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Robert Johns | 15 Jan, 2024
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Top 40 C# Interview Questions & Answers To Get Hired in 2024

 

Debuting in late 2001, C# is a type-safe language that’s easy to learn and primarily used for object-oriented programming. C# also adopts a component-oriented approach that allows applications to be built from new or predefined components with the .NET framework.

At its core, C# can be tied back to the C language family, but coders with experience in Java and Javascript will also be very familiar with C# syntax.

C# offers a range of features, including garbage collection, nullable types, and a unified type system where all types (including primitives like ints, chars, etc.) inherit from the Object class.

This article covers 40 C# interview questions and answers, ranging from basic to advanced. So whether you’re a new programmer with aspirations to become a C# developer or a seasoned pro making a job switch, you’re in the right place.

Download our C# Interview Questions and Answers PDF.

New to C# but want to learn? Check out our article on Learning C# as a Beginner.

Basic C# Interview Questions & Answers

1. What Is C#?

C# is a type-safe, object-oriented language used to create .Net applications with a component-oriented approach.

You can use C# to create apps for Microsoft Windows, cloud-based API services, mobile apps for iOs and Android, software for AI and Machine Learning (ML), blockchain apps, and serverless apps.

Note, this is the most fundamental C# interview question, so make sure you know it!

2. List Some Key C# Features.

C# Features

  • Simple language: Lacks pointers or direct memory modification vs. C++ or C
  • Type-safe: Ensures variable data types are correctly defined
  • Object-oriented: Define and use classes/objects
  • Component-oriented: Reuse existing components without coding from scratch 
  • Open-source: Can develop apps on multiple OS, inc. Mac and Linux
  • Interoperability: Can develop with managed and unmanaged 3rd party code
  • Structured-programming: Can break programs into functions
  • Rich library: Built-in functions speed up development
  • Fast: Compilation and execution are very quick

This is one of those simple C sharp interview questions that you should know in your sleep!

3. What Are Classes in C#?

As an object-oriented language, C# makes heavy use of classes. A class is a concept or blueprint to define how data will be structured, stored, and managed within a program.

Classes have properties and methods, which are referred to as class members. These could be private, which means that they are only accessible to the class, or public, which allows other parts of a program to access them. 

4. What Are Objects in C#?

Objects are class instances that can represent the characteristics of real-world entities (or imaginary!) by using class attributes and methods.

For example, if we have a program that handles books, we can create a Book class with two properties, the title, and the author. We can then instantiate multiple instances of the Book class to create different Book objects with various titles and authors.

5: Summarize the Different Class Types in C#.

While all classes in C# have a base type of Object, there are 4 distinct types.

  • Static Class: Cannot instantiate objects with the New keyword, and class members can only be accessed by using the actual class name
  • Abstract class: Restricted, which means it cannot be used to create objects directly, so we need to inherit from this via a sub-class if we want to access its members 
  • Partial Class: Allows methods, events, and properties to be split into separate/multiple .cs source files, which are combined into one class at compile time
  • Sealed Class: Prevents users from inheriting from the class, and restricts access to the class members such as properties or methods

6: Summarize the Four Access Modifiers in C#.

  1. Private: Only accessible from within the class
  2. Public: Accessible from anywhere in the code
  3. Internal: Only accessible at the current assembly point of the class
  4. Protected: Only accessible by class members and classes that inherit from it

7: Summarize the Four Steps for Compiling C# Code.

  1. Compile source code into a managed module
  2. Combine the newly created module from step 1 with assembly code
  3. Load the CLR (Common Language Runtime) 
  4. Execute the assembly with CLR 

8: What Is the Difference Between ‘Break’ & ‘Continue’ in C#?

  • Break: Used in loops (for, etc.) and switch statements, exits the iteration/switch and skips any remaining code in the loop or switch block
  • Continue: Only used in loops, skips any remaining code in the loop and starts the next iteration from the beginning of the loop

9: Summarize the C# Keywords Used for Error Handling.

  • Try:Used to signal a block of code that handles errors, followed by at least one catch 
  • Catch: Used to handle exceptions raised by code within a try block
  • Finally: Signals a block of code that almost always runs after all other code has executed, regardless of whether an error is handled
  • Throw: Used to throw an exception if a problem is encountered, and usually requires an if statement to check for a problem

10: What Are Accessors in C#?

The two accessors in C# are get and set. These are used to retrieve or assign values for the private fields related to a property within a class or struct.

