Tip on Debugging Stable Composition Problems in MEF

My debugging strategy is to develop a hypothesis or two, pick the easiest most valuable one to test, test it in the easiest manner, and go on. I’m working to disprove the hypothesis because this is far easier to do and helps avoid me getting stuck on a particular notion of what is wrong. A test is more valuable if it cuts out a greater portion of the possibilities for failure, which John Robbins calls the divide and conquer approach.

I bring this up to explain that I rarely use the reporting MEF provides to solve stable composition errors. Maybe one day I will, but its much faster for me to find the missing part and begin adding the “AllowDefault=true” parameter to the Import attribute. I’m not being chaotic or random, it just works for me.

This morning I remained a bit stumped by a stable composition error. I realized it was a stable composition error because it went away when I said “AllowDefault=true”. But, my own reporting and other tests showed that the implementation was clearly in the container. So, what was going on.

A passing hypothesis flitted through my head “what would happen if there were two?”

It was a quick test to put a break point right after composition competed and request the exports for the part. Voila there were two!

Two tips:

- Stable composition fails when cardinality is not correct. A big word say Import fails when there are zero or more than one match.

- If you have multiple discovery directories, it is very easy for common assemblies to wind up in more than one directory. This results in multiple matches, and why I had the cardinality failure.

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