C# Dot Net Interview Preparation Part 12 | Top 6 Frequently Asked Interview Questions and Answers

Question 67: What is the difference between abstract class and interface in C#?

Answer:

• abstract class: Can have both implemented and abstract (unimplemented) methods.
• interface: Only contains method declarations (before C# 8), but C# 8+ allows default implementations.

Code Example:

abstract class Animal
{
    public abstract void MakeSound(); // Abstract method
    public void Sleep() => Console.WriteLine("Sleeping...");
}

interface IBird
{
    void Fly(); // No implementation (before C# 8)
}

class Dog : Animal
{
    public override void MakeSound() => Console.WriteLine("Bark!");
}

class Sparrow : IBird
{
    public void Fly() => Console.WriteLine("Flying high!");
}
        

Question 68: What is a delegate in C#?

Answer: "A delegate is a type that holds a reference to methods with a specific signature."

Code Example:

delegate void MyDelegate(string message);

class Program
{
    static void ShowMessage(string msg) => Console.WriteLine(msg);

    static void Main()
    {
        MyDelegate del = ShowMessage;
        del("Hello, Delegates!");
    }
}
        

Question 69: What is the difference between Stack and Queue in C#?

Answer:

Feature Stack<T> Queue<T> Order LIFO (Last In, First Out) FIFO (First In, First Out) Methods Push(), Pop() Enqueue(), Dequeue()

Code Example:

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        Stack<int> stack = new Stack<int>();
        stack.Push(1);
        stack.Push(2);
        Console.WriteLine(stack.Pop()); // Output: 2

        Queue<int> queue = new Queue<int>();
        queue.Enqueue(1);
        queue.Enqueue(2);
        Console.WriteLine(queue.Dequeue()); // Output: 1
    }
}
        

Question 70: What is async and await in C#?

Answer: "async and await are used to write asynchronous (non-blocking) code, improving performance."

Code Example:

using System;
using System.Threading.Tasks;

class Program
{
    static async Task Main()
    {
        await PrintMessage();
        Console.WriteLine("Finished!");
    }

    static async Task PrintMessage()
    {
        await Task.Delay(2000); // Simulate delay
        Console.WriteLine("Hello, Async!");
    }
}
        

Question 71: What is the difference between Equals() and == in C#?

Answer:

• == compares values (default for value types).
• Equals() compares object references (unless overridden).

Code Example:

class Program
{
    static void Main()
    {
        string str1 = "Hello";
        string str2 = "Hello";

        Console.WriteLine(str1 == str2); // True (compares values)
        Console.WriteLine(str1.Equals(str2)); // True (compares values for strings)
    }
}
        

Question 72: What is LINQ in C#?

Answer: "LINQ (Language Integrated Query) is used to query collections and databases in a readable way."

Code Example:

using System;
using System.Linq;

class Program
{
    static void Main()
    {
        int[] numbers = { 1, 2, 3, 4, 5 };

        var evenNumbers = numbers.Where(n => n % 2 == 0);

        foreach (var num in evenNumbers)
            Console.WriteLine(num); // Output: 2, 4
    }
}
        

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