  • Get: Used to retrieve the value of a private field
  • Set: Used to assign a value to a private field

C# Property

11: Demonstrate the Different Ways to Add Comments in C#.

This is one of the most fundamental C# programming interview questions, and it should be automatic for any prospective developer.

Single line comment (//):

// single line comment example

XML comment (///):

/// XML comment example

Multi-line comment (/* */):

/*
multi-line comment example
*/

12: What Are Static Declared Variables in C#?

Each C# object has member variables that have a particular scope. We can use static declared variables to ensure that each instance of a class object has the same value. This is possible because each class object shares the same instance of the static variable for the class.

The name static declared variables relates to an unchanging value (static keyword) that is strongly typed (declared), but usually, these are just called static variables.

13: What Are Value Types & Reference Types in C#?

Value Type: Directly stores the variable data value inside its allocated memory space

Reference Type: Does not directly store the variable data in its memory location but instead stores a reference to another memory address that holds the variable data value, so the equivalent of a pointer

Value Types & Reference Types in C#

14: What Are Namespaces in C#?

Namespaces are used to organize code and provide a way to separate code that may utilize the same names.

The .NET framework uses namespaces to separate and organize its extensive collection of provided classes, but you can also use custom namespaces to organize your own code and limit the scope of method and class names within your projects.

Want to boost your interview prep? Refresh key concepts by checking out the 10 Best C# Books

Intermediate C# Interview Questions

15: Why Would You Implement the ‘using’ Statement in C#?

To manage and automatically release resources once an object has finished its processing. Specifically, by implementing the using statement, we implicitly ensure that the .Dispose() method is called to automatically release unused resources. 

16: What’s the Difference Between Dispose() and Finalize()?

Both methods have a similar purpose, namely to release unused resources. However, unlike the .Dispose() method, .Finalize() does not guarantee the garbage collection of any unused and released resources.

17: What’s the Difference Between Boxing & Unboxing in C#?

Both boxing and unboxing are techniques for converting types in C#.

  • Boxing: Refers to an implicit process that is used to convert a variable from a Value Type to a Reference Type data type
  • Unboxing: Refers to the opposite of the boxing process, meaning that it represents an explicit conversion process where we convert a Reference Type data type into a Value Type variable

18: Summarize the Options to Pass Parameters With Methods in C#.

  • Value parameter: The default option, parameters that are passed into a method will result in a new copy being stored in memory, and any changes to this copy will not be reflected on the original value parameter
  • Reference parameter: Stores the memory address of a parameter within the method argument, so any changes to the argument are reflected on the original parameter in memory
  • Output parameter: This is used when you want to a method to return multiple values

19: How Do You Use Nullable Types in C#?

In order to assign a null value to Value Types in C#, we have to use the Nullable Type. This can be done with either the Nullable keyword or the ‘?’ operator shorthand, as shown below.

It is not possible to use the Nullable Type with vartype variables because these are not explicitly defined but rather implicitly defined by the value assigned to the variable.

Note that this is one of those C# important questions you need to understand properly.

Nullable Type example:

Nullable<int> i = null;
int? i = null;

20: What Are C# Constructors?

Constructors are a special type of function member from a class that shares the same class name. They are automatically invoked whenever a new object instance of the class is created, and this process also invokes the data members of the class. This may involve passing parameters into the class constructor if it is parameterized.

21: What Is a C# Destructor?

These C# class methods are used to destroy class instances when they’re no longer required. Destructor methods are not called explicitly within a program, but rather the .NET garbage collection calls these methods implicitly when an object is no longer accessible (not in scope).

To define a destructor, we use the class name and precede this with a tilde, as shown in the example below.

Destructor code example:

class Example {

  ~Example() {
    Console.WriteLine("Destructor has been called.");
  }
}

22: Summarize the C# Non-Generic Collection Types.

Each non-generic collection can be used to store different types as they are not strongly typed.

  • ArrayList: Similar to an array, does not have a specific size, and can store any number of elements
  • HashTable: Stores key-value pairs for each item, does not have a specific size, can store any number of elements, key objects must be immutable
  • SortedList: Combination of ArrayList and HashTable, data stored as key-value pairs, items sorted by keys, items accessed by key or index, does not have a specific size, can store any number of elements
  • Stack: Simple Last-In-First-Out (LIFO) structure, does not have a specific size, can store any number of elements, elements are pushed onto the stack and popped off from the stack
  • Queue: First-In-First-Out (FIFO) structure, does not have a specific size, can store any number of elements, elements are enqueued into the queue and dequeued from the queue

23: What Is a Jagged Array in C#?

