2023 Top Interview Questions for C# Developers - IQCode

C# Language Overview

C# is an object-oriented programming language developed by Microsoft and used with the .NET framework to create websites, applications, and games. It has become popular because of its ease of use, wide range of development options, and larger target audience.

Compared to other programming languages, C# is comparatively easier to learn and use. It offers some fantastic features, such as automatic garbage collection, interfaces, and more, making it an excellent choice for building better applications. Applications created using C# have an advantage due to collaboration with Microsoft and a wider target audience.

Since C# is widely used, it's essential to prepare for interviews with basic and advanced C# questions. Below you will find 50 frequently asked and comprehensive questions and answers on C#.

C# Basic Interview Questions:

1. How is C# different from C?

What is Common Language Runtime (CLR)?

Common Language Runtime (CLR) is a component of the Microsoft .NET framework that provides an environment for executing .NET applications. It is responsible for managing the execution of code and facilitating memory management, thread management, and exception handling. The CLR also provides features such as garbage collection for automatic memory management, just-in-time (JIT) compilation for optimal performance, and interoperability between different programming languages that target the .NET framework. Overall, the CLR plays a crucial role in making the .NET platform a powerful and flexible development platform.

Understanding Garbage Collection in C#

Garbage Collection in C# is a built-in feature that automatically releases memory that is no longer referenced by any active objects in the program. This helps in ensuring efficient memory usage and preventing memory leaks in the program. When the garbage collector runs, it identifies the objects that are no longer being used and frees up the associated memory. This process is done automatically by the runtime environment and does not require any explicit deallocation of memory by the programmer.

Types of Classes in C#

In C#, there are several types of classes. These include:

1. Static Classes - These are classes that cannot be instantiated and contain only static methods.

2. Abstract Classes - These are classes that cannot be instantiated and are created to serve as base classes that other classes can inherit from. They contain both abstract and non-abstract methods.

3. Sealed Classes - These are classes that cannot be inherited from.

4. Partial Classes - These are classes that can be split among multiple source files.

5. Nested Classes - These are classes that are defined inside another class. They have access to all members of the containing class and can be either static or non-static.

It's important to choose the right type of class for the job to ensure your code is organized, efficient, and easy to maintain.

Managed and Unmanaged Code

Code is the set of instructions that makes up a program. There are two types of code: managed and unmanaged code. Managed code is code that runs inside a runtime environment, like the .NET Framework, which manages many aspects of the code, like memory allocation and garbage collection. Unmanaged code, on the other hand, runs directly on the operating system and has to manage its own memory. This makes unmanaged code faster but more prone to errors.

Differences Between Abstract Classes and Interfaces

Abstract classes and interfaces are both used to achieve abstraction in Java, but there are some key differences between the two:

  • An abstract class can have abstract and non-abstract methods, while an interface can only have abstract methods.
  • An abstract class can have method implementations, while an interface cannot.
  • An abstract class can have constructors, while an interface cannot.
  • A class can only extend one abstract class, but it can implement multiple interfaces.
  • An abstract class is meant to be extended by a subclass, while an interface is meant to be implemented by a class.
// Example of an abstract class
public abstract class Animal {
   public void eat() {
      System.out.println("This animal is eating");
   public abstract void makeSound();

// Example of an interface
public interface Vehicle {
   void start();
   void stop();
   void accelerate();
   void brake();

Differences Between Ref and Out Keywords

In C#, the




keywords are used for passing arguments by reference. The main differences are:

1. Initialization: The


keyword requires the variable to be initialized before it is passed as a reference, whereas the


keyword does not require the variable to be initialized before being passed as a reference.

2. Usage: When using the


keyword, the parameter must be initialized before the method call, whereas with the


keyword, the parameter is initialized within the method.

3. Return value: A method can return a value with the


keyword, while with the


keyword, the method returns a value only through the parameter.


public void ExampleMethod(ref int num)
    num = num * 2;

public void AnotherExampleMethod(out int num)
    num = 10;
    num = num + 5;

int x = 5;

// Using ref keyword
ExampleMethod(ref x);

// Using out keyword
AnotherExampleMethod(out x);

In the above example, the


multiplies the value of a number passed as a reference by 2, whereas the


initializes the parameter within the method, adds 5 to it, and then uses the modified value.

Understanding Extension Methods in C#

Extension methods in C# are a useful feature that allows adding new functionality to existing types without modifying the original code or creating a new derived class. They are defined as static methods in a static class and are used as if they were instance methods of the extended type. They can be applied to interfaces, classes, and even to types defined as sealed or as already existing in the .NET framework. Extension methods help to keep code organized, reduce complexity and make the code more readable.

