Top 70+ C# Interview Questions and Answers [2024]

Intermediate-Level C# Interview Questions

Here are some intermediate-level C# interview questions designed for experienced developers. These questions delve into more advanced topics in C# programming, including memory management, object-oriented concepts, and modern C# features.

32. What Is Encapsulation in C#?

It is a fundamental concept in object-oriented programming. It means combining data and methods into a single unit or class while hiding internal details and restricting access to them. Encapsulation helps prevent unintended changes to data and simplifies code management, making it easier to understand. This is a key topic in C# and is often asked in C# interview questions.

33. Explain Polymorphism

Polymorphism allows methods or objects to take on different forms. In C#, you can achieve polymorphism through method overriding and method overloading. This enables methods to behave differently based on the object using them.

For instance, a Shape class could have a Draw() method that the Circle and Square classes override to provide their implementations. Understanding the concept of polymorphism is essential for developers, and it is often asked in C# interview questions.

34. What Is a Delegate in C#?

A delegate is a type that references methods with a specific signature, allowing methods to be passed as arguments to other methods. This promotes flexibility and reusability in code, particularly for callbacks and event handling. Delegates are a common topic that appears in C# interview questions as it's repeated to the concept of event-driven programming.

35. What Does Garbage Collection Involve in C#?

Garbage collection is an automated process that reclaims memory allocated to objects that are no longer needed. The .NET runtime's garbage collector scans for such objects and releases the memory, preventing memory leaks and ensuring efficient memory usage. This is crucial for developers, as manual memory management is not required. Questions around garbage collection often appear in C# interview questions as it is related to memory management.

36. Explanation of Extension Methods in C# and Their Functionality

Extension methods in C# allow you to add functionality to existing types without modifying their original code. Defined as static methods in a static class, they use this keyword to specify the type they are extending.

This is particularly useful when you need to enhance types you cannot modify directly. Extension methods are frequently asked in C# interview questions as they relate to code flexibility.

37. What Are Partial Classes in C#?

Partial classes in C# allow a class’s definition to be spread across multiple files, with the parts combined during compilation.

You declare a partial class using the partial keyword, which can be helpful for managing large classes across different files. These classes are important for developers to know, and it's often featured in C# interview questions that focus on code organization and structure.

public partial class ClassName  
{
    // Code here
}

This feature is useful for breaking down large classes, interfaces, or structures across different files. You can also add nested partial classes if needed.

38. What Do the System.String and System.Text.StringBuilder Classes Refer to?

In C#, a String is immutable, meaning it cannot be modified once created. Each change creates a new string object in memory, which can affect performance during frequent modifications.

The StringBuilder class is designed for dynamic string operations, enabling more efficient handling of frequent changes by modifying the same object. This is a common topic in C# interview questions focused on memory management.

39. Explain the Difference Between String and StringBuilder in C#?

Below are the differences between String and StringBuilder:

40. What Is Reflection in C#?

Reflection in C# allows for runtime inspection of types, assemblies, modules, and more. Through reflection, you can retrieve metadata such as methods, properties, constructors, and events. This is done using the System.Reflection namespace is valuable for dynamic object inspection and tool creation. Reflection is a critical topic in C# and is often asked in many C# interview questions.

With reflection, you can inspect elements such as:

• Assemblies
• Modules
• Methods (MethodInfo)
• Constructors (ConstructorInfo)
• Properties (PropertyInfo)
• Fields (FieldInfo)
• Events (EventInfo)
• Parameters (ParameterInfo)

Reflection is valuable for scenarios involving dynamic object inspection and tool development.

41. What Different Types of Methods Be Overloaded in C#?

Method overloading occurs when methods share the same name but differ in their parameters, enabling the method to handle different data types or parameter counts. Methods can be overloaded by:

• Changing the number of parameters.
• Reordering parameters.
• Using different data types for parameters.

This is a common concept in C# and is often covered in C# interview questions.