Jagged Array in C#

A jagged array is made up of elements that are also arrays, but each of these member arrays can be a different size. Because of this composition, we can refer to a jagged array as an ‘array of arrays’.

24: Summarize the Process of File Handling in C#

File handling can refer to the process of opening, reading, writing, and appending to and from files. The two most common file-handling operations are reading and writing, and in each case, the file’s data is converted into a stream of bytes.

When writing to a file, we use the output stream, and when reading from a file, we use the input stream. We use the System.IO namespace to access static methods for file handling.

25: What Is an Interface Class in C#?

This is an abstract class that only contains public abstract methods and properties.

These methods and properties are only declared and have no definition or fields as these are defined within the class that the abstract class interface inherits. It is best practice to name an interface class with ‘I’ to identify it as an interface visually.

26: Describe the C# Struct Data Type.

The struct is a value type data type in C# that inherits from System.Value. These are used to store data structures and usually small quantities of data. Structs can use parameterized constructors, and they are declared with the struct keyword.

You can use the new keyword to create a struct, which will require parameters to be passed (if defined in the constructor), else you can create a struct without the new keyword, which will leave any struct members unassigned.

27. What’s the Difference Between a Class and a Struct?

Struct

Class

Value Types that are allocated on the stack or on inline (more performant)

Reference Types that are allocated on the heap and garbage-collected

Used for small data structures

Used for complex data structures

Create with or without the new keyword

Create with the new keyword

Cannot have a destructor

Can have a destructor

Cannot inherit from another class or struct

Can inherit from another class or be the base for another class to inherit from

Assignment to variables creates a copy of the data, and operations on one variable will not affect others

Assignment to variables creates a copy of the object reference, and any operation on one variable will affect others

Want to become a C# developer? Check out the Best C# Courses to boost your skills and enhance your resume.

C# Interview Questions for Experienced Professionals

28: What’s the Difference Between Managed & Unmanaged Code?

Managed code was developed with the .NET framework using C# (or VB.NET) and is directly executed with the common language runtime (CLR). The CLR manages object creation, object deletion, and memory management processes in these scenarios.

Unmanaged code is any codebase that was developed without the .NET framework, which means that code execution is not handled by the CLR. Unmanaged code is sometimes referred to as unsafe code since the CLR does not handle it, and it provides low-level access, which, if not programmed correctly, can lead to memory leaks or other bugs.

Note: This is one of the most common C# .Net interview questions, so be sure to know the difference between managed and unmanaged code.

29: What Is a Virtual Method in C#?

A method that we can redefine in a derived class, which means it can have different implementations in the base (original) and derived classes.

These are used when we want to extend the functionality of an original method from a base class. We use the virtual keyword to create a virtual method in the base class, which allows the functionality to be overridden in the derived class. This is, therefore, an example of polymorphism.

You cannot use the virtual keyword with static, abstract, or private methods in a base class. 

30: What is Reflection in C#?

A process for describing metadata for types, fields, and methods in your code at runtime by using the System.Reflection namespace.

This can be useful to ‘reverse engineer’ your code after it executes, thus allowing you to observe and examine important metadata for the different elements within your code.

31: What Is Serialization in C#?

A conversion process that changes a class object’s data into a byte stream. This can then be transported over a network or stored in memory, a file, or a database.

Primarily, serialization is used to save an object’s state, allowing us to restore this later. To reverse the process, we need to use deserialization. We have various serialization types available to us.

  • Binary: Convert object to binary bytes, useful for network transfer or local storage
  • SOAP: Convert object to byte stream for network transfer over varied architectures
  • XML: Convert object to XML format, useful for network transfer or local storage
  • JSON: Convert object to a JSON string, useful for data exchange between apps

32: Summarize Thread Pooling in C#.

Thread Pooling in C#

When we initialize a multithreaded application, thread pooling is used to create a collection of threads that can be reused when new tasks require a thread. This removes the need to create new threads for each new task.

Each of the threads within the pool is designed for a particular task, and it waits in the pool until that task type arises. Whenever a thread completes its task, it returns the pool to await future assignments to new tasks.

33: Can You Use ‘this’ in a C# Static Method?

No, because ‘this’ returns a reference to the current instance of the same class, and static methods do not belong to any particular instance because they can be called by the class name alone.

One exception is with Extension Methods, as we can use ‘this’ with the method's first parameter.

34: What Is Constructor Chaining in C#?

This is a way to share initialization code between different constructors within the same class or from a base class via inheritance in the derived class.

