Why does the assembly reference gets loaded successfully when using MEF?

Question

Contrary to the usual issues of MEF users, I'm not trying to figure out why a dependency of an assembly I load with MEF is not resolved. I am solving the opposite problem - trying to find out why the dependency is loaded.

The situation is that I have an ASP.NET application which uses MEF to dynamically load extension assemblies residing outside the application directory. There may be several versions of the same extension assembly in different directories I probe with MEF, each with its own set of dependencies and these dependencies again may be multiple versions of a single assembly (i.e. extension A.dll, 1.0.0.0 has a dependency D.dll, 1.0.0.0 and an extension B.dll, 2.0.0.0 has a dependency D.dll, 2.0.0.0).

It works in a sense that all the extensions and all their dependencies are loaded correctly. And I don't know why. I have no special settings in my web.config file. Where in the official documentation on assembly resolving does it say that it should work? It doesn't say anything about looking at the same directory the requesting assembly is located at.

I was hoping the Fusion Log Viewer would tell me how the runtime locates the dependencies but even though it shows me the binding requests for all the different versions of a given dependency, when I click on it to see the full log, I only see failed bindings (and yes, I have it set to log all binding requests, not just the failed ones).

Can anyone point me to the part of an official documentation explaining why this scenario works?

Answer 1

The main resolution with MEF is determined by the ComposablePartCatalog used. For example, if you use DirectoryCatalog, all assemblies within the specified directory get probed and found.

By looking at the CompositionContainer, you should be able to find the catalogs that are in place, which in turn should provide some details as to why the parts are composing properly.

It is also possible that your dependencies are being found due to the rules defined in "Other Locations Probed":

If an assembly uses the LoadFrom method to reference another assembly, the calling assembly's location is considered to be a hint about where to find the referenced assembly. If a match is found, that assembly is loaded.

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If you are using a DirectoryCatalog to find the assemblies within a directory, it internally loads every assembly within that folder. This will cause all of your dependencies to also be loaded into the process when the DirectoryCatalog is constructed.