Where to start... Mutation testing is described as modifying a program in small amounts and then executing the original 'passing' tests that exercise that code and then watching them fail. It is a way of making sure your tests are actually testing what you believe they are testing.
Setting the stage... So how can we do this with .NET? Well first we need to know what tests execute what code and we can use OpenCover for that when it is using it's tracking by test feature. With that feature we can see which tests execute which sequence points and also see what branches were exercised, it is this later information we can take advantage of when creating a mutation testing utility.
New toys to play with... Now this mutation tester is going to be working at the IL level and as such we could use the JIT (Just-in-time) compilation feature that is used with OpenCover (and PartCover). However that would mean a complicated instrumentation that we would then have to control which path we would want to exercise, or we could have simpler instrumentation but that would require the process under test (e.g. nunit, mstest, ...) to be stop and started each time to allow new code to be exercised. With .NET 4.5 (in preview at the time of writing) there is a re-JIT compilation feature that we could use instead and this would allow us to use simple instrumentation without needing to stop and start the process under test. There are a number of limitations of re-JIT but after reviewing them (several times) I don't think any are actually show stoppers.
However to make the Re-JIT useful we need a way of executing a test or tests repeatedly without having to restart the application under test and this isn't possible with nunit and mstest. However it should be possible to use the test runners from AutoTest.Net if we host them directly or in a separate process that can be communicated with.
A plan... So the flow will be something like this (I wonder how will this will stand up to the test of time) I haven't looked at the latest profiler API in-depth but documentation on MSDN) and David Broman's Blog seem to indicate this should be possible.
- Run OpenCover to produce an XML file with a list of what tests exercised what branches
- For each branch point =>All it needs is a name...All of this will be hosted on GitHub under OpenMutate. Let the games begin....