If we have multiple constructors within the same class, we can chain these with the :this() keyword, which allows us to flexibly implement various constructors within the same class.

If we want to chain constructors via inheritance, we can use the :base() keyword, which maps the child class constructor to the constructor of the parent class.

35: Summarize the Thread Lifecycle in C#.

  1. Unstarted State (New): New instance of the Thread class is initialized and not started
  2. Runnable State: The Start() method is called, and the thread is ready to run
  3. Running: A thread has been selected by the scheduler to run
  4. Not Runnable: Sometimes referred to as WaitSleepJoin, a thread enters this state when we call Wait() or Sleep() and also when it calls Join() on another thread
  5. Dead State: A thread has completed its task, or the Abort() method has been called; this is the last stage of a thread’s lifecycle

Thread Lifecycle in C#

36: Why Are Async and Await used in C#?

If we have a program that requires methods to be run independently of the primary process, we need to use asynchronous programming. This allows us to run processes and, when needed, make them wait without blocking the rest of the program.

To do this, we use the Async keyword to create an asynchronous method and Await to run it without blocking our program.

37: What Is an Indexer in C#?

Sometimes referred to as smart arrays, indexers are used to create a class or struct that behaves like a virtual array.

By doing this, instances of the class or struct can be accessed using the standard array [] accessor, allowing them to be indexed like an element of an array.

38: What’s the Difference Between ‘ref’ and ‘out’ in C#?

Both the ref and out keywords allow us to pass in arguments by reference to a method. By doing this, we can ensure that any changes we make to the argument within the body of the method or function will be reflected in the original variable and maintained outside the method’s scope.

  • Ref: You must initialize parameters before passing to ref, but you don’t need to initialize or assign the parameter before returning it to the calling method. This is often used to pass arguments you want to modify in a function and maintain the changes.
  • Out: You don’t need to initialize parameters before passing to out, but you do have to ensure they are initialized before returning to the calling method. This is often used when you have a method that you want to design to return multiple values.

Note this is one of those C# technical interview questions for experienced developers that you should be able to answer without blinking.

39: What Is a Singleton in C#?

A singleton is a special type of class (which also applies to other languages) that we use to ensure that we can only create a single instance of the class. We also need to ensure that we provide a global access point.

Logically, when using this design pattern, we don’t parameterize the constructor since we will not create multiple class instances with different fields. We also ensure the constructor is private.

40: What Are C# Delegates?

These are the C# equivalent of a C++ function pointer, with the primary difference being that C# delegates are type-safe. These are a Reference Type data type that can be used to treat other functions like data. We can pass functions as parameters and handle any associated callback functions and events.

Want to increase your chances of landing a C# developer job? Consider adding one of the Best C# Certifications to your resume.

Bonus Tips for C# Interviews

  • Company Research: Do they have a blog with insights about their culture, major company milestones, exciting projects, or any other talking points that might come up during the interview? Make sure you know who you’re going to be working for.
  • Clarify Key Details: If you’re unsure about the interview structure, process, timing, or anything else, contact the recruiter or hiring manager to ask any and all questions you may have. You can also look on sites like Glassdoor for information from previous interviews and candidates.
  • Soft Skills: Being prepared for all things technical is a strong starting point, but don’t be caught off guard by questions relating to soft skills, including teamwork, communication, dealing with difficult situations, etc. Generally, you should expect behavioral and situational questions to test your soft skills and general personality.

Download our C# cheat sheet as a supplementary resource for your interview prep.

Conclusion

C# is a type-safe language that’s easy to learn and well-suited to object-oriented and component-oriented programming with the .NET framework. While its roots are linked to the C language family, coders with Java and Javascript familiarity will be comfortable with C# syntax.

Whether you’re looking for an entry-level software development position or an experienced C# developer eyeing a new role, you’ll want to refresh your memory of the essential C# concepts. 

This article has covered 40 C# interview questions and answers, with topics ranging from complete beginner to C# interview questions for 5 years of experience or more. Any time you can invest in studying these C# interview questions is time well spent to ensure you’re ready for anything!

Interested in becoming a C# developer but want to boost your skills? Check out the Complete C# Masterclass

 

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By Robert Johns

Technical Editor for Hackr.io | 15+ Years in Python, Java, SQL, C++, C#, JavaScript, Ruby, PHP, .NET, MATLAB, HTML & CSS, and more... 10+ Years in Networking, Cloud, APIs, Linux | 5+ Years in Data Science | 2x PhDs in Structural & Blast Engineering

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