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vb2008中对控件的“null”值因如何判断

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Access中有一个微软的函数你可以借鉴一下。
Public Function IsNothing(varToTest As Variant) As Integer
' Tests for a "logical" nothing based on data type
' Empty and Null = Nothing
' Number = 0 is Nothing
' Zero length string is Nothing
' Date/Time is never NothingIsNothing = True
Select Case VarType(varToTest)
Case vbEmpty
Exit Function
Case vbNull
Exit Function
Case vbBoolean
If varToTest Then IsNothing = False
Case vbByte, vbInteger, vbLong, vbSingle, vbDouble, vbCurrency
If varToTest <> 0 Then IsNothing = False
Case vbDate
IsNothing = False
Case vbString
If (Len(varToTest) <> 0 And varToTest <> " ") Then IsNothing = False
End SelectEnd Function