Interface declaration syntax 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 or structs that implement the interface must provide the implementation.

beginner

How do you declare an interface in C#?

Use the interface keyword followed by the interface name and a block containing method/property signatures. For example:

public interface IExample { void DoWork(); }

intermediate

Can interfaces contain fields or constructors in C#?

No, interfaces cannot contain fields or constructors. They only declare method, property, event, or indexer signatures without implementation.

intermediate

What access modifier is used for interface members in C#?

Interface members are implicitly public and cannot have access modifiers. All members declared in an interface are public by default.

beginner

How does a class implement an interface in C#?

A class implements an interface by using a colon <code>:</code> followed by the interface name, then providing implementations for all interface members. Example:<br><pre>public class Worker : IExample { public void DoWork() { /* code */ } }</pre>

Which keyword is used to declare an interface in C#?

A interface

B class

C struct

D implements

The interface keyword is used to declare an interface in C#.

Can an interface in C# contain method implementations?

A Yes, always

B Only default interface methods (C# 8.0+)

C Only private methods

D No, never

Starting with C# 8.0, interfaces can have default implementations for methods, but traditionally interfaces only declare signatures.

What access modifier do interface members have by default?

A public

B protected

C private

D internal

Interface members are implicitly public and cannot have other access modifiers.

How does a class indicate it implements an interface?

A Using the keyword 'inherits'

B Using the keyword 'implements'

C Using a colon ':' followed by the interface name

D Using angle brackets '<>'

In C#, a class implements an interface by using a colon ':' followed by the interface name.

Which of these can NOT be declared inside an interface?

A Events

B Properties

C Method signatures

D Fields

Interfaces cannot contain fields; they only declare signatures for methods, properties, events, and indexers.

Explain how to declare an interface in C# and how a class implements it.

Think about the contract nature of interfaces and how classes fulfill that contract.

What are the limitations of interfaces in C# regarding members they can contain?

Consider what an interface represents and what it cannot do.

Practice

1. What is the main purpose of an interface in C#?

easy

A. To define a contract with method and property signatures only

B. To implement all method bodies for a class

C. To store data like variables and constants

D. To create an instance of a class directly

Solution

1. Step 1: Understand what an interface is

   An interface only declares method and property signatures without implementations.

2. Step 2: Compare with other options

   Interfaces do not implement methods or store data; they define a contract for classes.

3. Final Answer:

   To define a contract with method and property signatures only -> Option A

4. Quick Check:

   Interface purpose = contract definition [OK]

Hint: Interfaces declare methods, they don't implement them [OK]

Common Mistakes:

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

2. Which of the following is the correct syntax to declare an interface named IMyInterface in C#?

easy

A. interface IMyInterface { void MyMethod(); }

B. class IMyInterface { void MyMethod(); }

C. interface IMyInterface() { void MyMethod(); }

D. interface IMyInterface[] { void MyMethod(); }

Solution

1. Step 1: Check the keyword and name format

   Interfaces use the keyword interface followed by the name without parentheses or brackets.

2. Step 2: Validate method declaration inside interface

   Methods inside interfaces have only signatures ending with semicolons, no bodies.

3. Final Answer:

   interface IMyInterface { void MyMethod(); } -> Option A

4. Quick Check:

   Correct interface syntax = interface IMyInterface { void MyMethod(); } [OK]

Hint: Use 'interface Name { }' without parentheses or brackets [OK]

Common Mistakes:

• Using class keyword instead of interface
• Adding parentheses after interface name
• Using brackets [] after interface name

3. What will be the output of the following code?

interface IExample { void Show(); }
class Demo : IExample {
  public void Show() { Console.WriteLine("Hello Interface"); }
}
class Program {
  static void Main() {
    IExample obj = new Demo();
    obj.Show();
  }
}

medium

A. Compilation error: Show method missing

B. Hello Interface

C. Runtime error: Cannot create interface instance

D. No output

Solution

1. Step 1: Understand interface implementation

   The class Demo implements IExample and provides the Show method.

2. Step 2: Analyze the Main method

   An object of Demo is created and assigned to an IExample reference, then Show() is called, printing the message.

3. Final Answer:

   Hello Interface -> Option B

4. Quick Check:

   Interface method call prints message [OK]

Hint: Interface methods must be implemented to avoid errors [OK]

Common Mistakes:

• Assuming interfaces can be instantiated directly
• Forgetting to implement interface methods
• Expecting no output without method body

4. Identify the error in the following interface declaration:

interface ITest {
  void Run() {}
}

medium

A. Missing semicolon after method declaration

B. Method name must be lowercase

C. Interface name must start with lowercase 'i'

D. Interfaces cannot have method bodies

Solution

1. Step 1: Check method declaration in interface

   Interfaces only declare method signatures without bodies (no curly braces).

2. Step 2: Validate other syntax rules

   Method names can be any case; interface names usually start with uppercase 'I'. Semicolon is required after signature.

3. Final Answer:

   Interfaces cannot have method bodies -> Option D

4. Quick Check:

   Interface methods = signatures only [OK]

Hint: No method bodies allowed inside interfaces [OK]

Common Mistakes:

• Adding method bodies inside interfaces
• Confusing naming conventions with syntax errors
• Omitting semicolon after method signature

5. You want to declare an interface IVehicle with two methods: Start() and Stop(). Which of the following is the correct way to declare it and implement it in a class Car?

hard

A. interface IVehicle { void Start(); void Stop(); }
class Car { public void Start() { } public void Stop() { } }

B. interface IVehicle { void Start() {} void Stop() {} }
class Car : IVehicle { }

C. interface IVehicle { void Start(); void Stop(); }
class Car : IVehicle { public void Start() { Console.WriteLine("Car started"); } public void Stop() { Console.WriteLine("Car stopped"); } }

D. interface IVehicle { void Start(); void Stop(); }
class Car : IVehicle { void Start() { } void Stop() { } }

Solution

1. Step 1: Declare interface with method signatures only

   IVehicle must declare Start() and Stop() without bodies.

2. Step 2: Implement interface methods publicly in class

   Car must implement both methods with public access and provide method bodies.

3. Step 3: Check other options for errors

   interface IVehicle { void Start() {} void Stop() {} } class Car : IVehicle { } has method bodies in interface (invalid). interface IVehicle { void Start(); void Stop(); } class Car { public void Start() { } public void Stop() { } } does not implement interface. interface IVehicle { void Start(); void Stop(); } class Car : IVehicle { void Start() { } void Stop() { } } implements methods but lacks public modifier, causing error.

4. Final Answer:

   Correct interface and class implementation with public methods -> Option C

5. Quick Check:

   Interface methods declared; class implements publicly [OK]

Hint: Interface methods need public implementation in classes [OK]

Common Mistakes:

• Adding method bodies inside interface
• Not implementing interface in class
• Omitting public modifier in class methods