Interface as contract mental model in C Sharp (C#) - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an interface in C#?

An interface in C# is like a contract that defines a set of methods and properties without implementing them. Classes that use the interface promise to implement those methods and properties.

beginner

Why do we say an interface acts as a contract?

Because it sets clear rules (methods and properties) that any class implementing it must follow, ensuring consistent behavior across different classes.

intermediate

Can a class implement multiple interfaces in C#?

Yes, a class can implement multiple interfaces, meaning it agrees to follow multiple contracts at the same time.

intermediate

What happens if a class does not implement all members of an interface it declares?

The C# compiler will give an error because the class breaks the contract by not providing all required methods or properties.

intermediate

How does using interfaces improve code flexibility?

Interfaces allow different classes to be used interchangeably if they follow the same contract, making code easier to extend and maintain.

What does an interface in C# define?

A A class with full method implementations

B A variable type

C A set of method and property signatures without implementation

D A method that runs automatically

An interface only defines method and property signatures, acting as a contract without implementation.

If a class implements an interface, what must it do?

A Implement all methods and properties declared in the interface

B Ignore the interface methods

C Only implement some methods

D Inherit from another class

Implementing an interface means the class must provide code for all its methods and properties.

Can a class implement more than one interface in C#?

A Yes, it can implement multiple interfaces

B No, only one interface is allowed

C Only if interfaces are related

D Only if the class is abstract

C# allows a class to implement multiple interfaces, combining multiple contracts.

What is the main benefit of using interfaces?

A They replace classes

B They store data

C They speed up program execution

D They allow different classes to be used interchangeably

Interfaces enable polymorphism by allowing different classes to follow the same contract and be used interchangeably.

What error occurs if a class does not implement all interface members?

A Warning only

B Compiler error

C No error

D Runtime error

The compiler enforces the contract and will raise an error if the class does not implement all required members.

Explain the idea of an interface as a contract in C# using a real-life example.

Think about agreements you make in daily life that require you to do certain things.

Describe how interfaces help make code more flexible and easier to maintain.

Consider how different devices can use the same charger type because they follow the same standard.

Practice

1.

What does an interface in C# represent?

easy

A. A contract that defines methods a class must implement

B. A class that contains method implementations

C. A variable type that stores data

D. A method that runs automatically

Solution

1. Step 1: Understand the role of an interface

   An interface defines a set of method signatures without implementations.

2. Step 2: Compare with classes

   Classes implement interfaces by providing method bodies, fulfilling the contract.

3. Final Answer:

   A contract that defines methods a class must implement -> Option A

4. Quick Check:

   Interface = contract for methods [OK]

Hint: Interfaces define method rules, not code bodies [OK]

Common Mistakes:

• Thinking interfaces contain method code
• Confusing interfaces with classes
• Believing interfaces store data

2.

Which of the following is the correct way to declare an interface in C#?

?

easy

A. interface IAnimal { void Speak(); }

B. class IAnimal { void Speak(); }

C. interface IAnimal() { void Speak(); }

D. interface IAnimal { void Speak() {} }

Solution

1. Step 1: Check interface declaration syntax

   Interfaces use the keyword 'interface' followed by the name and method signatures without bodies.

2. Step 2: Identify correct method signature

   Method declarations in interfaces do not have bodies, so no curly braces after method.

3. Final Answer:

   interface IAnimal { void Speak(); } -> Option A

4. Quick Check:

   Interface syntax = keyword + method signatures [OK]

Hint: Interfaces have method signatures only, no bodies [OK]

Common Mistakes:

• Adding parentheses after interface name
• Using class keyword instead of interface
• Providing method bodies inside interface

3.

What will be the output of the following code?

interface IWorker { void Work(); }
class Employee : IWorker {
  public void Work() { Console.WriteLine("Employee working"); }
}
class Robot : IWorker {
  public void Work() { Console.WriteLine("Robot working"); }
}
class Program {
  static void Main() {
    IWorker w = new Robot();
    w.Work();
  }
}

medium

A. No output

B. Employee working

C. Compilation error

D. Robot working

Solution

1. Step 1: Identify the object type assigned to interface variable

   The variable 'w' is of type IWorker but assigned a new Robot instance.

2. Step 2: Determine which Work() method runs

   Calling w.Work() runs Robot's Work method, printing "Robot working".

3. Final Answer:

   Robot working -> Option D

4. Quick Check:

   Interface variable calls actual object's method [OK]

Hint: Interface calls method of assigned object's class [OK]

Common Mistakes:

• Assuming interface variable calls Employee method
• Expecting compilation error due to interface
• Thinking no output will print

4.

Identify the error in this code snippet:

interface IShape {
  double Area();
}
class Circle : IShape {
  public double Area() {
    return 3.14 * radius * radius;
  }
}

medium

A. Interface method cannot return double

B. Missing radius field or property in Circle class

C. Circle class should not implement IShape

D. Area method should be void

Solution

1. Step 1: Check Circle class members

   The method Area uses 'radius' but no radius variable or property is declared in Circle.

2. Step 2: Understand interface method return type

   Interface method returning double is valid; no error there.

3. Final Answer:

   Missing radius field or property in Circle class -> Option B

4. Quick Check:

   Undefined variable 'radius' causes error [OK]

Hint: Check all variables used are declared [OK]

Common Mistakes:

• Thinking interface methods can't return values
• Believing class can't implement interface
• Assuming method return type must be void

5.

You want to create a system where different devices can Start() and Stop() but each device does it differently. How should you use interfaces to design this?

hard

A. Create a base class Device with Start and Stop methods and inherit it

B. Write Start and Stop methods directly in each device class without interface

C. Define an interface IDevice with Start and Stop methods, then implement it in each device class

D. Use abstract classes only, no interfaces

Solution

1. Step 1: Understand interface purpose

   Interfaces define a contract for methods without implementation, perfect for different device behaviors.

2. Step 2: Apply interface to devices

   Define IDevice with Start and Stop, then each device class implements these methods with its own details.

3. Final Answer:

   Define an interface IDevice with Start and Stop methods, then implement it in each device class -> Option C

4. Quick Check:

   Interface = shared method rules, different implementations [OK]

Hint: Use interfaces for shared method names, different code [OK]

Common Mistakes:

• Using base class limits flexibility
• Skipping interface loses contract benefits
• Confusing abstract classes with interfaces