42. Differentiate Between Late Binding and Early Binding in C#?

C# supports two types of binding: early and late binding.

• Early Binding: Occurs at compile time. The object type and its methods or properties are known beforehand. Early binding offers better performance and fewer runtime errors since everything is validated at compilation. It is associated with static objects.
• Late Binding (Dynamic Binding): Occurs at runtime. The object type and its methods or properties are determined during execution. Late binding uses dynamic objects and virtual methods, making it slower due to runtime lookups. It provides more flexibility but can lead to runtime errors.

The above two types are important concepts in C# and are often covered in most of the C# interview questions.

43. What Sets IEnumerable Apart From IQueryable in C#?

IEnumerable is intended for iterating through a group of items. It works with data stored in memory and handles queries within the client's environment. Utilize IEnumerable when dealing with data that has already been loaded into memory.

IQueryable, an extension of IEnumerable, is specifically created to query data sources such as databases and can translate queries. Performing queries on the server side can be beneficial for processing large volumes of data. Understanding these differences is crucial as it occur in many C# interview questions.

44. How to Join Two Lists in C#?

In C#, two lists can be joined using the AddRange() method, which appends the elements of one list to another. The added list may contain null elements, but the list itself cannot be null. This topic is often covered in C# interview questions as it is related to working with collections.

45. How to Intersect Two Lists in C#?

To find common elements between two lists, use the Enumerable.Intersect() method from the System.Linq namespace. This is a common technique discussed in C# interview questions about manipulating collections and optimizing algorithms.

46. In C#, Explain the Purpose and Internal Working of Async and Await. How Does It Relate to the Task Parallel Library (TPL)?

The async and await keywords in C# simplify writing asynchronous code. These keywords rely on the Task Parallel Library (TPL) to manage tasks, allowing the program to execute other code while awaiting the completion of asynchronous operations.

• async: Marks a method as asynchronous.
• await: Pauses the method until the awaited task is completed without blocking the main thread.

47. What Is an Indexer in C#, and When Would You Use One?

An indexer in C# is a unique property that enables an object to be accessed using array syntax. This is particularly handy when you want to provide array-like access to a class or struct that may not use an actual array to store its data.

Use indexers when:

• You want to allow array-style access to a class that manages a list or collection of items.
• You're working with a class that represents a structured collection, like a database table or a custom collection.
• You aim to simplify access to complex internal data structures.

48. What Is the Distinction Between the Dispose() and Finalize() Methods?

There is a slight difference between Dispose() and Finalize() methods which are explained below.

Dispose() is utilized to explicitly free unmanaged resources. It needs to be invoked manually, usually inside a using statement or directly by the developer. It guarantees that resources are quickly cleaned up.

Finalize(), also referred to as a destructor, is used for releasing unmanaged resources, but it is invoked by the garbage collector. Finalize does not ensure prompt resource release since it is called before an object is recycled by the garbage collector.

Understanding these differences will help developers use these methods effectively, and it is often covered in most of the C# interview questions.

49. What Are Get and Set Accessor Properties?

In C#, Get and Set are parts of properties. Get retrieves the value of a property, and Set changes the value. They manage how private fields are accessed and updated in a class. Questions related to property accessors are common to appear in C# interview questions.

50. What Are C# Attributes and Why Are They Important?

C# attributes are special tags you can add to code elements like classes and methods. They provide extra information about these elements, which can be accessed later using Reflection. This helps in adding metadata or customizing behavior without changing the code itself, and it's an important topic in many C# interview questions.

51. What Is Dependency Injection and How Is It Implemented in C#?

Dependency Injection provides a class with the necessary objects rather than allowing the class to create them internally. This aids in increasing the flexibility and manageability of code.

In C#, Dependency Injection is commonly achieved through:

• Constructor Injection: Passing the needed objects to a class through its constructor.
• Property Injection: Setting the needed objects through public properties.
• Method Injection: Providing the needed objects as parameters to methods.

This is a key aspect in C# programming, and it is frequently covered in many C# interview questions.

