Default Interface Implementations in C#: Where Inheritance Goes to Troll You

Default interface implementations in C# let interfaces ship methods with default bodies, which helps API evolution but can hide tricky behavior when inheritance enters the picture. The article shows a service that returns the interface default after a base class is added, even though the concrete class still defines the method.

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• Default interface implementations were introduced in C# 8.0 and let interfaces define methods with default bodies.
• The feature helps add methods without breaking existing implementations and improves interoperation with platforms like Android and iOS.
• The article focuses on a case where this behavior becomes subtle when combined with inheritance and dependency injection.
• A service named MyService receives an IFoo and calls GetValue() to print the result.
• IFoo provides a default GetValue() that returns "IFoo".
• Foo defines its own GetValue() that returns "Foo", and a unit test against Foo passes.
• The console app also prints "Foo" before any refactor.
• After introducing FooBase : IFoo and making Foo inherit from it, the tests still pass but the program output changes to "IFoo".
• The explanation given is that the CLR builds a method table for type resolution and follows the inheritance chain when resolving interface calls.
• Once Foo no longer directly implements IFoo, the runtime looks to FooBase, finds no GetValue() there, and falls back to the interface default implementation.
• The article recommends understanding how default methods behave with inheritance and avoiding unnecessary inheritance.
• It suggests preferring composition over inheritance when shared logic is needed.
• It also recommends explicitly declaring the interface on derived classes and providing overrides when needed.
• The article advises testing through the interface instead of relying on a concrete class or var.
• It mentions static analysis tools such as Roslyn analyzers and SonarAnalyzer for .NET.
• It ends by recommending documentation and careful review for changes involving default methods.