'MS Access Table
        tableType = DLookup("[Type]", "[MSysObjects]", "[Name] = '" & TableName & "'")
        DoCmd.SetWarnings False
        DoCmd.RunSQL "DELETE * FROM [" & TableName & "]" 'delete all records from table
        If KeyColumn <> "" Then
            DoCmd.SetWarnings True
            If tableType = 1 Then 'Resident/Local
                Set db = CurrentDb
                db.Execute "ALTER TABLE [" & TableName & "] ALTER COLUMN [" & KeyColumn & "] COUNTER(1,1)"
            Else 'Linked Table
                Set wsp = DBEngine.Workspaces(0)
                Set db = wsp.OpenDatabase(DLookup("[Database]", "[MSysObjects]", "[Name] = '" & TableName & "'")) 'ERROR THROWN ON THIS LINE
                db.Execute "ALTER TABLE [" & TableName & "] ALTER COLUMN [" & KeyColumn & "] COUNTER(1,1)"  'reset the autonumber column value
            End If
            DoCmd.SetWarnings False
            Set db = Nothing
        End If
    End If
    Exit Function

请注意，我检查了TableName变量，它不是null，而且是一个有效的表名

Answer 1

这条线上发生了几件事。除了检查TableName是否为null之外，您还应该检查wsp是否为空？ Set wsp = DBEngine.Workspaces(0)因任何原因失败了吗？

dlookup失败了吗？ DLookup("[Database]", "[MSysObjects]", "[Name] = '" & TableName & "'")。如果该值为null，则表示您将null传递给wsp.OpenDatabase()，这会出现错误：

Answer 2

您的DLOOKUP在MSYSObjects的“数据库”字段中返回一个没有值（NULL）的行。在即时窗口和executs中键入以下内容。您需要添加代码以允许这种可能性。

我认为你正试图获得完整的路径？

?(DLookup("[Database]", "[MSysObjects]", "[Name] = '" & "Table1" & "'"))