"无效使用null"用于返回变量的vba函数

Question

我正在尝试编写一个VBA函数,该函数使用SELECT查询从Access表中获取值.这是代码:

Function getTableValue(uniqueID As Long, tableName As String, _ 
idField As String, tableField As String) As Variant

Dim db As Database
Dim rs As Recordset
Dim selectSQL As String

selectSQL = "select * from " & tableName & " where " & idField & "=" & uniqueID

Set db = OpenDatabase(dbPath)
Set rs = db.OpenRecordset(selectSQL)

If rs.RecordCount > 0 Then
    rs.MoveLast
    rs.MoveFirst
    getTableValue = rs.Fields(tableField)
End If

End Function

如果指定的字段tableField是日期/时间类型且字段为空,则getTableValue()返回"无效使用空"错误.我认为让函数返回Variant将允许我返回Null值; 我该怎么做来处理这些空日期？

Answer 1

事实证明函数的Null值被传递给Date/Time变量.我通过使用以下内容修复了这个问题:

dim myDate as date
dim myVar as variant

myVar=getTableValue(idNum,"thisTable","idField", "valueField")
if isNull(myVar) then
 myDate=0
else
 myDate=myVar
endif

虽然它不检查是否myVar返回Date,但其valueField值的类型为Date/Time,所以没关系.