Instructions on how to test dev versions of coverlet against CoreFx

Author: petli

README.md

@@ -434,12 +434,39 @@ These steps must be followed before you attempt to open the solution in an IDE (
 
 ### Performance testing
 
-There is a performance test for the hit counting instrumentation in the test project `coverlet.core.performancetest`.  Build the project with the msbuild step above and then run:
+There is a simple performance test for the hit counting instrumentation in the test project `coverlet.core.performancetest`.  Build the project with the msbuild step above and then run:
 
     dotnet test /p:CollectCoverage=true test/coverlet.core.performancetest/
 
 The duration of the test can be tweaked by changing the number of iterations in the `[InlineData]` in the `PerformanceTest` class.
 
+For more realistic testing it is recommended to try out any changes to the hit counting code paths on large, realistic projects.  If you don't have any handy https://github.com/dotnet/corefx is an excellent candidate.  [This page](https://github.com/dotnet/corefx/blob/master/Documentation/building/code-coverage.md) describes how to run code coverage tests for both the full solution and for individual projects with coverlet from nuget.  Suitable projects (listed in order of escalating test durations):
+
+* System.Collections.Concurrent.Tests
+* System.Collections.Tests
+* System.Reflection.Metadata.Tests
+* System.Xml.Linq.Events.Tests
+* System.Runtime.Serialization.Formatters.Tests
+
+Change to the directory of the library and run the msbuild code coverage command:
+
+    dotnet msbuild /t:BuildAndTest /p:Coverage=true
+
+Look for a line like this:
+
+    ----- start 18:13:36,59 ===============  To repro directly: =====================================================
+
+It is followed by a `pushd` command into the artefact directory, and a command to run coverlet.  Run these to get to see the coverlet output and especially the test duration. E.g.:
+
+    C:\...\corefx\artifacts\tools\coverlet "System.Collections.Concurrent.Tests.dll" --target ...
+
+To run with a development version of coverlet call `dotnet run` instead of the installed coverlet version, e.g.:
+
+    dotnet run -p C:\...\coverlet\src\coverlet.console\coverlet.console.csproj -c Release -- "System.Collections.Concurrent.Tests.dll" --target ...
+
+
+
+
 
 ## Code of Conduct