Make the dependency injection thread safe

[Greybird]

Hi,

I just wanted to share this code, which I developed for our company to solve issue SpecFlowOSS/SpecFlow#657 from Specflow, using NUnit runner along with an Unity 3 plugin.
This change to make the dependency injection thread-safe allowed us to parallellize at the feature level without any issue, while it was constantly failing before the fix.

Of course, the fix is pretty crude, as its aim was to have no public surface change, but I though it was better to share it if it could be of use.

[gasparnagy]

@Greybird Thx. Unfortunately the diff cannot show the differences properly here, so it is hard to get an overview. Could you please summarize the change in a few sentences?

[Greybird]

Yes sure!

Basically

• features from the ObjectContainer are moved to the ObjectContainerUnsafe, Unsafe meaning not thread-safe
• a new ObjectContainer class wraps an instance of the ObjectContainerUnsafe and delegates calls to it, after locking.

This design was chosen because it allowed:

• in-place replacement for SpecFlow usage using a binding-redirect
• locking in the interface implementation methods, without having to consider actual inner workings of the previously-named ObjectContainer.

Of course, the key is locking, and a simpler design could be used provided depending libraries code can be modified.

The goal of this PR is mainly to indicate that the SpecFlow issue reported is actually solvable through proper locking on the DI.

Hope this helps!

Arnaud