Common Interview 🚀 Questions for C# Developers

R M Shahidul Islam Shahed
15 min readApr 10

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Surprisingly, even senior developers seem to not know the answers to these questions. The key to success in a C# interview is to be well-prepared, confident, and ready to demonstrate your skills and experience.

Common interview questions for C# developers

Here are some common interview questions for C# developers:

#1: What is C# and why is it used?

C# is a modern, multi-paradigm programming language developed by Microsoft as a part of the .NET framework. It was first introduced in 2000 and has since become one of the most widely-used programming languages in the world.

C# is designed to be a simple, yet powerful language that is easy to learn and use. It is widely used for developing Windows desktop applications, mobile apps, and web applications, as well as game development, cloud-based services, and enterprise software.

C# is a versatile language that supports a wide range of programming paradigms, including object-oriented programming (OOP), functional programming (FP), and generic programming. It also includes a powerful set of libraries and frameworks that enable developers to build complex applications with ease.

One of the biggest advantages of C# is that it can be used to create applications that run on multiple platforms, including Windows, macOS, and Linux, through the use of .NET Core. Additionally, C# is constantly evolving and adding new features, making it an ideal choice for modern software development.

#2: What is the difference between an abstract class and an interface?

An abstract class and an interface are two key concepts in object-oriented programming that are used for creating reusable code. While they share some similarities, they have some key differences as well.

An abstract class is a class that cannot be instantiated on its own, but is meant to be inherited by other classes. It can contain both abstract and non-abstract (concrete) methods and properties, and may also include implementation code. Subclasses that inherit from an abstract class must implement any abstract methods in the abstract class or be declared as abstract themselves. An abstract class can also have constructor methods, instance variables, and non-abstract methods.

An interface, on the other hand, is a contract that specifies a set of methods and properties that a class must implement if it wants to be considered to be of that interface type. It contains only method and property signatures and does not include any implementation code. A class can implement multiple interfaces, but it cannot inherit from multiple classes. Interfaces are often used to define the public API of a class or a library, and to enable polymorphism by allowing objects to be treated as instances of different types that share a common interface.

// Abstract class
public abstract class Shape
{
public abstract double GetArea();
}

// Interface
public interface IDrawable
{
void Draw();
}

// Concrete class that implements both Shape and IDrawable
public class Circle : Shape, IDrawable
{
private double radius;

public Circle(double radius)
{
this.radius = radius;
}

public override double GetArea()
{
return Math.PI * radius * radius;
}

public void Draw()
{
// Code to draw a circle
}
}

In this example, Shape is an abstract class that has an abstract method GetArea(). This means that any concrete class that inherits from Shape must provide an implementation for GetArea(). On the other hand, IDrawable is an interface that has a single method Draw(). Any class that implements IDrawable must provide an implementation for Draw().

Circle is a concrete class that implements both Shape and IDrawable. It provides an implementation for GetArea() by calculating the area of a circle, and an implementation for Draw() by drawing a circle.

In summary, the key difference between an abstract class and an interface in C# is that an abstract class can have implementation code and can contain fields, while an interface can only define method signatures and constants. Additionally, a class can inherit from only one abstract class but can implement multiple interfaces.

#3: What is the difference between a value type and a reference type?

In C#, data types can be classified into two categories: value types and reference types.

Value types are data types that store their values directly in memory, and they are allocated on the stack. Examples of value types in C# include primitive types such as int, float, double, and char, as well as struct types.

Reference types, on the other hand, store a reference or a pointer to the memory location where the actual data is stored. They are allocated on the heap. Examples of reference types in C# include class types, interface types, delegate types, and array types.

The main difference between value types and reference types is the way they are stored and passed around in memory. When you assign a value type to a variable, a copy of the value is created and stored in that variable. Any changes made to the value in the variable do not affect the original value. On the other hand, when you assign a reference type to a variable, you are creating a reference to the original data, and any changes made to the data through that variable will affect the original data.

// Value type example
int x = 10;
int y = x; // y gets a copy of the value in x
y = 20; // changing y does not affect x
Console.WriteLine($"x = {x}, y = {y}"); // Output: x = 10, y = 20

// Reference type example
int[] arr1 = new int[] { 1, 2, 3 };
int[] arr2 = arr1; // arr2 points to the same location as arr1
arr2[0] = 4; // changing arr2 changes arr1 as well
Console.WriteLine($"arr1[0] = {arr1[0]}, arr2[0] = {arr2[0]}"); // Output: arr1[0] = 4, arr2[0] = 4

In the above example, x and y are both value types, and assigning y to x creates a copy of the value in x. Changing y does not affect the original value of x.

