Major RB Mystery Solved Canvas Does Not Allow Drawing

rbuser

Hello All:

I am running an older version of Reportbuilder 6.03 using Delphi 5. When
my application runs under remote desktop I occasionally get a "Canvas Does
Not Allow Drawing" Error. I could never figure out why it happened. To cure
it I was rebooting the server - the reboot corrected the problem alot of the
time. However, recently rebooting one client's computer did not cure the
"Canvas Does Not Allow Drawing" Error.

After googling this error, I have discovered the problem. The problem is
that the default driver under the Remote Desktop user profile was invalid.
My code assigns a valid print driver to the ppReport component (im not sure
exactly when in the sequence this happens) - but it appears that the older
version of ReportBuilder uses the default windows printer driver for some
functionality.

If I switch the windows default printer driver to a valid printer
driver - the reportbuilder error disappears and all printing is executed
correctly. If I switch the windows printer driver back to the invalid
printer driver the program again produces the "Canvas Does Not Allow
Drawing" Error. This occurs despite the fact that I assign the printer
driver to the valid printer driver in the ppReport1 component. If the
windows default driver in the Remote Desktop session points to an invalid
printer driver, Reportbuilder 6.03 seems to produce that error.

Nard - does this make sense ??? Is this something that should be logged
into the RBWiki ???

I own RB12 and will upgrade my software to RB12 in the next few weeks -
Im hoping per the Wiki that RB12 will eliminate this issue. Thank you very
much. I really love the RB product.

Neil Huhta
Profit Monster Data Systems LLC

nardmoseley

"Canvas does not allow drawing" is a Delphi exception that indicates an
invalid handle to a windows device context (Canvas.Handle). This is most
often caused by the application leaking memory/resources, which is why
restarting the app or rebooting may resolve the problem. As you mention
this exception can also be caused by a reference to a invalid driver.

1. I recommend running the app under a tool that can detect resource/memory
leaks. FastMM is the memory manager included with the latest Delphi releases
and it can be configured to debug resource/memory leaks. AQTime is another
such tool.

2. RB caches the list of available printers. If drivers are being
installed/uninstalled while the app is running, you can programmatically
refresh the list using the following code

uses
ppPrintr;

ppPrinters.Refresh;


RB 12 has an improved architecture that is designed to give a more specific
error messages for some printer related issues. I don't know whether it will
help your specific case or not. If the remote desktop is
installing/uninstalling drivers then doing the refresh shown above should
help. If the app is leaking memory/resources, then fixing the leaks should
help.


-
Nard Moseley
Digital Metaphors
www.digital-metaphors.com

rbuser

Nard:

There is absolutely not a resource leak here - this is primarily a quirk
between Windoiws remote desktop services and version 6 and earlier of
ReportBuilder.

#1) I routinely use AQTime for leak testing.
#2) I am not adding a printer driver form my application ever (no need
to refresh the application).

I can open a remote desktop session which has an invalid printer driver
assigned as the default printer (this is the quirk in remote desktop
services - you should NOT so easily be able to have invalid printer drivers
this-but it seems to be a common problem in Remote Desktop). Next, I take
any app with ReportBuilder 6.x and assign a different printer driver and
then try to print.

Here is what happens - it appears that Reportbuilder grabs the default
printer driver to do some processing - but - since the default printer
driver is not valid - it produces the "Canvas does not allow drawing" error
.

Next, I switch the default printer in the windows session to a valid
printer driver, then re-run the report - same app - same session - and now -
it prints perfectly.

I tested this with my big application and I also tested it with a 1 table
1 form application - same results both applications.

What my point is - is as follows:

When running in a Remote session - one should be aware that an invalid
default printer in the session will cause ANY software trying to get a
handle to it - to throw the "Canvas Does Not Allow Drawing" error.

My assertion is that these circumstances suggest that ReportBuilder is
ignoring my assignment of a specific driver (but only during pre-print
processing) - and is trying to access the default printer driver during
pre-printing processing.

My first question is "Does Reportbuilder 6 do some pre-printing
processing by grabbing the default printer driver (I suspect yes) ???

My second question is "Has the current Version 12 of ReportBuilder
eliminated the use of the default printer driver for pre-printing processing
(I also suspect yes)???

I discovered this cause by parousing the MSDN library and newsgroups -
there seems to be some amount of problems with default printers in Remote
Desktop.

I really think you should put a note in the Wiki - when this message
comes up in remote desktop - suggest that the users test for an invalid
printer handle by switching the default printer and re-running the report.

Such a note would have saved me alot of work.

Neil Huhta

rbuser

You might also want to search this newsgroup for messages with subjects
containing "NTDLL".

I had a print issue with ReportBuilder 12.03 when the Windows print
spooler service was not enabled (Windows Server 2008).

Enabling the print spooler service resolved the issue I was experiencing.