Generics in C#

Generics provide a way to create reusable code by writing methods, classes, and structures that can be used with any type. In C#, you can create generic classes, interfaces, methods, delegates, and even constraints on type parameters. By using generics, you can write flexible and efficient code that can work with different data types without the need for duplicated code. For example, you can create a generic list class that can hold any type of data, such as integers, strings, or objects. The benefit of using generics is that you can write the code once and use it for multiple types, thus saving time and reducing errors.

Difference between Array and ArrayList in C#

In C#, arrays and ArrayLists are both used to store collections of data, but there are some key differences between them.

1. Data Type: An array can only store data of a specific data type, while an ArrayList can store data of any data type.

2. Size: The size of an array is fixed when it is created, while the size of an ArrayList can be dynamically resized.

3. Access: An array can be accessed directly using an index value, while an ArrayList requires a method call to access a specific element.

4. Performance: Arrays can be faster than ArrayLists when accessing elements directly, but ArrayLists can be faster when adding or removing elements because of their dynamic resizing capabilities.

In most cases, it's best to use an array when you know the size of the collection beforehand and need direct access to its elements, while an ArrayList is better when the size of the collection is unknown or needs to be frequently changed.

Understanding Inheritance in C# and its Support for Multiple Inheritance

Inheritance is a concept in object-oriented programming where a class (called the child class) derives properties and behaviors from another class (called the parent class). The child class can then add or modify its own properties and behaviors as needed.

In C#, inheritance is supported through the use of the "class" keyword, followed by the child class name and the parent class name in parentheses after a colon. For example:

public class ChildClass : ParentClass
    // Child class properties and behaviors

C# does not support multiple inheritance, which is the ability for a child class to derive from multiple parent classes. This is because multiple inheritance can lead to issues with ambiguity and conflicts between different inherited properties and behaviors.

To work around this, C# introduced the concept of interface, which allows a class to inherit multiple interfaces while still only inheriting from a single parent class. An interface is a collection of method, property, and event definitions that a class can implement as needed.

Overall, understanding inheritance and interfaces is important for creating efficient and structured code in C#.

C# Advanced Interview Question: Boxing and Unboxing

Boxing and unboxing in C# is the process of converting a value type to a reference type and vice versa, respectively. Boxing is done when a value type is assigned to an object type variable, while unboxing is done when an object type is assigned to a value type variable.

Boxing is achieved through the creation of an object instance and copying the value of the value type into the object instance. On the other hand, unboxing is done by extracting the value of the value type from the object instance. The process of boxing and unboxing can have an overhead performance cost, particularly in cases where it's done frequently in loops or in performance-critical code. It's important to consider this overhead when working with boxing and unboxing in C#.

For example:


int i = 123;
object obj = i;    // boxing

int j = (int)obj;  // unboxing

In the code above, the variable `i` is of type `int` and is boxed by assigning it to an `object` instance (`obj`). The value of `i` is copied to `obj`. Later, `obj` is unboxed by casting it to `int` and assigning it to the variable `j`.

It's important to note that some of the newer versions of C# have introduced various features and language constructs to help avoid unnecessary boxing and unboxing. In general, avoiding boxing and unboxing can help improve the performance of your C# code.

Properties in C#

In C#, properties are used to provide access to the private fields or variables of a class. They allow us to read, write, or modify private field values in a controlled manner. Properties are similar to methods but are accessed like fields. They consist of a get and/or set accessor, which specify how the property's value can be retrieved or modified.

Here's an example of how to define a property in C#:

public class Person {
    private string name;

    public string Name {
        get { return name; }
        set { name = value; }

In this example, we define a `Person` class with a private `name` field and a public `Name` property. The `get` accessor returns the `name` field value, and the `set` accessor sets the `name` field value to the provided value. This allows us to control how the `name` field is accessed and modified outside of the class. Properties provide a more elegant way to access and modify the fields of a class, and are widely used in C# programming.

Understanding Partial Classes in C#

Partial classes in C# are classes that can be defined in multiple parts within the same namespace, assembly, and file. This allows developers to split the definition of a class across multiple files, which can be useful for organizing and maintaining large codebases.

When defining a partial class, each part must use the "partial" keyword, and have the same name, accessibility level, and type parameters. The parts are then combined into a single class by the compiler at compile-time.