In contrast, arr1 and arr2 are both reference types and assigning arr2 to arr1 creates a reference to the same array in memory. Changing the first element of arr2 also changes the first element of arr1, since they are both pointing to the same memory location.

#4: What is LINQ and how is it used in C#?

LINQ (Language Integrated Query) is a feature of C# that provides a uniform way to query data from different data sources, such as collections, databases, and XML documents. With LINQ, you can write queries using a syntax that is similar to SQL, and the compiler will translate those queries into executable code.

LINQ can be used to query objects of various types, such as arrays, lists, and other collections. It provides a set of standard query operators, such as Where, Select, GroupBy, and Join, which can be used to filter, transform, group, and join data.

Here is an example of using LINQ to query a collection of objects:

// Define a collection of books
var books = new List<Book>
{
new Book { Title = "The Great Gatsby", Author = "F. Scott Fitzgerald", Year = 1925 },
new Book { Title = "To Kill a Mockingbird", Author = "Harper Lee", Year = 1960 },
new Book { Title = "1984", Author = "George Orwell", Year = 1949 },
new Book { Title = "Pride and Prejudice", Author = "Jane Austen", Year = 1813 }
};

// Query the collection using LINQ
var query = from book in books
where book.Year > 1900
orderby book.Year ascending
select book.Title;

// Execute the query and print the results
foreach (var title in query)
{
Console.WriteLine(title);
}

In this example, the LINQ query selects the titles of all books published after 1900 and orders them by the publication year in ascending order. The query is executed when the foreach loop is executed, and the titles are printed to the console.

#5: What is a delegate in C#?

In C#, a delegate is a type that represents a reference to a method with a specific signature. It is similar to a function pointer in C or C++, but with a number of additional features that make it more powerful and flexible.

Delegates are commonly used to implement callback functions or event handlers. They allow you to write code that can be executed by another part of the program, often in response to some kind of event or trigger. For example, you might use a delegate to specify what should happen when a button is clicked in a user interface, or to define a custom sorting function for a collection of data.

Here’s a simple example of a delegate in C#:

// Declare a delegate type with the signature of the method to be called
delegate void PrintMessage(string message);

// Define a method that matches the delegate signature
static void DisplayMessage(string message)
{
Console.WriteLine(message);
}

// Create an instance of the delegate and use it to call the method
PrintMessage printer = new PrintMessage(DisplayMessage);
printer("Hello, world!");

In this example, we define a delegate type called PrintMessage that takes a single parameter of type string and returns void. We then define a method called DisplayMessage that matches this signature, and create an instance of the delegate using this method. Finally, we call the delegate instance, passing in a string to be printed to the console. The delegate then calls the DisplayMessage method, which writes the string to the console.

#6: What are some advantages of using C# over other programming languages?

There are several advantages of using C# over other programming languages. Some of the key advantages include:

  1. Strong typing: C# is a strongly typed language, which means that every variable and object has a well-defined data type. This allows for better type checking at compile time, which can help catch errors earlier in the development process.
  2. Object-oriented programming: C# is an object-oriented programming language, which means that it is designed to work with objects and classes. This makes it easier to organize code and create reusable components.
  3. Interoperability: C# is designed to be interoperable with other programming languages and platforms. It can be used to create applications for a variety of platforms, including Windows, Linux, and macOS.
  4. Garbage collection: C# includes automatic garbage collection, which means that it automatically frees up memory that is no longer being used by the program. This makes it easier to write memory-safe code and reduces the risk of memory leaks.
  5. Language features: C# includes a number of advanced language features, such as LINQ, async/await, and nullable reference types, which make it easier to write expressive and efficient code.
  6. Large developer community: C# has a large and active developer community, which means that there is a wealth of resources available to developers, including libraries, frameworks, and tools. This can make it easier to find help and support when needed.

#7: What is the difference between a thread and a process?

In computing, a process is a container for a set of resources (such as memory, CPU time, and system handles) that are used when executing a program. A process is an instance of a program that is being executed, and it consists of one or more threads of execution. A thread is a lightweight process, and it is a basic unit of CPU utilization. A thread is scheduled independently by the operating system, and it runs within the context of its parent process.

