What is an Interface in C#?
💡 Concept: Interface
An interface in C# defines a contract that classes must adhere to. It contains only declarations of members—such as methods, properties, and events—without implementing any behavior (except default methods from C# 8.0+).
📘 Quick Intro
Interfaces enable polymorphism and code decoupling. A class can implement multiple interfaces, making them ideal for designing flexible systems, especially in testing, DI, and SOLID architecture.
🧠 Analogy
Imagine an interface as a job description—it lists what a person must be able to do (skills), but not how they do it. Different candidates (classes) can fulfill the same job (interface) in their unique ways.
🔧 Technical Explanation
- Declared using the
interfacekeyword. - Cannot contain fields or constructors.
- All members are public and abstract by default (until C# 8+).
- A class can implement multiple interfaces.
- Interfaces support default implementations (C# 8.0+).
🎯 Use Cases
- Defining shared behavior across unrelated classes.
- Creating plug-and-play designs that rely on abstraction.
- Enabling mocking and testing via dependency injection.
- Applying SOLID principles, especially Interface Segregation.
💻 Real Code Example
public interface IShape {
double GetArea();
}
public class Circle : IShape {
public double Radius { get; set; }
public double GetArea() => Math.PI * Radius * Radius;
}
public class Square : IShape {
public double Side { get; set; }
public double GetArea() => Side * Side;
}
❓ Interview Q&A
Q1: What is the purpose of an interface?
A: To define a contract of methods/properties a class must implement.
Q2: Can an interface contain fields?
A: No, only declarations (and default implementations from C# 8+).
Q3: Can a class implement multiple interfaces?
A: Yes, unlike classes, multiple interface inheritance is allowed.
Q4: Are interface methods public by default?
A: Yes, and cannot be made private.
Q5: Can an interface be instantiated?
A: No, only classes that implement it can be.
Q6: What is the use of interfaces in DI?
A: It allows substituting implementations easily for testing/mocking.
Q7: Can interfaces extend other interfaces?
A: Yes, using inheritance like: interface B : A.
Q8: Can interfaces have static members?
A: C# 8+ allows static members in interfaces.
Q9: Can interface methods be implemented directly?
A: No, unless using default implementation from C# 8+.
Q10: What is the difference between interface and abstract class?
A: Interfaces can’t have fields and allow multiple implementations; abstract classes provide partial implementation and allow fields.
📝 MCQs
Q1. What does an interface define?
- A constructor
- A class
- A contract
- A data type
Q2. Which keyword defines an interface?
- class
- interface
- contract
- struct
Q3. Can a class implement multiple interfaces?
- No
- Yes
- Only if sealed
- Only if static
Q4. Are interface members public by default?
- No
- Yes
- Private
- Internal
Q5. What version added default implementations to interfaces?
- C# 6.0
- C# 7.0
- C# 8.0
- C# 9.0
Q6. Can an interface contain fields?
- Yes
- Only constants
- No
- Only readonly
Q7. What is the use of interfaces in testing?
- Slows performance
- Restricts access
- Allows mocking
- Creates hierarchy
Q8. Can interfaces extend other interfaces?
- No
- Yes
- Only one
- Only abstract ones
Q9. Which is true about interface instantiation?
- Can be instantiated
- Only once
- Cannot be instantiated
- Instantiated with 'new'
Q10. Which is a key benefit of interfaces?
- Fields in interface
- Encapsulation
- Multiple inheritance
- Internal usage only
💡 Bonus Insight
Interfaces are essential for clean architecture and test-driven development. By abstracting functionality behind contracts, your application becomes easier to extend, refactor, and maintain.
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