Support for custom mutators

Is your feature request related to a problem? Please describe.

Follow up of https://github.com/stryker-mutator/stryker-net/issues/1561

There may be scenarios where the implemented mutators do not fit the application needs, nor the maintenance burden in the library justifies adding those mutators to core library. For that it would be nice to have some sort of plugin system: new mutators can be created by the community and used as opt-in.

The provided example is mutations for System.Linq.Expressions.Expression calls, which while might be a niche for most applications, they can be a must have in other scenarios.

Describe the solution you’d like

Ideas on top of the table:

• Lookup assemblies with types implementing IMutator interface.
• tbd

Describe alternatives you’ve considered

Additional context

[rouke-broersma]

I suggest a config option with custom mutator names (to add to default mutators) and searching in the project output folders (bin/debug) for dll’s matching a predefined format. This is how vstest for example also works (all vstest adapters must end in .testadapter.dll). As long as the dll is referenced in one of the projects such as the test project the dll will be copied to the output folder and will be available for us to load.

This would mean users can take advantage of nuget to distribute their custom mutators.

[dupdob]

Having add-in mutations could help corner cases, but could also be interesting (libraries could distribute their specific mutators). Bur, in order to avoid having to deal with the dependencies, I would suggest either:

• reduce or remove the IMutator dependencies with the rest of Stryker
• publishing the needed dependencies for mutators in a dedicated, very stable assembly.

Currently, the dependencies are IStrykerOptions and Mutation. I think Mutation could be replaced by a tuple of some sort. The problem is with IStrykerOptions, which is often updated. Any way, dealing with options for custom mutators requires a specific design.

Another solution could be to have a dedicated/simplified signature for custom mutators. This may be a good idea for a beta phase.