Implementing interfaces in C Sharp (C#) - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an interface in C#?

An interface in C# is a contract that defines a set of methods and properties without implementing them. Classes that implement the interface must provide the implementation for all its members.

beginner

How do you declare that a class implements an interface in C#?

You use a colon (:) after the class name followed by the interface name. For example: <br><code>public class MyClass : IMyInterface { }</code>

intermediate

Can a class implement multiple interfaces in C#? How?

Yes, a class can implement multiple interfaces by listing them separated by commas after the colon. For example:<br><code>public class MyClass : IFirst, ISecond { }</code>

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 must provide implementations for all interface members unless the class is declared abstract.

advanced

Explain explicit interface implementation in C#.

Explicit interface implementation means implementing interface members so they can only be accessed through the interface, not the class instance directly. This is done by prefixing the member with the interface name, like:<br><code>void IMyInterface.Method() { }</code>

How do you declare that a class implements an interface named IExample?

A public class MyClass : IExample { }

B public interface MyClass : IExample { }

C public class MyClass implements IExample { }

D public class MyClass inherits IExample { }

In C#, a class implements an interface using a colon (:), not 'implements' or 'inherits'.

What must a class do when it implements an interface?

A Implement none of the interface members

B Implement at least one interface member

C Implement all interface members

D Only declare the interface name

A class must implement all members of the interface unless it is abstract.

Can a class implement more than one interface?

A Yes, but only if interfaces inherit each other

B Yes, by separating interface names with commas

C No, only one interface is allowed

D No, interfaces cannot be implemented

C# allows multiple interfaces to be implemented by listing them separated by commas.

What is explicit interface implementation used for?

A To implement interface members with different names

B To make interface members public

C To inherit from multiple classes

D To hide interface members from the class's public API

Explicit implementation hides interface members so they are accessible only through the interface type.

If a class does not implement all interface members, what happens?

A Compilation error unless the class is abstract

B The program runs but with warnings

C The missing members are auto-implemented

D The interface is ignored

The compiler requires all interface members to be implemented unless the class is abstract.

Describe how to implement an interface in a C# class and what rules must be followed.

Think about the syntax and what the compiler expects.

Explain the difference between implicit and explicit interface implementation in C#.

Consider how you access the implemented methods.

Practice

1. What does it mean to implement an interface in C#?

easy

A. A class provides code for all methods declared in the interface.

B. An interface inherits from a class.

C. A class hides all methods of the interface.

D. An interface creates objects directly.

Solution

1. Step 1: Understand interface purpose

   An interface declares methods without code, setting a contract.

2. Step 2: Implementing means coding methods

   A class that implements the interface must write the code for all those methods.

3. Final Answer:

   A class provides code for all methods declared in the interface. -> Option A

4. Quick Check:

   Implementing interface = writing required methods [OK]

Hint: Implementing means writing all interface methods in the class [OK]

Common Mistakes:

• Thinking interfaces can create objects
• Believing interfaces inherit from classes
• Assuming methods are hidden, not implemented

2. Which of the following is the correct syntax to implement an interface IMyInterface in a class MyClass?

easy

A. interface MyClass : IMyInterface { }

B. class MyClass : IMyInterface { }

C. class MyClass implements IMyInterface { }

D. class MyClass inherits IMyInterface { }

Solution

1. Step 1: Recall C# interface syntax

   In C#, a class implements an interface using a colon (:), not 'implements' or 'inherits'.

2. Step 2: Check each option

   class MyClass : IMyInterface { } uses correct syntax: class MyClass : IMyInterface { }. Others use wrong keywords or declare interface as class.

3. Final Answer:

   class MyClass : IMyInterface { } -> Option B

4. Quick Check:

   Use colon (:) to implement interface [OK]

Hint: Use ':' to implement interface, not 'implements' or 'inherits' [OK]

Common Mistakes:

• Using 'implements' keyword like Java
• Trying to inherit interface with 'inherits'
• Declaring interface as a class

3. What will be the output of this code?

interface IGreet { void SayHello(); }
class Person : IGreet {
  public void SayHello() { Console.WriteLine("Hi!"); }
}
var p = new Person();
p.SayHello();

medium

A. No output

B. SayHello

C. Hi!

D. Compile error

Solution

1. Step 1: Understand interface and class

   The interface IGreet requires SayHello method. Person implements it by printing "Hi!".

2. Step 2: Trace code execution

   Creating Person object and calling SayHello prints "Hi!" to console.

3. Final Answer:

   Hi! -> Option C

4. Quick Check:

   Implemented method runs and prints output [OK]

Hint: Implemented method runs exactly as coded in class [OK]

Common Mistakes:

• Expecting method name printed instead of output
• Thinking interface prints output
• Assuming compile error without method body

4. Identify the error in this code:

interface IRun { void Run(); }
class Animal : IRun { }

medium

A. Class Animal must implement Run method.

B. Interface IRun cannot be empty.

C. Class Animal cannot implement interface.

D. No error, code is correct.

Solution

1. Step 1: Check interface requirements

   IRun interface requires a method Run() to be implemented.

2. Step 2: Verify class implementation

   Animal class implements IRun but does not provide Run() method, causing error.

3. Final Answer:

   Class Animal must implement Run method. -> Option A

4. Quick Check:

   Implement all interface methods [OK]

Hint: All interface methods must be in the class [OK]

Common Mistakes:

• Forgetting to implement interface methods
• Thinking interfaces must have code
• Assuming empty class is allowed

5. You have two interfaces:

interface IWalk { void Walk(); }
interface ITalk { void Talk(); }

How can a class Human implement both interfaces correctly?

hard

A. interface Human : IWalk, ITalk { }

B. class Human : IWalk { public void Walk() { } } class Human : ITalk { public void Talk() { } }

C. class Human implements IWalk, ITalk { }

D. class Human : IWalk, ITalk { public void Walk() { Console.WriteLine("Walking"); } public void Talk() { Console.WriteLine("Talking"); } }

Solution

1. Step 1: Understand multiple interface implementation

   A class can implement multiple interfaces by listing them separated by commas.

2. Step 2: Provide all required methods

   Human class must provide Walk() and Talk() methods with code.

3. Final Answer:

   class Human : IWalk, ITalk { public void Walk() { Console.WriteLine("Walking"); } public void Talk() { Console.WriteLine("Talking"); } } -> Option D

4. Quick Check:

   Multiple interfaces implemented with all methods [OK]

Hint: Separate interfaces with commas and implement all methods [OK]

Common Mistakes:

• Trying to declare multiple classes with same name
• Using 'implements' keyword (Java style)
• Declaring interface instead of class