C++ singletons and .NET AppDomains

[Yevgeny Menaker]

I am struggling with the following:
We have legacy classes written in C++ that are compiled to static
libraries. Some of these classes are singletons. In order to make
these classes available to C#, we wrapped them with Managed C++. The
wrapper classes works fine in most of the cases. However, if they are
used in ASP.NET Web Applications a problem may arise in the following
scenario:
Step 1: The first Web Application starts in its own AppDomain inside
aspnet_wp.exe process and creates wrapper singleton class, which in
turn creates underlying unmanaged C++ singleton.
Step 2: We change the web.config file of our Web Application. This
causes unloading AppDomain of the Web Application.
Step 3: Web Application starts again and a new AppDomain is loaded
upon HTTP request.
Step 4: A new AppDomain gets an instance of Singleton. Since the
underlying C++ singleton has a static variable holding pointer to
singleton instance, this instance lives until aspnet_wp.exe process
exits. So, when trying to access this instance from a new AppDomain
raises AppDomainUnloadedException, since we access memory that was
allocated for an old AppDomain.
The solution I found is using Remoting and create wrapper Singleton
together with the underlying C++ singleton in the "remote" AppDomain.
However, there is too much overhead. Another way is, of course, to
rewrite the unmanaged C++ class and make it non-singleton.

Are there any other cleaner solutions for this problem?

Many thanks,
Yevgeny Menaker
Author, Software Engineer.

[Alvin Bruney]

why don't you update your logic to cause the singleton instance to go away
when the application domain unloads?

--
Regards,
Alvin Bruney
Got tidbits? Get it here...
http://tinyurl.com/3he3b
"Yevgeny Menaker" <ge****@bezeqint.net> wrote in message
news:c1*************************@posting.google.co m...

I am struggling with the following:
We have legacy classes written in C++ that are compiled to static
libraries. Some of these classes are singletons. In order to make
these classes available to C#, we wrapped them with Managed C++. The
wrapper classes works fine in most of the cases. However, if they are
used in ASP.NET Web Applications a problem may arise in the following
scenario:
Step 1: The first Web Application starts in its own AppDomain inside
aspnet_wp.exe process and creates wrapper singleton class, which in
turn creates underlying unmanaged C++ singleton.
Step 2: We change the web.config file of our Web Application. This
causes unloading AppDomain of the Web Application.
Step 3: Web Application starts again and a new AppDomain is loaded
upon HTTP request.
Step 4: A new AppDomain gets an instance of Singleton. Since the
underlying C++ singleton has a static variable holding pointer to
singleton instance, this instance lives until aspnet_wp.exe process
exits. So, when trying to access this instance from a new AppDomain
raises AppDomainUnloadedException, since we access memory that was
allocated for an old AppDomain.
The solution I found is using Remoting and create wrapper Singleton
together with the underlying C++ singleton in the "remote" AppDomain.
However, there is too much overhead. Another way is, of course, to
rewrite the unmanaged C++ class and make it non-singleton.

Are there any other cleaner solutions for this problem?

Many thanks,
Yevgeny Menaker
Author, Software Engineer.