52. How Would You Implement Multiple Interfaces With the Same Method Name in the Same Class?

In C#, you can implement multiple interfaces that have the same method name, but you need to avoid ambiguity. If two interfaces have methods with the same name, you must use explicit interface implementation to differentiate between them.

For example, If you have the same method in different interfaces and you try to implement them in a class, the compiler won't know which method to call, which leads to errors. For this, you have to clearly show which method belongs to which interface by using explicit interface implementation. You can do this by explicitly stating the interface name during the method definition.

Let's say you have two interfaces ( IFirst and ISecond )with the same method, Execute(). You can implement these in a class and explicitly state which method belongs to which interface:

interface IFirst {
  void Execute();
}


interface ISecond {
  void Execute();
}


class ExampleClass : IFirst, ISecond {
  // IFirst's method implementation
  void IFirst.Execute() {
      Console.WriteLine("IFirst's method called");
  }


  // ISecond's method implementation
  void ISecond.Execute() {
      Console.WriteLine("ISecond's method called");
  }
}

Here, you can explicitly mention IFirst.Execute() and ISecond.Execute() to avoid confusion. When you create objects, you specify which interface the object is using, and only the corresponding method will be accessible.

If you call these methods, you will get different outputs based on which interface method is being used:

IFirst obj1 = new ExampleClass();
>> obj1.Execute(); // Output: IFirst's method called
>> 
>> ISecond obj2 = new ExampleClass();
>> obj2.Execute(); // Output: ISecond's method called

You could make the method public in the class, but that still wouldn't clarify which interface’s method is being called in a larger program. That’s why it’s better to use explicit interface implementation for clarity.

Therefore, we can say using multiple interfaces with the same method name requires careful naming to avoid mistakes and errors. Explicit interface implementation helps avoid confusion, and this question is frequently asked in C# interview questions.

53. What Is the Virtual Method, and How Is It Different From the Abstract Method?

An abstract method is defined in an abstract class, which cannot be instantiated. It only includes the method definition without any implementation. Any class that inherits from it must override the abstract method.

A virtual method, on the other hand, can be overridden in a derived class, but doing so is optional. Virtual methods must have a body with an implementation and can exist in both abstract and non-abstract classes.

Here’s an example:

public abstract class baseclass
{
    public abstract decimal getarea(decimal Radius); // Abstract method with no implementation.


    public virtual decimal interestpermonth(decimal amount) // Virtual method with implementation.
    {
        return amount * 12 / 100;
    }


    public virtual decimal totalamount(decimal Amount, decimal principleAmount) // Another virtual method with implementation.
    {
        return Amount + principleAmount;
    }
}


public class derivedclass : baseclass
{
    public override decimal getarea(decimal Radius) // Must override the abstract method.
    {
        return 2 * (22 / 7) * Radius; // Implementation for getarea.
    }


    public override decimal interestpermonth(decimal amount) // Overriding the virtual method.
    {
        return amount * 14 / 100; // New implementation for interestpermonth.
    }
}

In this example, the baseclass contains an abstract method getarea, which must be implemented by the derivedclass. The methods interestpermonth and totalamount are virtual, meaning they can be overridden, but the derived class can also choose to use the default implementations.

Additionally, consider this example:

abstract class twodshape
{
    public abstract void area(); // Abstract method with no body in the base class.
}


class twodshape2 : twodshape
{
    public virtual double area() // Virtual method that may or may not be overridden.
    {
        Console.WriteLine("AREA() may or may not be overridden.");
    }
}

In this case, the area method in the twodshape class must be overridden in any derived class, while the area method in twodshape2 is virtual and can be optionally overridden. Both methods are significantly important in C#, and this concept is often covered in C# interview questions.

54. Explain the “Continue” and “Break” Statement

The break statement: It terminates the loop immediately, regardless of the loop's condition. When the program hits a break, it leaves the loop and continues with the next line of code after the loop.

