The source code in this issue is designed for all versions of Visual Basic.
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
The non-null value will be returned as whatever data type was passed into NullConvert by the parameter vDest
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
The non-null value will be returned.
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]") RS.MoveFirst ' 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 RS.MoveNext Wend ' Close recordset and database RS.Close DB.Close End Sub
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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.
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