Partial classes are commonly used in code generation scenarios, where a tool generates some of the class code automatically, and developers can add their own code to the other part of the class. This separation makes it easier to regenerate the automatically generated code without losing any custom code.

Difference between Late Binding and Early Binding in C#

In C#, binding refers to the process of connecting a method call to the method implementation. The two types of bindings in C# are late binding and early binding.

Early binding is the process of binding a method call to its implementation at compile time. In early binding, the compiler knows at the compile time which method to invoke.

On the other hand, late binding occurs at runtime, where the method implementation is determined at runtime. In late binding, the method is not resolved until runtime, and therefore, it can result in slower program execution.

One advantage of late binding is that it allows for more flexibility in the program, as different objects can be used as parameters, and it also enables dynamic loading of assemblies.

However, early binding is preferred as it is less error-prone and provides better performance because the method implementation is known at compile time.

Arrays in C#

Arrays in C# are data structures that allow storage of multiple items of the same data type under one variable name. They are declared with a specified data type and size, and their elements can be accessed through an index starting at 0. Arrays are commonly used to store and manipulate large amounts of data efficiently.

Explanation of Indexers in C#

Indexers in C# are a type of property that allows an object to be indexed, i.e., to be accessed like an array. They are defined using the `this` keyword followed by a parameter list within square brackets. The parameter list can be of any data type, including reference types.

Indexers are useful when you need to access data in a class using an index, much like you would with an array. For example, you could create a `Person` class with an indexer that allows you to access a person's properties by their name.

Here's an example of how an indexer might look in code:

public class Person
    private string[] properties = {"Name", "Age", "Gender"};

    public string this[int i]
        get { return properties[i]; }
        set { properties[i] = value; }

In this example, we're using an integer index to access the person's properties array. We can then get or set the value at that index using the indexer.

Overall, indexers in C# provide a convenient way to access data in a class using an index or key.

Difference between the Equality Operator (==) and Equals() Method in C#

In C#, the equality operator (==) and the Equals() method are both used to compare two values for equality. However, there are some differences between these two approaches.

The equality operator (==) is a binary operator that compares the values of two operands for equality. It returns a boolean value indicating whether the operands are equal or not. This operator can be used with built-in value types, such as int and double, as well as with reference types, such as string and object.

On the other hand, the Equals() method is a method that is defined in the Object class, which is the base class for all types in C#. This method compares the values of two objects for equality. The default implementation of this method simply compares the references of the two objects. However, many classes override this method to provide their own implementation of how to compare two objects for equality. For example, the string class overrides the Equals() method to compare the contents of two strings for equality, rather than their references.

One important difference between these two approaches is how they handle null values. The equality operator (==) can be used to compare a null reference to another null reference, but using the Equals() method with a null reference will result in a NullReferenceException.

In summary, the equality operator (==) compares the values of two operands for equality, while the Equals() method compares two objects for equality. The implementation of the Equals() method can vary between different classes, while the equality operator (==) always compares values.

Different Ways to Overload Methods in C#

In C#, methods can be overloaded in multiple ways:

1. By changing the number of parameters in the method signature
2. By changing the data type of the parameters in the method signature
3. By changing the order of the parameters in the method signature

Overloading allows us to reuse the same method name with different parameter configurations. This can make our code more readable and maintainable.

What is Reflection in C#?

Reflection is a powerful feature in C# that allows a program to analyze and modify itself while it is running. It provides information about the program's types, methods, properties, and attributes during runtime by inspecting metadata. Reflection allows us to create extensible applications that can use arbitrary types and plug-ins that were not known at compile time. The System.Reflection namespace provides classes that allow you to obtain information about the types defined in your program, including their properties, methods, and events.

Difference between Constant and Readonly in C#

In C#, both `const` and `readonly` are used for creating constants, but there are some differences between them: - A `const` variable is a compile-time constant, which means its value cannot be changed once it is defined. It is implicitly static, so you can access it without creating an instance of its class. - A `readonly` variable can only be assigned a value at runtime or in a class constructor. It is not implicitly static, so you need to access it using an instance of its class.


public class Example {
    public const int ConstValue = 5;
    public readonly int ReadonlyValue;

    public Example(int value) {
        ReadonlyValue = value;

In the above example, `ConstValue` can be accessed using `Example.ConstValue` without creating an instance of the `Example` class, while `ReadonlyValue` can only be accessed using an instance of the `Example` class. Also, the value of `ConstValue` cannot be changed at runtime, while the value of `ReadonlyValue` can only be assigned in the class constructor.