Here’s an example:

#include <stdio.h>


int main() {
    int i = 0, j = 0;


    // Loop through the range [0, 5]
    for (int i = 0; i < 5; i++) {
        printf("i = %d, j = ", i);


        // Inner loop through the range [0, 5]
        for (int j = 0; j < 5; j++) {
            // Break when j equals 2
            if (j == 2)
                break;


            printf("%d ", j);
        }
        printf("
");
    }
    return 0;
}

Output:

i = 0, j = 0 1 
i = 1, j = 0 1 
i = 2, j = 0 1 
i = 3, j = 0 1 
i = 4, j = 0 1

In this program, the inner loop stops whenever j reaches 2.

The continue statement: It skips the rest of the current loop iteration and starts the next one. Then, the loop checks its condition again to decide if it should run the next cycle.

#include <stdio.h>


int main() {
    int i = 0, j = 0;


    // Loop through the range [0, 5]
    for (int i = 0; i < 5; i++) {
        printf("i = %d, j = ", i);


        // Inner loop through the range [0, 5]
        for (int j = 0; j < 5; j++) {
            // Skip the rest of the loop when j equals 2
            if (j == 2)
                continue;


            printf("%d ", j);
        }
        printf("
");
    }
    return 0;
}

Output:

i = 0, j = 0 1 3 4 
i = 1, j = 0 1 3 4 
i = 2, j = 0 1 3 4 
i = 3, j = 0 1 3 4 
i = 4, j = 0 1 3 4

In this program, the inner loop skips the iteration whenever j equals 2, so it prints the other values. It's important to note that using these statements is beneficial when dealing with loops like a switch statement, while, do-while. It's the most important topic in C#, and it's often covered in many C# interview questions.

55. What Is the Difference Between the System.Array.CopyTo() and System.Array.Clone()?

Below are the differences between the System.Array.CopyTo() and System.Array.Clone()

Experienced-Level C# Interview Questions

This set of C# interview questions is intended for individuals with a foundational understanding of C# who are ready to explore more advanced concepts. These questions will help deepen your knowledge of C# features and best practices.

56. What Is a Finalizer in C# and When Should You Use One?

A finalizer, sometimes called a destructor (similar to C++), is a method used for cleanup just before an object is destroyed by the garbage collector. It's primarily used to release unmanaged resources. Key points include:

• Defined using a tilde (~) followed by the class name.
• Cannot be directly called, nor can they take parameters or have access modifiers.
• Intended for cleanup of unmanaged resources (e.g., file handles, database connections).

Finalizers should be used sparingly, as they can hinder performance and slow down garbage collection. In most cases, implementing the Disposable interface is preferred for manual resource cleanup. Understanding the balance between these techniques is crucial for developers, and it's often covered in most of the C# interview questions.

57. How Does the ‘Yield’ Keyword Work in C# and When Is It Useful?

The yield keyword in C# is used in iterator methods to create sequences of values without the need to store all the elements in memory at once:

• yield return allows a method to return one element at a time when needed instead of returning all at once
• The method maintains its state between calls, making it efficient for handling large datasets.
• It saves memory by not requiring the entire collection to be stored at once.

This question often focuses on how the yield keyword optimizes performance for large data collections, highlighting its efficiency in memory management and its frequently asked in many C# interview questions.

58. Differentiate Between Covariance and Contravariance in C#?

Below is the detailed difference between covariance and contravariance in C#:

59. What Are the Built-In Exceptions in C#?

C# has several built-in exceptions, all derived from the System.Exception class.

Some of the key exceptions include:

• System.IO.IOException: For input/output errors.
• System.IndexOutOfRangeException: When attempting to access an invalid array index.
• System.ArrayTypeMismatchException: When the data type does not match the array type.
• System.NullReferenceException: For attempting to use a null object.
• System.DivideByZeroException: When dividing by zero.
• System.InvalidCastException: Errors during type casting.
• System.OutOfMemoryException: When there isn’t enough memory.

These exceptions frequently appear in C# interview questions focused on error handling and application stability.

