What is an interface in C#?
PICTURE THIS: DATA SPLIT
Fit on train, tune on val, report on test once.
Simple meaning
An interface defines a contract of members a type must implement.
WHY — OOP instead of guessing?
Why interviewers care about OOP:
who only read docs from people who shipped.
and tied to .NET work.
Name the idea, why it exists, then one short example.
End with when you use it and one common pitfall.
STEPS — What happens step by step?
Before you speak the answer, walk the interviewer through these steps:
- 1An interface defines a
contract of members a type must implement.
- 2It enables polymorphism and
testing with mocks.
- 3How it works
Classes can implement many interfaces.
- 4Give an example
One tiny concrete case you can say aloud.
- 5Common mistake
What juniors usually get wrong.
- 6Close
When you pick this over the alternative.
EXAMPLE — See it in action
Here's a short line you can speak, broken into clear beats:
Note: Adapt this scaffold to your own project — keep it under 60–90 seconds.
Key takeaway
An interface defines a contract of members a type must implement. It enables polymorphism and testing with mocks.