In simpler terms, a process is a program that is being executed, and a thread is a unit of execution within that program.

The key difference between a thread and a process is that threads share the same memory space and system resources as their parent process, while processes have their own separate memory space and system resources. This means that inter-thread communication is generally faster and more efficient than inter-process communication, as threads can directly share data without the need for serialization or other forms of inter-process communication. However, this also means that a bug or crash in one thread can potentially bring down the entire process, while a bug or crash in one process will not affect other processes running on the system.

In C#, threads are used extensively for concurrent programming and can be created and managed using the System.Threading namespace. Processes can be created and managed using the System.Diagnostics namespace.

Here’s an example that demonstrates the difference between a thread and a process in C#:

using System;
using System.Threading;

class Program
{
static void Main(string[] args)
{
// Create a new thread
Thread t = new Thread(WorkerMethod);

// Start the thread
t.Start();

// Do some work in the main thread
for (int i = 0; i < 5; i++)
{
Console.WriteLine("Main thread is running.");
Thread.Sleep(1000);
}
}

static void WorkerMethod()
{
// Do some work in the worker thread
for (int i = 0; i < 5; i++)
{
Console.WriteLine("Worker thread is running.");
Thread.Sleep(1000);
}
}
}

In this example, the Main method creates a new thread using the Thread class and starts it by calling its Start method. The WorkerMethod method represents the code that will be executed in the new thread.

While the worker thread is running, the main thread continues to run the loop and prints out “Main thread is running” every second. Meanwhile, the worker thread prints out “Worker thread is running” every second.

This example demonstrates that the two threads are executing concurrently, which is a characteristic of threads. On the other hand, if you were to run another instance of this program, it would represent a separate process running independently of the first instance.

#8: How do you handle exceptions in C#?

In C#, exceptions can be handled using a try-catch block. The code that is likely to generate an exception is placed in the try block. If an exception is thrown, the catch block is executed, allowing you to handle the exception and take appropriate actions.

Here’s an example:

try
{
// Code that may generate an exception
int x = 10 / 0; // This will generate a divide by zero exception
}
catch (DivideByZeroException ex)
{
// Code to handle the exception
Console.WriteLine("An exception occurred: " + ex.Message);
}

In this example, the code in the try block will generate a divide by zero exception. The catch block catches the exception and prints an error message to the console.

You can also use the finally block to specify code that should be executed regardless of whether an exception is thrown or not:

try
{
// Code that may generate an exception
int x = 10 / 0; // This will generate a divide by zero exception
}
catch (DivideByZeroException ex)
{
// Code to handle the exception
Console.WriteLine("An exception occurred: " + ex.Message);
}
finally
{
// Code to be executed regardless of whether an exception is thrown or not
Console.WriteLine("The try-catch block has finished executing.");
}

In this example, the finally block will be executed regardless of whether an exception is thrown or not.

#9: What is the difference between an asynchronous method and a synchronous method?

In C#, a synchronous method performs an operation that blocks the calling thread until it completes, while an asynchronous method allows the calling thread to continue execution while the operation runs in the background.

When a synchronous method is called, the calling thread is blocked until the method returns a result or throws an exception. During this time, the thread is not available to handle other work. In contrast, when an asynchronous method is called, the method starts running on a separate thread or thread pool, and the calling thread is free to continue executing other code.

Asynchronous methods are useful when you need to perform long-running operations, such as network or disk I/O, without blocking the calling thread. They can improve the responsiveness and scalability of applications, especially in client-server scenarios.

In C#, asynchronous methods are typically implemented using the async and await keywords. These keywords allow you to write code that looks like synchronous code, but actually runs asynchronously in the background.

Here is an example of a synchronous method in C# that adds two numbers:

public int Add(int a, int b)
{
return a + b;
}

And here is an example of an asynchronous method in C# that does the same thing using the async and await keywords:

public async Task<int> AddAsync(int a, int b)
{
return await Task.Run(() => a + b);
}

Note that the Task.Run method is used to execute the addition operation on a separate thread, allowing the calling thread to continue with its work while the operation completes in the background. The await keyword is used to asynchronously wait for the result of the operation to become available before returning it to the caller.

#10: What is the difference between a property and a field in C#?

In C#, a field is a variable that belongs to a class or a struct, and is used to store data that can be accessed by the class or struct members.