60. How to Implement a Custom Exception Class in C#?

Developing a personalized exception class in C# enables you to manage precise errors that pertain to your program.

Here’s how:

• Inherit from the Exception Class: Define a class that inherits from the Exception base class or a more specific one if needed.
• Include Constructors: Add constructors that call the base class constructors. This helps pass error messages or any relevant data up the chain.
• Override or Add Properties: You can override the Message property or add custom properties to provide more detailed error information.

This question focuses on how you handle exceptions using custom classes and the ability to manage precise error cases, it's one of the most important questions that appear in C# interview questions.

61. What Is the Singleton Design Pattern Involved in C#?

In C#, the Singleton pattern ensures that a class will only have one instance throughout the application’s lifetime. This design simplifies the management of shared resources.

Key features include:

• Private Constructor: Restricts the creation of additional instances from outside the class.
• Sealed Class: Prevents inheritance to ensure that no other class can derive from the singleton.
• Static Field: Stores the single instance of the class.
• Public Static Method: Provides a global access point to retrieve the singleton instance.

Understanding Singleton is important for developers, especially in scenarios involving application-wide states or shared resource handling. This question is frequently asked in many C# interview questions.

Subscribe to our LambdaTest YouTube Channel to get more videos on various design patterns.

62. What Are Events?

In C#, events are mechanisms that signal when something of interest happens, allowing objects to respond accordingly. They are built on top of delegates, which are references to methods.

Here's how events work:

• Registration: Objects that need to respond to an event register their handlers (methods) for that event.
• Notification: When the event occurs, all registered handlers are invoked.
• Delegates: Serve as the foundation for events, representing the methods that will be called when the event is triggered.

Mastery of event-driven programming must be the focus of every C# developer, and it's often asked in C# interview questions as it relates to events that play a significant role in building interactive applications.

63. What Is the Concept of Generics in C#?

Generics in C# 2.0 enables developers to create classes, methods, and interfaces with unspecified types. This allows for the creation of code that works with any type of data, including standard types like int or string, as well as user-defined types.

Before generics, collections in C# could only store objects of type object, which led to type safety issues and required explicit type casting. Generics improve performance by allowing type parameters to be specified, ensuring type checking happens at compile time, thus reducing runtime errors and avoiding unnecessary casting.

The role of generics is often highlighted in C# interview questions due to their impact on type safety and performance in applications.

64. What Does POCO Mean?

POCO refers to Plain Old CLR Objects, which are classes that don't rely on any framework-specific base class, functioning as standard .NET classes. These entities, also called persistence-ignorant objects, can support most LINQ queries that are used with Entity Object-derived entities.

Understanding POCO objects is essential for developers, and it is often covered in C# interview questions, as it's particularly in the context of Entity Framework and ORM-based designs.

65. When Does System.OutOfMemoryException Occurs in C#?

System.OutOfMemoryException occurs in C# when the system is unable to allocate memory for new objects.

This typically happens in cases such as:

• Trying to allocate large objects that exceed the available memory.
• Memory is becoming highly fragmented.
• A memory leak consumes too much memory.
• The GC.Collect() method fails to free up memory.

Understanding when and why this exception occurs is essential for developers and is often covered in C# interview questions.

66. What Is The First Thread That Is Created And Executed Inside A Process In C#?

The first thread created and executed in a C# process is the Main thread, which starts by executing the Main() method of the program. Mastering this is crucial for understanding multi-threading, an important topic in C# interview questions for developers.

67. Why Do We Use The Array.SyncRoot Property In C#?

The Array.SyncRoot property in C# is used in multi-threaded applications to provide a synchronization lock for arrays. It ensures thread safety when multiple threads are reading from or writing to an array simultaneously.

This question is often covered in C# interview questions as it relates to thread safety and synchronization.

68. Do Mixed Arrays Still Exist In C#?

Yes, C# allows the use of mixed arrays where elements of different types can be stored, but only if the array is defined as object[]. Although possible, it's typically avoided in favor of type safety, which is a common question in C# interview questions related to best practices in type-safe programming.

