Visual Basic Code of the Week (COTW)
Issue #10
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Requirements for this Issue

The source code in this issue is designed for all versions of Visual Basic.

In this Issue

One of the problems encountered by developers using databases in Visual Basic are Null values. If you try to assign Null values to typed variables such as String or Long you receive a Type Mismatch error. This issue introduces one way around this problem.


Function NullConvert(vDest As Variant, v As Variant) As Variant


Return Value

The non-null value will be returned as whatever data type was passed into NullConvert by the parameter vDest

Supplemental Null Conversion Routines

Function NullConvertDate(v As Variant) As Date
Function NullConvertCurrency(v As Variant) As Currency
Function NullConvertLong(v As Variant) As Long
Function NullConvertNumber(v As Variant) As Variant
Function NullConvertString(v As Variant) As String
Function NullConvertBoolean(v As Variant) As Boolean


Return Value

The non-null value will be returned.

Sample Usage

This subroutine does not really do anything useful except demostrate the various ways the NullConvert functions can be called. Note that you must pass the destination variable to the NullConvert routine so that it can return the correct data type.

Sub ProcessDatabase()
    Dim DB As Database
    Dim RS As Recordset
    Dim sString As String
    Dim lLong As Long
    Dim cCurrency As Currency

    ' Open database
    Set DB = OpenDatabase(App.Path & "\0010.mdb")

    ' Get recordset for processing
    Set RS = DB.OpenRecordset("Select * from [testtable]")


    ' loop until we have no more records
    While Not RS.EOF

        ' Data Type Specific Functions

        ' sString will either be "" or it's value from the database
        sString = NullConvertString(RS.Fields("VString").Value)

        ' lLong and cCurrency will either be 0 or it's value from the database
        lLong = NullConvertLong(RS.Fields("VLong").Value)
        cCurrency = NullConvertCurrency(RS.Fields("VCurrency").Value)

        ' Generic NulConvert function
        sString = NullConvert(sString, RS.Fields("VString").Value)
        lLong = NullConvert(lLong, RS.Fields("VLong").Value)

        ' This method of calling NullConvert will only work in VB 4 or later
        NullConvert cCurrency, RS.Fields("VCurrency").Value


    ' Close recordset and database
End Sub

Source Code

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Comments on the Source Code

This is not necessarily the most efficient way to handle Null values. For strings you can simply do something like this:

sDest = "" & RS.Fields("StringField")

These routines attempt to simplify the methods necessary to handle null values.

If you have any questions that you can not seem to figure out, drop us a email at and we will do our best to help you out.


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