When would you use an interface instead of an abstract base class in C#, and when is a base class the better choice?

Choose an interface or abstract base class in C#

Chooses contracts and shared implementation based on substitutability, state ownership, evolution, and inheritance constraints.

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TL;DR

I use an interface for a capability contract and an abstract base class for a genuine hierarchy with shared state or implementation. A payment port or clock is usually an interface because the application needs behavior without shared object state. A framework template with a stable algorithm and carefully chosen extension points may justify an abstract base class. C# allows a type to implement several interfaces but inherit from only one class, so choosing a base class consumes an important design option.

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Contracts, shared implementation, state, constructors, multiple interfaces, inheritance, default methods, and versioning.

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I use an interface when consumers need a capability contract that unrelated types can implement. I use an abstract base class when derived types share a genuine identity, protected state, lifecycle, and implementation that belongs in one inheritance hierarchy.
I use an interface for a capability contract and an abstract base class for a genuine hierarchy with shared state or implementation. A payment port or clock is usually an interface because the application needs behavior without shared object state. A framework template with a stable algorithm and carefully chosen extension points may justify an abstract base class. C# allows a type to implement several interfaces but inherit from only one class, so choosing a base class consumes an important design option.
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I do not use a base class only to share a few helper methods; composition or a separate collaborator is often clearer. Interface default implementations can evolve small behavior, but they do not turn an interface into a stateful base type. A base class must document constructor requirements, protected members, virtual behavior, and invariants. Derived classes can otherwise become tightly coupled to details that callers never see.

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Choosing a base class only to reuse helper code.
Putting mutable shared state on an interface abstraction.
Creating deep inheritance hierarchies.
Making every base member protected and virtual.

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