Creating a Singleton that is a cross AppDomain Singleton -- dotnet clr

Ken Kolda
9/14/2004 8:54:47 AM

See comments inline below...

[quoted text, click to view]
(http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dncscol/ht
ml/csharp05162002.asp)
[quoted text, click to view]

If you store the plugin objects in your main AppDomain, then you've defeated
the purpose of creating a separate AppDomain in that, even if you unload the
other AppDomain, you'll still have the plugin assemblies loaded. In order
for the plugin assemblies only to be loaded in the second AppDomain you must
be sure that your primary AppDomain never obtains a direct reference to a
method/class/type/etc. from one of those assemblies.

[quoted text, click to view]

In the sample code it demonstrates how to do this. In fact, there are two
different objects which "live" in the second AppDomain but are used by the
main AppDomain thru a proxy: the RemoteLoader and the FunctionList. Notice
that both of these object derive from MarshalByRefObject, which means when a
reference to the object crosses the AppDomain boundary, only a proxy is
created in the main AppDomain. Any method calls on the object are forwarded
to the second AppDomain and executed there.

[quoted text, click to view]

You can create a singleton to hold the plugins, but it must live in the
second AppDomain. If you wish, you can make this singleton a
MarshalByRefObject and then access it from the main AppDomain. The main
complication is in getting a reference to this object from the main
AppDomain. For that, you'd probably instantiate a class in the second
AppDomain using CreateInstanceAndUnwrap() (the way the example code creates
the RemoteLoader) and invoke a method that returns a reference to the
singleton.

[quoted text, click to view]

The FunctionList object provided in the sample code does just this. Is there
some additional behavior you need that this isn't providing (or couldn't
just be added on to what's there)?

Ken

kevin_ferron NO[at]SPAM hotmail.com
9/14/2004 5:03:33 PM

Thanks for you reply, comments inline..

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Ok, this was probably the conceptual barrier i was facing.

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Well the main difference is that in Erics example is a collection of
functions
whereas the way i have mine is that i need a collection of objects..

So am I correct that really what i want to do is be calling a proxy
class in the main AppDomain that acts as an interface to each of the
objects that im marshalling from the secondary AppDomain?

[quoted text, click to view]

Ken Kolda
9/15/2004 8:06:21 AM

What I would advise is that you create an abstract base class from which all
of yur plugin objects must derive, e.g.

public abstract class Plugin : MarshalByRefObject
{
// ... Your plugin's interface methods go here
}

This would ensure that every plugin must be a MarshalByRefObject so you
could obtain references to the plugin objects from your main AppDomain and
invoke methods on them without having them get loaded in that AppDomain.

If that's not feasible, then I'd create a wrapper class that derives from
MarshalByRefObject and implements the same interface as your plugins. The
wrapper would simply delegate calls to an internally held plugin object,
e.g.

public class PluginWrapper : MarshalByRefObject, IPlugin
{
private Plugin innerPlugin;

public PluginWrapper(Plugin plugin) { innerPlugin = plugin; }
public SomePluginMethod() { innerPlugin.SomePluginMethod(); }
}

You would then pass the wrapper back to the main AppDomain so it could
invoke the plugin methods thru the wrapper.

Hope that helps -
Ken

[quoted text, click to view]

Ravichandran J.V.
9/21/2004 5:05:07 AM

Delegates do not marshal and should not be marshalled as marshalling of
delegates is not a successful way of implementing remoting. I have
presumed some points in the above answer, like, you are using delegates
to encapsulate methods and have created a factory of methods and using
just one instance etc.

Please do clarify.

with regards,

J.V.Ravichandran
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jvravichandran
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qry=Ravichandran+J.V.&cob=aspnetpro
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