[RESOLVED] Need help to understand what this simple code does

Hi!

I need your help to truly undestand the following code:

Code:

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Function NullNum(ByRef X As Object) As Object
        Dim s As Object
     
        On Error GoTo NullNum_Err
                NullNum = 0
                s = CStr(X)
                NullNum = X
                If NullNum Is Nothing Then NullNum = 0
        NullNum_Err:
End Function

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My questions are:
1. What reason is the cast of X from Object to String for, if the result is assigned to an Object "s"? Is this where we would expect an Exception(Error) to be thrown if X cannot be cast to string? Or is this totally redundant, as "s" is not used anywhere?
2. Unfortunately none of the functions in the code are commented. So I am not even sure I undestand all possible use of this function. My primary idea would be that this function checks if the argument is a number, and would return the number if true and 0 if false. But if this function is called with a boolean expression, then it would return the boolean value of the expression, if the function is called with any object that can be casted to string, then it would return that object. So what does this function do really? I know it sounds stupid and I'm new to VB, I just want to understand this function as it is used massively in the whole project.

Besides, I am now upgrading the project to VB.NET, and the database connection will be upgraded to ADO.NET. So if this function just needs to check if a string argument is a number, then I will figure out a better way to do it in VB.NET.

In the project the function is called everywhere like in this function:

Code:

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Function GetValue(ByRef my_id As Object) As Object
        Dim r As Object
               
        r = conn.Execute("select firstcol from tablename where id = " & CStr(my_id))
       
        If Not r.EOF Then
                       
                GetValue = NullNum(r("firstcol"))
                       
                If IsDbNull(r("firstcol")) Then
                               
                        GetValue = -1
                End If
        End If
               
        r.Close()
        r = Nothing
End Function

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1) Yes, it is to cause an error if the value can't be converted to a string.


2) I agree with your analysis of it, so it is a rather strange function.

Based on various quality issues in the code for GetValue, I don't think you can rely on the behaviour of NullNum matching the intended design (and you can't even tell what the design is :sick: ).

Assuming that the field firstcol is numeric, the GetValue routine should be rewritten to something like this (which doesn't need NullNum):

Code:

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Function GetValue(ByRef my_id As Object) As Integer
        GetValue = -1
        Dim r As ADODB.Recordset

        r = conn.Execute("select firstcol from tablename where id = " & CStr(my_id))
       
        If Not r.EOF Then
                If Not r("firstcol") Is Null Then
                        GetValue = r("firstcol")
                End If
        End If
               
        r.Close()
        Set r = Nothing
End Function

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...if for some reason you need to determine the difference between no record existing and there being a record with a null value, it will need to be changed slightly.

Thank you very much! I needed to know that there is no special VB mechanism in this function that I dont undestand, before I change it.

The typical method to test a string to see if is numeric in VB6 is to use the IsNumeric() function
In VB.Net you would use the Integer.TryParse() method or whatever type of number you are looking to use.