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.