Awesome Open Source
Awesome Open Source


Instrumenting coverage tool for .net (framework 2.0+ and core) and Mono, reimplemented and extended almost beyond recognition from dot-net-coverage, plus a set of related utilities for processing the results from this and from other programs producing similar output formats.

Never mind the fluff -- how do I get started?

Start with the Quick Start guide

The latest releases can be downloaded from releases, but the easiest (and most automated) way is through the nuget packages.

What's in the box?

For Mono, .net framework and .net core, except as noted

  • AltCover, a command-line tool for recording code coverage (including dotnet and global tool versions)
  • MSBuild tasks to drive the tool, including dotnet test integration
  • An API for the above functionality, with Fake and Cake integration
  • A PowerShell module for PowerShell 5.1 and PowerShell Core 6+ containing a cmdlet that drives the tool, and other cmdlets for manipulating coverage reports
  • A coverage visualizer tool

NuGet Packages

Why altcover?

As the name suggests, it's an alternative coverage approach. Rather than working by hooking the .net profiling API at run-time, it works by weaving the same sort of extra IL into the assemblies of interest ahead of execution. This means that it should work pretty much everywhere, whatever your platform, so long as the executing process has write access to the results file. You can even mix-and-match between platforms used to instrument and those under test.

In particular, while instrumenting .net core assemblies "just works" with this approach, it also supports Mono, as long as suitable .mdb (or .pdb, in recent versions) symbols are available. One major limitation here is that the .mdb format only stores the start location in the source of any code sequence point, and not the end; consequently any nicely coloured reports that take that information into account may show a bit strangely.

Why altcover? -- the back-story of why it was ever a thing

Back in 2010, the new .net version finally removed the deprecated profiling APIs that the free NCover 1.5.x series relied upon. The first version of AltCover was written to both fill a gap in functionality, and to give me an excuse for a ground-up F# project to work on. As such, it saw real production use for about a year and a half, until OpenCover reached a point where it could be used for .net4/x64 work (and I could find time to adapt everything downstream that consumed NCover format input).

Fast forwards to autumn 2017, and I get the chance to dust the project off, with the intention of saying that it worked on Mono, too -- and realise that it's déja vu all over again, because .net core didn't yet have profiler based coverage tools either, and the same approach would work there as well.

Continuous Integration

Build GitHub Build statusBuild history AppVeyor Build status Build history
Test coverage Coveralls Coverage Status AppVeyor Test status


See the Wiki page for details


See the current project and long term research items for details; though ad hoc items not in the projects will get added as inspiration or need arise.

All To do and On Hold items are implicitly up for grabs and Help Wanted; most of the current project items are XML manipulation or GUI programming.

Possible retirement/obsolescence of support

tl;dr -- legacy framework/Mono support is not going away any time soon.

Despite earlier ruminations on the subject, as .net 4.7.2 can consume netstandard2.0 libraries (everything but the recorder), and .net core 2+ can consume net20 libraries (the recorder), legacy framework/Mono support continues after the release of .net 5 and until such a time as it is no longer possible to retain those API levels. Framework builds apart from the minimum (executable entry-points and the recorder) remain until I have suitable replacements for Framework-only static analysis tooling (i.e. can convince FxCop to consume netstandard20).



All platforms

It is assumed that the following are available

.net SDK version as per global.json, or later minor version (dotnet) -- try
PowerShell Core 7.1.0 or later (pwsh) -- try

The build may target netstandard2.0 or netcoreapp2.0/2.1 for deliverables, and net5.0 for unit tests, but does not need any pre-5.0 runtimes to be installed (roll-forward policies are in place).

Note: F# compiler code generation changes may cause incompatibilities due to some of the IL inspection performed by AltCover and its self-tests (e.g. by, at 5.0.201, generating non-closure function objects as static fields rather than locally instantiated objects)


You will need Visual Studio VS2019 (Community Edition) v16.10.3 or later with F# language support (or just the associated build tools and your editor of choice). The NUnit3 Test Runner will simplify the basic in-IDE development cycle.

For GTK# support, the GTK# latest 2.12 install is expected -- try -- while the latest releases of the GTK#3 libraries will download the native support if the expected version is not detected.


It is assumed that mono (version 6.12.x or later) and dotnet are on the PATH already, and everything is built from the command line, with your favourite editor used for coding.


Start by setting up with dotnet tool restore; this sets up local tools including dotnet fake. Then dotnet fake run ./Build/setup.fsx to do the rest of the set-up.

Normal builds

Running dotnet fake run ./Build/build.fsx performs a full build/test/package process.

Use dotnet fake run ./Build/build.fsx --target <targetname> to run to a specific target.

If the build fails

If there's a passing build on the CI servers for this commit, then it's likely to be one of the intermittent build failures that can arise from the tooling used. The standard remedy is to try again.

Unit Tests

The tests in the AltCover.Test project are broadly ordered in the same dependency order as the code within the AltCover project (the later Runner tests aside). While working on any given layer, it would make sense to comment out all the tests for later files so as to show what is and isn't being covered by explicit testing, rather than merely being cascaded through.

Note that some of the unit tests expect that the separate build of test assemblies under Mono, full .net framework and .net core has taken place; these tests will be marked ignore when running the unit tests under net472 and pass without doing anything under net5.0 (as Expecto has no ignore option) if the build is not complete and thus those expected assemblies are not found e.g. in Visual Studio from clean, or after a build to targets like Analysis that only build code to be analysed.

Thanks to

  • AppVeyor for allowing free build CI services for Open Source projects
  • Coveralls for allowing free services for Open Source projects

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