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Find API Breaking Changes in your .NET Libraries and Frameworks

If you are developing a framework, the last thing you want to happen when releasing a new version of your product is to break the code of your clients because of an API breaking change. For example, you want to make sure that all public methods you had in previous versions are here in the next version, unless you tagged some of them with System.ObsoleteAttribute for a while before deprecation.

Let’s underline that we are talking here of syntactic breaking changes that can be all detected with tooling as we are about to see. On the other hand semantic breaking changes are code behavior changes that will break your client code. Those breaking changes cannot be found by a tool, but are typically caught by a solid test suite.

One key feature of NDepend is to be able to compare all results against a baseline snapshot. Code querying can explore changes since the baseline. For example this code query detects methods of an API that are not anymore visible from clients:

This query code is pretty simple and readable but let’s detail it a bit:

  • The warnif count > 0 prefix transforms this code query into a code rule
  • We iterate on methods in the baseline thanks to codeBase.OlderVersion().Application.Methods
  • We want methods that were publicly visible in the baseline and not just public. Being declared as public is not enough for a method to be publicly visible. A public method can still be declared in a class declared as internal.
  • We don’t warn for public methods that were deemed as obsolete in the baseline
  • We handle both situations: A) publicly visible methods removed since the baseline and B) publicly visible methods not publicly visible anymore.

This query is pretty similar than the source of the default rule API Breaking Changes: Methods that is a bit more sophisticated to handle more situations, like when the public method return type changes. This rule also presents results in a polished way.

Similar rules are available for publicly visible types and publicly visible fields.

Some other sorts of breaking changes are detected when for example, a publicly visible interface or base class changes: in such situation clients code that implement such interface or derive from such abstract class will be broken.

Some rules also detect when a serializable types gets broken and when an enumeration Flags status change.

Real Experiments

From comparing NHibernate v5.2.x against NHibernate v4.1.x here are breaking changes found: 36 types, 995 methods, 205 fields (including enumeration values) and 121 interfaces or abstract classes have been changed. I guess most of those breaking changes are about elements that were not intended to be seen by API consumers in the baseline version but a detailed analysis would be needed here.

NHibernate v5.2 vs v4.1 Breaking Changes

I’ve also compared .NET Core v2.2.5 with .NET Core v3.0 Preview 8 but hopefully didn’t find any breaking change. However NDepend has an heuristic code query to find types moved from one namespace or assembly to another and here more than 200 types were matched like the class ArrayList moved from the assembly System.Collections.NonGeneric.dll to the assembly System.Private.CoreLib.dll. Hopefully such changes are invisible to the clients consuming .NET Core as a NuGet package.

ArrayList moved from System.Collections.NonGeneric.dll to System.Private.CoreLib.dll

Find new APIs consumed and APIs not used anymore

A related and interesting topic is to review new APIs used by an application or APIs not used anymore. This is possible from the NDepend > Search Elements by Changes panel with the buttons Third Party Code Elements – Used Recently / Not Used Anymore. For example the screenshot below shows new API consumed by the application eShopOnWeb:

New APIs used by eShopOnWeb

Before releasing a new version, this is interesting to have a glance at API consumption changes. Doing so often sheds light on interesting points.

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 and also did manage some academic and professional courses on the platform and C#.

Over my consulting years I built an expertise about the architecture, the evolution and the maintenance challenges of large & complex real-world applications. It seemed like the spaghetti & entangled monolithic legacy concerned every sufficiently large team. As a consequence, I got interested in static code analysis and started the project NDepend.

Today, with more than 12.000 client companies, including many of the Fortune 500 ones, NDepend offers deeper insight and full control on their application to a wide range of professional users around the world.

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


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