1 Scenario where C# interface is indispensable?

Interview Question: Tell one scenario where C# interface  is indispensable. For the scenario, only C# interface is the solution and abstract class cannot be used.

Answer: Scenario is multiple interface inheritance. C# object oriented programming does not support multiple class inheritance, means, a class cannot inherit more than one class in C#. But, a class can inherit multiple interfaces.

Multiple interface inheritance example in C#

//Create Ist interface
interface ICreditCard
{
    void validateCard();
}
//Create 2nd interface
interface ITransaction
{
    void startTransaction();
}

//Mastor credit card class will inherit multiple interfaces
class MastroCreditCard : ICreditCard, ITransaction
{

    public void validateCard()
    {
        Console.WriteLine("Validating Credit Card");
    }
    public void startTransaction()
    {
        Console.WriteLine("Starting transaction");
    }
    public void transactionStatus()
    {
        Console.WriteLine("Transaction completed");
    }
}

//Test
class Program
{
    static void Main(string[] args)
    {
        MastroCreditCard card = new MastroCreditCard();
        card.validateCard();
        card.startTransaction();
        card.transactionStatus();
    }
}

Output:

Validating Credit Card
Starting transaction
Transaction completed

So, if a class has a requirement of multiple inheritance in C#, then only interfaces are solution not an abstract class.

Recommended another nice interview question here: what would you prefer between an interface and an abstract class in C# if both offer contracts only.