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Not planning now to migrate your .NET 4.8 legacy, is certainly a mistake

2020 will see the achievement of the massive remodeling of the .NET platform initiated by Microsoft in November 2014 with the introduction of .NET Core 1, with the promise of an open-source, a multi-platform and a modernizable framework (thanks to no rock-solid backward compatibility constraint) – everything that the .NET Framework isn’t. This U-turn in the Microsoft plans for .NET is part of the new Microsoft’s strategy initiated by Satya Nadella that became CEO of Microsoft in February 2014, succeeding to Steve Ballmer.

.NET 5 will be released in November this year. Within 6 years Microsoft will have succeeded a complete platform shift like no other. The .NET Core brand was here to make clear that two .NET platforms were living side by side. But now we know that the .NET Framework 4.8 won’t evolve anymore and that all Microsoft efforts will be put on .NET Core continuation with well scheduled releases ahead. Let’s use the term .NET OSS to designate [.NET Core, .NET 5, .NET 6…. [ in the remainder of this post.

We can expect an early beta of .NET 5 before July 2020. Today in January 2020 the .NET 5.0 milestone is 72% achieved.

By now, the way to get prepared to .NET 5 and later is to migrate to .NET Core 3.1. Despite the branding change from .NET Core 3.1 to .NET 5 it is no mystery that .NET 5 will be mostly based on the actual .NET Core platform.

The cost of migration from .NET 4.8 to .NET OSS can get pretty high, especially if the legacy relies on some deprecated APIS (like WCF, WWF, WebForms or AppDomain). Thus it may seems attractive to stick with .NET 4.8 if your application is intended to run on Windows only.

Why it is not a good idea to not anticipate the migration now?

.NET 4.8 won’t evolve but some security patches will be provided for as long as we can foresee. However we can predict that .NET 4.8 will be quickly considered as a thing-of-the-past:

  • Developer mindset: .NET OSS and also C# will evolve. There will come a point where it’ll feel pretty awkward for .NET programmers working with 4.8 to not be able to use all the new goodies. Could you imagine programming with C#3 nowadays?
  • Third-Party Libraries: The .NET 4.8 / .NET OSS increasing gap will push open-sourced libraries authors toward .NET OSS. The cost of maintaining two code bases will be too high for an OSS developer. If your .NET 4.8 application consumes some OSS libraries, not migrating it will put you in an awkward situation where you’ll have to maintain the OSS code consumed yourself! Certainly serious commercial libraries will be maintained on both platforms for a longer period of time, but not forever.
  • Performance: We can expect more and more performance improvements with .NET OSS.
  • Tooling: Tools will continue to evolve and with time, less and less tool will support .NET 4.8 application.

Recently I’ve discussed with Jean-Baptiste Evain that develops the OSS library Cecil. Jb is also responsible for UnityVS at MS. Here at NDepend we’re relying on Cecil for more than a decade. Cecil processes compiled .NET assemblies bytes and thus will obviously benefit from Span<T> only available on .NET Core. This concrete situation illustrates well the points mentioned above:

  • By using Span<T> Jb is not enthusiast to have to maintain two versions of Cecil, one relying on Span<T> and the .NET 4.8 one.
  • Even though these two versions will co-exist because it is too early to discard the .NET 4.8 version of Cecil used by many serious projects, it is a matter of a few years until .NET 4.8 Cecil version gets deprecated.
  • Without Span<T> the .NET 4.8 version of Cecil will be slower.

Our case

NDepend is still running on .NET 4.8. NDepend is a CI tool, a standalone UI tool, an Azure DevOps extension and a Visual Studio extension. Developing an extension is a sensitive situation because we need to align our platform with the platform of the host. VS is such a massive application that I don’t expect it to run on .NET 5 in 2021. On the other hand VS is evolving so quickly nowadays that this possibility is not totally excluded. It is also possible that Microsoft takes an incremental approach and that the main VS process (devenv.exe) will remain on .NET Fx 4.8 for a while, while children processes run on .NET OSS (VS runs with quite a few children processes!).

At this point the reasonable move for us is to anticipate the migration to .NET OSS mostly by compiling as much code as possible against .NET Standard 2.0, supported by both .NET Fx 4.7.2+ and .NET Core. We also need to make sure that our WPF and Winforms code will be easily movable (which shouldn’t be a problem since most of the WPF/Winforms APIs are supported by .NET 3.1). We are also mulling over on having our own child process(es) but all the UI part must remain in the main VS process.

We also keep in mind that it will be tricky to support the future VS version running on .NET OSS and previous VS versions running on .NET v4.8 for a few years.


Those like us still working on a large .NET 4.8 legacy are entering into a turbulence zone for the years to come. However for all the reasons explained above, we can expect that in not so long (2023? 2025?) successful applications still running on .NET 4.8 will be the exception. Certainly not anticipating legacy migration now is likely a strategic mistake.

My dad being an early programmer in the 70's, I have been fortunate to switch from playing with Lego, to program my own micro-games, when I was still a kid. Since then I never stop programming.

I graduated in Mathematics and Software engineering. After a decade of C++ programming and consultancy, I got interested in the brand new .NET platform in 2002. I had the chance to write the best-seller book (in French) on .NET and C#, published by O'Reilly (> 15.000 copies) and also did manage some academic and professional courses on the platform and C#.

Over the years, I gained a passion for understanding structure and evolution of large complex real-world applications, and for talking with talented developers behind it. As a consequence, I got interested in static code analysis and started the project NDepend.

Today, with more than 8.000 client companies, including many of the Fortune 500 ones, NDepend offers deeper insight and understanding about their code bases to a wide range of professional users around the world.

I live with my wife and our twin babies Léna and Paul, in the beautiful island of Mauritius in the Indian Ocean.


  1. We have an application that uses WCF. What will be replacing WCF in the .Net Core world?

  2. Span is available in .NET 4.8 via a NuGet package, although it’s missing some of the deeper framework integration.

    The bigger issue for us is the complete lack of support for SQL Server Reporting Services in Core. No way to view or export a report, since the only library provided by MS uses WebForms.

  3. @Patrick – alas not of the options really provide replacement for WCF or WF or a host of other items. Yes, there are alternates that approximate some of the functionality, but when dealing with large (millions of lines of code) systems that have deep complexity and inter-system [e.g. mainframe] interactions, I have yet to find any viable “migration” route – only a basic scrap and redesign which would encompass man-decades of work.

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