On the other hand, a property is a member of a class or a struct that provides a flexible way to read, write or compute the value of a private field. Properties are usually implemented using a getter and a setter method, which can be used to retrieve or set the value of the underlying field.

The main difference between a field and a property is that a property provides a layer of abstraction between the field and its users. This means that, in some cases, you can control how a property is accessed and modified, while with a field, you have no control over how it is accessed or modified.

Another important difference between a field and a property is that properties can be used to validate and enforce constraints on the values that are assigned to them, while fields cannot.

Here is an example:

public class Person
{
private string _name;
public string Name
{
get
{
return _name;
}
set
{
if (string.IsNullOrEmpty(value))
{
throw new ArgumentException("Name cannot be null or empty.");
}
_name = value;
}
}

private int _age;
public int Age
{
get
{
return _age;
}
set
{
if (value < 0 || value > 120)
{
throw new ArgumentException("Age must be between 0 and 120.");
}
_age = value;
}
}

// This is a field, not a property
private string _address;
}

In this example, the Name and Age properties provide a way to read and write the values of the private fields _name and _age, while enforcing certain constraints on the values. The _address field, on the other hand, is accessed directly, without any abstraction or validation.

#11: What is the difference between an inner join and an outer join in SQL?

In SQL, joins are used to combine data from two or more tables. Inner join and outer join are two types of joins in SQL.

An inner join returns only the rows that have matching values in both tables. In other words, it only returns the rows where the join condition is satisfied in both tables. It is the most commonly used join type in SQL.

Here is an example of an inner join:

SELECT orders.order_id, customers.customer_name
FROM orders
INNER JOIN customers ON orders.customer_id = customers.customer_id;

In this example, all the rows from the customers table are returned, along with the matching rows from the orders table. If there are no matching rows in the orders table, NULL values are returned for the order_id column.

#12: What is the difference between an ArrayList and a List in C#?

In C#, both ArrayList and List are used to store a collection of items, but there are some key differences between them.

  1. Type safety: List is a generic class, which means you can specify the type of elements it will contain at compile time. This makes it type-safe, whereas ArrayList is not type-safe and can contain any type of object.
  2. Performance: List is faster than ArrayList because it is type-safe and avoids boxing and unboxing of value types. When you add a value type to an ArrayList, it is boxed into an object, which can cause performance issues.
  3. Flexibility: ArrayList is more flexible than List because it can contain any type of object. On the other hand, List is more restrictive because you have to specify the type of elements it will contain at compile time.
  4. Capacity: ArrayList increases its capacity by 50% when it needs to grow, whereas List doubles its capacity when it needs to grow. This means that List may waste more memory than ArrayList if it has to grow frequently.

In summary, List is the preferred option in most cases because it is type-safe and faster than ArrayList. However, ArrayList may still be useful in scenarios where you need more flexibility or when dealing with legacy code that uses ArrayList.

Here is an example showing the difference between ArrayList and List<T> in C#:

using System;
using System.Collections;

class Program
{
static void Main(string[] args)
{
// ArrayList example
ArrayList arrayList = new ArrayList();
arrayList.Add("one");
arrayList.Add("two");
arrayList.Add(3);

Console.WriteLine("ArrayList:");
foreach (object obj in arrayList)
{
Console.WriteLine(obj);
}

// List example
List<string> list = new List<string>();
list.Add("one");
list.Add("two");
// The following line will generate a compile-time error because a List<string> only accepts strings:
// list.Add(3);

Console.WriteLine("\nList:");
foreach (string str in list)
{
Console.WriteLine(str);
}
}
}

In this example, we create an ArrayList and a List<string>. We add elements to both collections, including an int in the ArrayList. When we iterate over the ArrayList, we see that it contains three elements of different types. When we try to add an int to the List<string>, we get a compile-time error because a List<string> only accepts strings. When we iterate over the List<string>, we see that it contains only two strings. This shows that a List<T> is type-safe, whereas an ArrayList can hold elements of any type. Additionally, because a List<T> is type-safe, it can offer better performance than an ArrayList.

Conclusion

These are just a few examples, and the specific questions may vary depending on the company and the job position. It’s important to note that the specific interview questions may vary depending on the company and the job requirements. It’s always a good idea to research the company and the position you’re interviewing for to get a sense of what kind of questions to expect. Additionally, it’s important to practice coding problems and review your C# skills before an interview to ensure you’re prepared to answer technical questions.

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R M Shahidul Islam Shahed

.NET Developer, Author at Codecanyon