Using a VFP DLL in a dotNet system
If you want to migrate a Visual FoxPro system into dotNet then you will obviously have
to rewrite everything in the new language, probably C# or Basic if you're sticking with
Microsoft. If the FoxPro database is a large one then the rewrite will take a lot of
time and effort and you will have to bring a completely new system into operation with a
sudden-death changeover. Even with the best of testing this is still a risky business and
one way of managing the risk is to convert some of your existing FoxPro system into a DLL
that can be called from dotNet. This allows you to commission the new system in a number
of stages. The first might be a database that is just a series of VB or C# forms which
call VFP for data handling and business logic; subsequent stages would see the classes
of the VFP DLL being replaced by native dotNet classes. The overall project might take a
little longer but the user would be using the new forms for a lot of this time and the
improved feedback should lead to a better final result.
Writing a VFP DLL
The functionality of the DLL must be wrapped into a class and this must be a custom
class of type OLEPUBLIC. The example described below opens the Customer table and
finds the name of the company for a given customer id.
The first step is to define the class and to give it a property that will hold the
customer name. A real world class would of course have properties to match every field
of the table:
Define Class
Customer
As Custom OlePublic
Company = Space(40)
Enddefine
The DLL can't have any user interface so we can't have messages telling us what Fox is
doing. We can turn these off in the Init method of the class:
Procedure Init
On Error
Set Console Off
Set Notify Off
Set Safety Off
Set Talk Off
Set Notify Off
Endproc
The DLL will have to open the customer table and set it into Cust_ID order so that
we'll be able to find the customer later. The calling program will need to know
whether the DLL has managed to open the table so we'll return the record count:
Procedure OpenTable
Select 0
Use C:\Projects\Customer
Set Order To Cust_ID
Return Reccount()
Endproc
The SetName method of the class will take the customer ID as a parameter and look
for the matching record:
Procedure
SetName()
Lparameter tcID
Local llFound As Boolean
If Seek(tcID)
This.Company = Customer.Company
llFound = .T.
Else
This.Company = Space(40)
llFound = .F.
Endif
Return llFound
Endproc
The next steps
Once you've got the basic class written the next stages are to
test and
build
the DLL and then to
call
it from Visual Basic or C#.
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