69. Differentiate Between Task.Run and Thread.Start. Also, Explain In What Situation Would You Choose One Over The Other?

Task.Run is part of Task Parallel Library which is designed for running CPU-intensive tasks one at a time on a thread pool. It is made simpler to manage tasks such as scheduling, canceling, and handling post-task actions. It also handles errors automatically and allows you to link tasks with async/await.

Thread.Start, however, creates a separate operating system thread, which uses more system resources. Threads created this way aren’t pooled, so you need to manage them more manually, including joining them and handling exceptions.

When to use:

• Use Task.Run when you want to offload tasks to a thread pool for better resource management or when working with async/await for asynchronous code.
• Use Thread.Start when you need full control over a thread, such as adjusting priorities, setting apartment states (STA/MTA), or running long-term tasks that don’t fit well with the thread pool.

Choosing between Task.Run and Thread.Start depends on whether you need resource efficiency (use Task.Run) or more granular thread control (use Thread.Start). This distinction is often covered in C# interview questions as it’s related to multi-threading and task management.

70. What Are Generics In C#?

Generics in C# is one of the most powerful features you'll encounter. They enable you to create type-safe data structures, which can significantly enhance performance and lead to high-quality code. With generics, you can reuse data processing algorithms without having to duplicate type-specific code, making your development process more efficient.

You can think of generics as being similar to templates in C++, but they differ in both implementation and capabilities. The concept of type parameters allows you to create methods and classes that delay the specification of data types until the class or method is declared and instantiated by your client code.

Using generic types often results in better performance compared to regular system types because they minimize the need for boxing, unboxing, and type casting of variables or objects. When you create a generic class, you specify the parameter types, giving you greater control and flexibility in your code. This way, you can ensure that your applications are both efficient and maintainable.

public class GenericList<T>
{
    private T[] items = new T[100]; // Creates an array to hold items of type T
    private int count = 0;


    public void Add(T item) // Method to add an item of type T
    {
        items[count] = item; // Adds the item to the array
        count++;
    }


    public T Get(int index) // Method to retrieve an item by index
    {
        return items[index]; // Returns the item at the specified index
    }
}

In this example, GenericList<T> is a generic class that uses T as a placeholder for any type of data. The Add method lets you add an item of that type, and the Get method allows you to retrieve an item using its index. This setup makes the code flexible and safe to use with different data.

71. What Are Custom Exceptions?

Custom exceptions in C# are special error classes you create yourself by building on the existing Exception class. They help you handle errors in a way that fits your application's needs. To make a custom exception, you need to define a class that inherits from Exception and can include constructors for custom messages if you want. For instance, you might create a class called InvalidAgeException to deal with incorrect age inputs.

In your application, you can use this custom exception to check input. If the age entered is zero or less, you can throw the InvalidAgeException. Then, in your main method, you can catch this exception to show the user a helpful error message. This makes it easier to understand and manage errors.

using System;


public class InvalidAgeException : Exception
{
    public InvalidAgeException(string message) : base(message) { }
}


class Program
{
    static void Main()
    {
        try
        {
            int age = -1; // Example of invalid input
            if (age <= 0) throw new InvalidAgeException("Age must be greater than 0.");
        }
        catch (InvalidAgeException ex)
        {
            Console.WriteLine(ex.Message);
        }
    }
}

This code demonstrates the effective creation and use of a custom exception. Developers need to know how to create custom exceptions, and it’s one of the most common questions that appear in C# interview questions.

Conclusion

These top 70+ C# Interview questions and answers for 2024 are a valuable resource to excel in C# interviews. Getting ready for these questions will demonstrate your technical expertise, problem-solving aptitude, and understanding of current C# practices. Give attention to grasping important C# ideas, keeping up-to-date with the latest features, and refining problem-solving skills. Balancing both theory and practical skills will improve your chances of success in the interview.