Testing a VFP DLL
A DLL can't run by itself and you can only test it by making a call to the DLL. It's
easiest to start by calling the class from within FoxPro itself. This will make sure
that there are no internal errors of logic. Before doing that though
we'll need to be able to handle the errors in the FoxPro code.
Errors
A DLL cannot have any user interface. If the DLL tries to produce any form of output
you'll get an error 2031 "User-interface operation not allowed at this time". This of
course includes any error message so the DLL must have comprehensive error-handling.
Debugging becomes impossible if all error messages are suppressed so my own preference
is to trap every error and to write details to a text file.
The Error method of the class will run whenever there is an error in the class and
this code will write the basic details of the error to a text file:
Procedure Error
Lparameters nError, cMethod, nLine
Local lcError
Local lcLine
Local lcMarker
Local lcMethod
Local lcTimeStamp
lcMarker = Replicate("-", 35)
lcTimeStamp =
"Error at …..:"
+
Ttoc(Datetime())
lcLine =
"Line ………:"
+
Alltrim(Str(nLine, 6, 0))
lcError =
"Error ……..:"
+
Alltrim(Str(nError, 6, 0))
lcMethod =
"Method …….:"
+ cMethod
Set Textmerge On
Set Textmerge To
c:\Error.txt
Additive
\<<lcMarker>>
\<<lcTimeStamp>>
\<<lcError>>
\<<lcLine>>
\<<lcMethod>>
Set Textmerge To
Set Textmerge Off
Return
Endproc
I'm using TextMerge to add lines to the error log. Note that the lines within the
TextMerge aren't indented. If they were then the indents would be output to the text
file too. The code above generates error log entries looking like this:
-----------------------------------
Error at .....:19/11/10 11:56:46
Error ........:36
Line .........:20
Method .......:init
With a bit more work you can use the Program() function to
loop through the call stack and show how the program reached the error.
Testing the class from VFP
A simple program can create the class and call the two methods:
Local
lcCustID
As String
Local
loCust
As
Customer
Close Databases All
Set Procedure To Customer.prg
loCust =
Createobject("customer")
Messagebox(loCust.OpenTable())
lcCustID =
INPUTBOX("Enter customer ID")
If (loCust.Getname(lcCustID))
Messagebox(loCust.Company)
Else
Messagebox("Cannot find "
+ lcCustID)
Endif
Return
Run the program with a known customer ID such as "RANCH" and with one that doesn't
exist. Edit the text of customer.prg with a silly syntax error to make sure that the
error will be detected - I used SET SAFELY ON.
The next steps
The first step in this process was to
write
a class as the basis for a DLL. The next two steps are to
build
the DLL and then to
call
it from Visual Basic or C#.
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