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Steve McLellan - 27 May 2004 00:24 GMT

Hi,

I've just discovered that accessing Image.Height (and probably other
properties) in a multi-threaded environment can cause access problems (as a
test, you can spawn a hundred threads and get them all to perform a couple
of multiplications on an image's height and width ). I know I can prevent
simultaneous access, but is there any reason why Image can't be accessed
like this? And is this common for all .NET objects? I know the docs say 'not
thread-safe' but not being safe for multiple reads seems bizarre.

Steve

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Justin Rogers - 27 May 2004 00:34 GMT

http://msdn.microsoft.com/library/default.asp?url=/library/en-us/gdicpp/GDIPlus/
sec_gdiplus.asp?frame=true

Info on thread safety. Internally Height always calls methods to get the height
and no caching is performed. Since each call is a method, there might be some
work, the return of an ObjectBusy status, or any number of other items that
might go wrong.

Signature

Justin Rogers
DigiTec Web Consultants, LLC.
Blog: http://weblogs.asp.net/justin_rogers

> Hi,
>
[quoted text clipped - 7 lines]
>
> Steve

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Steve McLellan - 27 May 2004 00:43 GMT

OK, cheers. Just the sort of thing I need to find out at 1am :-) Is this
common to the framework? I'm never likely to know what's cached and what
isn't for a given property.... and surely this is unnecessary if there are
only read operations going on? I'm still not sure why exactly that could
ever be a problem.

Steve

http://msdn.microsoft.com/library/default.asp?url=/library/en-us/gdicpp/GDIPlus/
sec_gdiplus.asp?frame=true

> Info on thread safety. Internally Height always calls methods to get the height
> and no caching is performed. Since each call is a method, there might be some
[quoted text clipped - 12 lines]
> >
> > Steve

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Justin Rogers - 27 May 2004 00:51 GMT

Image is abstracting you from a bunch of method calls. In GDI+ each object
is simply a pointer or handle, and you call methods to get the information from
those handles. If Image cached the values for you, then it would have a bunch
of state work to handle for things like moving to the next frame or other events
that could possibly change the height or width of the underlying image.

This generally isn't a huge problem across the framework, but is common when
API's are created that are wrapping an unmanaged API. The unmanaged API's
simply have semantics that don't translate 1 for 1 into the .NET world.

Signature

Justin Rogers
DigiTec Web Consultants, LLC.
Blog: http://weblogs.asp.net/justin_rogers

> OK, cheers. Just the sort of thing I need to find out at 1am :-) Is this
> common to the framework? I'm never likely to know what's cached and what
[quoted text clipped - 3 lines]
>
> Steve

http://msdn.microsoft.com/library/default.asp?url=/library/en-us/gdicpp/GDIPlus/
sec_gdiplus.asp?frame=true

> > Info on thread safety. Internally Height always calls methods to get the
> height
[quoted text clipped - 19 lines]
> > >
> > > Steve

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Steve McLellan - 27 May 2004 01:07 GMT

Splendid explanation. That does kind of make sense, and it's a good job we
spotted it now (and didn't put it into our 'strange but true' bug file) and
not after release. My TV show "When multithreading goes bad" is coming on
leaps and bounds.

Thanks again, and I hope it's not as late where you are as it is here :-)

Steve

> Image is abstracting you from a bunch of method calls. In GDI+ each object
> is simply a pointer or handle, and you call methods to get the information from
[quoted text clipped - 13 lines]
> >
> > Steve

http://msdn.microsoft.com/library/default.asp?url=/library/en-us/gdicpp/GDIPlus/
sec_gdiplus.asp?frame=true

> > > Info on thread safety. Internally Height always calls methods to get the
> > height
[quoted text clipped - 19 lines]
> > > >
> > > > Steve

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