问题
I understand that a MEF CompositionContainer creates and keeps instances of classes. I don't know under what circumstances a CompositionContainer has a class instance in its bowels.
- Can anybody list operations performed on the CompositionContainer or methods of the CompositionContainer class that cause the CompositionContainer to store an instance of a class within the CompositionContainer.
- Is it possible to view class instances held within a CompositionContainer in the debugger or any other fashion?
回答1:
The CompositionContainer will keep references to all shared parts for the lifetime of the CompositionContainer. (The default CreationPolicy is Any for both imports and exports, which means by default all parts will be shared unless otherwise specified.)
References to NonShared parts will be kept if the part implements IDisposable. The reference will be released when the root export that was pulled from the container is released (if that export was from a NonShared part). Exports can be released either by calling CompositionContainer.ReleaseExport, or ExportLifetimeContext.Dispose for exports created with an ExportFactory.
I don't think there's any simple way to view what's held by the CompositionContainer. The source code is available so you could theoretically dive into it and figure out exactly where it's stored.
回答2:
In regards to your second question (#2 above) ...
Using the QuickWatch Window (Shift + F9) or a regular Watch window, copy the following in there:
((System.ComponentModel.Composition.Hosting.CompositionContainer)(this.Container))._catalogExportProvider._activatedParts
The line above assumes that the object you are stopped on has a "this.Container" property, which is the CompositionContainer of the scope you are referring to.
From there, you'll get an array of ActivatedParts. You then navigate the dictionary of Parts. Find the Part Definition you want to find the instance of, and expand its "Non-Public Members". There you will find the CachedInstance, and this will be the instance of your "Shared" exported part that's been created.
I think Parts that are exported NonShared and not IDisposable aren't cached or held onto at all. At least that's the behavior I've seen.
来源：https://stackoverflow.com/questions/5633774/instance-of-class-held-by-composition-container-in-mef