Concept questionMedium~7 min

Interview question

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.

1Say itAnswer out loud first

2CompareCompare with Core answer

3RefineRefine with Stronger and Advanced

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.

Full answer below

Practice focus

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

Answer first

Answer path

Core answer first, then checkpoints toward a stronger answer.

Start
Core answer
Start here. This is the direct spoken answer to practice first.
Why this question matters
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.
Checkpoint
Stronger answer
Optional upgrade with examples, trade-offs, and sharper phrasing.
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.
Finish
Advanced answer
Optional deeper angle for production judgment and nuance.
In production I consider compatibility and ownership. Adding an abstract member breaks every derived type, while adding a virtual member with a default may preserve compilation but alter behavior. Public base classes are difficult to evolve because external subclasses may depend on protected internals. I keep inheritance hierarchies shallow, make extension points explicit, and write contract tests for all implementations. If the relationship is only code reuse, I choose composition and keep the consumer-facing interface small.

Review signals

Use these to catch weak phrasing, shallow reasoning, and risky claims.

Common mistakesCandidateCommon mistakesMistakes that weaken the answer or signal shallow understanding.Open details

Details

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.

Red flagsCandidateSigns the object design is fragileSignals that an interviewer may treat as risky.Open details

Details

Derived classes depend on undocumented call order.
A base-class change breaks unrelated consumers.
The relationship does not represent a stable is-a concept.
Callers require concrete-type checks after using the abstraction.

Follow-up prompts

Quick interviewer prompts inside this drill.

Follow-up questionsCandidateFollow-up questionsLikely next questions after the main answer.Open details

Details

Treat these as the next interviewer turn. Answer one out loud, then save the gap in your practice notes.

What do default interface methods change?
Why is public inheritance hard to version?
When is composition better for code reuse?

Continue practice

No explicit flow link is set yet, so these are nearby drills from the same path or taxonomy.

Abstraction vs encapsulationConcept question / Easy / ~5 minContinue Access modifiersConcept question / Easy / ~5 minContinue ActionResult vs Typed ResultsComparison question / Medium / ~7 minContinue

Concept questionMedium~7 min

Interview question

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.

1Say itAnswer out loud first

2CompareCompare with Core answer

3RefineRefine with Stronger and Advanced

TL;DR

Full answer below

Practice focus

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

Answer first

Answer path

Core answer first, then checkpoints toward a stronger answer.

Start
Core answer
Start here. This is the direct spoken answer to practice first.
Why this question matters
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.
Checkpoint
Stronger answer
Optional upgrade with examples, trade-offs, and sharper phrasing.
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.
Finish
Advanced answer
Optional deeper angle for production judgment and nuance.
In production I consider compatibility and ownership. Adding an abstract member breaks every derived type, while adding a virtual member with a default may preserve compilation but alter behavior. Public base classes are difficult to evolve because external subclasses may depend on protected internals. I keep inheritance hierarchies shallow, make extension points explicit, and write contract tests for all implementations. If the relationship is only code reuse, I choose composition and keep the consumer-facing interface small.

Review signals

Use these to catch weak phrasing, shallow reasoning, and risky claims.

Common mistakesCandidateCommon mistakesMistakes that weaken the answer or signal shallow understanding.Open details

Details

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.

Red flagsCandidateSigns the object design is fragileSignals that an interviewer may treat as risky.Open details

Details

Derived classes depend on undocumented call order.
A base-class change breaks unrelated consumers.
The relationship does not represent a stable is-a concept.
Callers require concrete-type checks after using the abstraction.

Follow-up prompts

Quick interviewer prompts inside this drill.

Follow-up questionsCandidateFollow-up questionsLikely next questions after the main answer.Open details

Details

Treat these as the next interviewer turn. Answer one out loud, then save the gap in your practice notes.

What do default interface methods change?
Why is public inheritance hard to version?
When is composition better for code reuse?

Continue practice

No explicit flow link is set yet, so these are nearby drills from the same path or taxonomy.

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

Practice focus

Answer path

Why this question matters

Review signals

Follow-up prompts

Continue practice

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

Practice focus

Answer path

Why this question matters

Review signals

Follow-up prompts

Continue practice