Difference Between String and StringBuilder in C#

In C#, a string is an immutable sequence of characters while a StringBuilder is a mutable sequence of characters that can be modified without creating a new object.

Strings cannot be modified once created, which means any operations like appending, inserting, or replacing a character in a string create a new copy of that string in memory. This makes string manipulation less efficient and more memory-consuming.

StringBuilder, on the other hand, allows for efficient string manipulation because it dynamically allocates memory as the string grows or shrinks, minimizing the need for creating new objects.

In summary, use Strings when you need to work with fixed text. Use StringBuilder when you need to frequently modify and manipulate a string.

Reverse a String in C#

Here's the code to reverse a string in C#:

    using System;

    class Program {
        static void Main(string[] args) {
            string originalString, reversedString = "";
            Console.WriteLine("Enter a string to be reversed:");
            originalString = Console.ReadLine();
            for (int i = originalString.Length - 1; i >= 0; i--) {
                reversedString += originalString[i];
            Console.WriteLine("Reversed string is: " + reversedString);

This program asks the user to enter a string and then iterates through each character in the string in reverse order, adding them to another string to build up the reversed version. Finally, it prints out the reversed string to the console.

Reverse Order of Words in C#

using System;

class Program {
   static void Main() {
      string str = "Hello World";
      string[] words = str.Split(' '); //split string at spaces
      Array.Reverse(words); //reverse the array
      string reversedStr = string.Join(" ", words); //join the words back into a string
      Console.WriteLine(reversedStr); //print the reversed string

This program takes a string "Hello World" and splits it into an array of words using the split method. Then, it reverses the array using the 'Array.Reverse' method. Finally, it joins the words back into a string using the 'string.Join' method and prints the reversed string.

C# program to check if a string is palindrome or not

using System;

public class Program {
    public static void Main() {
        Console.WriteLine("Enter a string: ");
        string str = Console.ReadLine();
        int length = str.Length;
        bool isPalindrome = true;

        for(int i = 0; i < length/2; i++) {
        	if(str[i] != str[length-i-1]) {
        		isPalindrome = false;

        if(isPalindrome) {
        	Console.WriteLine("The given string is a palindrome.");
        } else {
        	Console.WriteLine("The given string is not a palindrome.");

This is a C# program to check whether a given string is palindrome or not. A Palindrome String is one that remains the same when its characters are reversed.

The program takes input string from the user, iterates through the string, and compares the first character to the last character, and proceeds with this comparison till the middle of the string is reached. If all the characters in the string match, then the given string is palindrome else it is not.

C# Program to Find a Substring from a Given String

using System;

class Program
    static void Main(string[] args)
        string inputString, subString;
        Console.WriteLine("Enter a string: ");
        inputString = Console.ReadLine(); //reading the input string
        Console.WriteLine("Enter the substring: ");
        subString = Console.ReadLine(); //reading the substring to search for
        if (inputString.Contains(subString)) //Checking if the substring exists in the input string
            Console.WriteLine("The substring '{0}' is found in '{1}'", subString, inputString);
            Console.WriteLine("The substring '{0}' is not found in '{1}'", subString, inputString);
        Console.ReadKey(); //keep console window open

In this C# program, we take user input for a string and a substring. We then use the Contains() method to check if the substring exists in the input string. If it does, we output a message saying the substring is found in the input string and if it doesn't, we output a message saying the substring is not found in the input string.

Check if a Positive Integer is Prime or Not in C#

Here's a C# program to determine whether a positive integer is a prime number or not.

using System;

class PrimeChecker
    static void Main()
        int num, i, flag = 0;
        Console.Write("Enter a positive integer to check if it's prime: ");
        num = int.Parse(Console.ReadLine());

        if (num == 1 || num == 0)
            Console.WriteLine(num + " is NOT a prime number.");
            for (i = 2; i <= num / 2; ++i)
                if (num % i == 0)
                    flag = 1;

            if (flag == 0)
                Console.WriteLine(num + " is a prime number.");
                Console.WriteLine(num + " is NOT a prime number.");

In this program, we first prompt the user to enter a positive integer. We then use a for loop to loop through all possible factors of the number until we reach the number itself (or half of the number, as factors greater than half of a number cannot be less than 2). If the number is divisible by any other number except 1 or itself, it is not a prime number. If the for loop finishes iterating without finding a factor other than 1 and itself, then the number is a prime number.

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