Q259211: PRB: COMRETURNERROR() Only Works Properly if Error Has Occurred

Article: Q259211
Product(s): Microsoft FoxPro
Version(s): WINDOWS:6.0
Operating System(s): 
Keyword(s): kbCOMt kbvfp600 kbGrpDSFox kbDSupport kbCodeSnippet
Last Modified: 12-APR-2000

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The information in this article applies to:

- Microsoft Visual FoxPro for Windows, version 6.0 
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SYMPTOMS
========

The COMRETURNERROR() function populates the COM exception structure with
information that Automation clients can use to determine the source of Visual
FoxPro Automation errors. However, if this function is called without an error
having occurred, it does not properly populate the structure. Visual FoxPro
clients display the information that was sent, but other clients may display
only a generic "Automation Error" message.

RESOLUTION
==========

One resolution to this problem is to always call the COMRETURNERROR function
from the Error method of your Automation server. The ERROR cErrorMessage command
throws Error 1098 (User-defined error) when used. You can test for this in the
Error method and call the COMRETURNERROR function with the cErrorMessage
command.

STATUS
======

This behavior is by design.

MORE INFORMATION
================

Steps to Reproduce Behavior
---------------------------

1. Create a new project and save it as "ThrowErr.pjx" (without the quotation
  marks).

2. On the the Code tab, click New.

3. Save the following code as "ThrowErr.prg" (without the quotation marks):

  DEFINE CLASS RetErr AS custom OLEPUBLIC

  	Name = "RetErr"
  	
  	PROCEDURE ThrowErrBad
  		COMRETURNERROR("ThrowErr", "109")
  	ENDproc
  	
  	PROCEDURE ThrowErrGood
  		ERROR "109"
  	ENDproc
  	
  	PROCEDURE Error
  		LPARAMETERS nError, cMethod, nLine

  		IF nError = 1098
  		   COMRETURNERROR("ThrowErr", MESSAGE())
  		ENDif
  	ENDproc
  ENDdefine

4. On the Build menu, select Multi-threaded COM server, and save it as
  "ThrowErr.dll" (without the quotation marks).

5. To test this in FoxPro, run the following code in the Command window. You
  should see the same error box pop up both times.

  loServer = CREATEOBJECT("ThrowErr.RetErr")
  loServer.ThrowErrBad()
  loServer.ThrowErrGood()

6. To test this in Microsoft Access, create a new database and a new form. Put
  two CommandButtons named Command1 and Command2 and three Labels named Label1,
  Label2, and Label3 on the form.

7. In Command1, put the following code:

  Private Sub Command1_Click()
  On Error Resume Next
  Set x = CreateObject("ThrowErr.RetErr")
  Call x.ThrowErrBad
  Label1.Caption = Str(Err.Number)
  Label2.Caption = Err.Source
  Label3.Caption = Err.Description
  End Sub

8. In Command2, put the following code:

  Private Sub Command2_Click()
  On Error Resume Next
  Set x = CreateObject("ThrowErr.RetErr")
  Call x.ThrowErrGood
  Label1.Caption = Str(Err.Number)
  Label2.Caption = Err.Source
  Label3.Caption = Err.Description
  End Sub

9. Run the form, click Command1, and then click Command2. Command1 displays 440
  for the Error Number, the name of the database for the Error Source, and
  Automation Error for the Error Description.

  Command2 displays 98 for the Error Number, ThrowErr for the Error Source, and
  109 for the Error Description.

  Further clicks on Command1 display the correct error information.

REFERENCES
==========

For more information about COMRETURNERROR(), the ERROR command, or building COM
servers, please see the Visual FoxPro documentation.

Additional query words:

======================================================================
Keywords          : kbCOMt kbvfp600 kbGrpDSFox kbDSupport kbCodeSnippet 
Technology        : kbVFPsearch kbAudDeveloper kbVFP600
Version           : WINDOWS:6.0
Issue type        : kbprb

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