Interface declaration syntax in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of interface method call

What is the output of this C# code when calling obj.ShowMessage()?

C Sharp (C#)

interface IMessage
{
    void ShowMessage();
}

class Greeting : IMessage
{
    public void ShowMessage()
    {
        System.Console.WriteLine("Hello from interface implementation!");
    }
}

class Program
{
    static void Main()
    {
        IMessage obj = new Greeting();
        obj.ShowMessage();
    }
}

A Compilation error: interface method cannot be called

B Hello from interface implementation!

C Runtime error: method not implemented

D No output

💡 Hint

Remember that classes implementing interfaces must provide method bodies.

The class Greeting implements the interface IMessage and provides the ShowMessage method. Calling ShowMessage on the interface reference calls the implemented method, printing the message.

❓ Predict Output

intermediate

Interface property declaration and usage

What will be the output of this program?

C Sharp (C#)

interface ICar
{
    string Model { get; set; }
}

class Car : ICar
{
    public string Model { get; set; }

    public Car(string model)
    {
        Model = model;
    }
}

class Program
{
    static void Main()
    {
        ICar myCar = new Car("Tesla Model S");
        System.Console.WriteLine(myCar.Model);
    }
}

A Tesla Model S

B Compilation error: property not implemented

C Runtime error: property access failed

D null

💡 Hint

Check how the property Model is implemented in the class.

The class Car implements the interface property Model with get and set accessors. The constructor sets the Model to "Tesla Model S". Printing myCar.Model outputs the string.

❓ Predict Output

advanced

Explicit interface implementation output

What is the output of this code?

C Sharp (C#)

interface IAlpha
{
    void Display();
}

interface IBeta
{
    void Display();
}

class MyClass : IAlpha, IBeta
{
    void IAlpha.Display()
    {
        System.Console.WriteLine("Alpha Display");
    }

    void IBeta.Display()
    {
        System.Console.WriteLine("Beta Display");
    }
}

class Program
{
    static void Main()
    {
        MyClass obj = new MyClass();
        ((IAlpha)obj).Display();
        ((IBeta)obj).Display();
    }
}

A

Alpha Display
Beta Display

B

Beta Display
Alpha Display

C Compilation error: ambiguous method call

D Runtime error: method not found

💡 Hint

Explicit interface implementations require casting to call.

The class MyClass explicitly implements Display for both interfaces. Calling Display via IAlpha reference prints "Alpha Display" and via IBeta reference prints "Beta Display".

❓ Predict Output

advanced

Interface inheritance and method call output

What will this program print?

C Sharp (C#)

interface IBase
{
    void Show();
}

interface IDerived : IBase
{
    void Show();
}

class Implementation : IDerived
{
    public void Show()
    {
        System.Console.WriteLine("Show method in Implementation");
    }
}

class Program
{
    static void Main()
    {
        IBase obj = new Implementation();
        obj.Show();
    }
}

A No output

B Compilation error: method Show must be implemented twice

C Runtime error: ambiguous method call

D Show method in Implementation

💡 Hint

Interface inheritance means IDerived includes IBase's members.

IDerived inherits from IBase, so Show is declared once. Implementation provides one Show method that satisfies both interfaces. Calling Show prints the message.

🧠 Conceptual

expert

Interface declaration syntax error identification

Which option contains a syntax error in the interface declaration?

A interface IExample { void DoWork(); }

B interface IExample { int Calculate(int x); }

C interface IExample { public void DoWork(); }

D interface IExample { string Name { get; set; } }

💡 Hint

Interface members cannot have access modifiers like 'public'.

In C#, interface members are implicitly public and cannot include access modifiers. Option C incorrectly uses 'public' inside the interface.

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