A code coverage library, used for Test Impact Analysis (or Continuous Testing) in our internal testing framework, as a replacement for vstest's inefficient data collectors.

SG Coverage works by weaving IL codes into project's assemblies using its Instrumenter type in the SG.CodeCoverage project. It injects a AddHit(int typeId, int methodId) method call that counts number of times the method visited, and creates a json map file that maps id of assemblies, types and their methods to their file paths, during this process.

The AddHit(int, int) method resides in the SG.CodeCoverage.Recorder project, which is an auto generated assembly for each instrumentation phase (inspired by AltCover, Coverlet and MiniCover) and holds the total number of types instrumented and an in-memory visit counter.

1. Instrument assemblies using Instrumenter class. This process outputs a json map file.
2. When any method in the project is called, instrumented assemblies make sure that the RecordingControllerServer is running and collecting data, and then sends the hit records to the server.
3. After running each test, you can call RecorderControllerClient.SaveHitsAndReset(string outputPath). This will send a request to server to save the visited types and methods as a binary file like hits.test1.bin, then resets the counter.
4. After collecting test hits, you can find the visited files by merging the json map file and the binary hit file. This can be easily done using DataCollector.GetVisitedFiles method which does this task.

This is not exactly a sequence diagram, but helps you get the idea, and as they say: "a picture worth a thousand words":