TOP 20 C# INTERVIEW QUESTIONS AND ANSWERS YOU NEED TO KNOW

Are you preparing for a C# interview and looking for some commonly asked questions to practice? Look no further! In this blog post, we have compiled a list of the top 20 C# interview questions that are frequently asked by hiring managers. Whether you are a beginner or an experienced C# developer, these questions will help you brush up on your knowledge and prepare for your upcoming interview. So let’s get started and see how well you fare on these essential C# interview questions.

Table of Contents

Can you describe the difference between a value type and a reference type?

In C#, a value type is a data type that represents a simple value, such as an integer or a floating-point number. These values are stored directly in the location in memory where the variable is declared. In contrast, a reference type is a data type that stores a reference to the value’s memory address. This means that when you assign a reference type to a variable, you are not creating a new copy of the value, but rather creating a new reference to the same memory location.

Here is an example of a value type and a reference type in C#:

// This is a value type.
int number = 10;

// This is a reference type.
string text = "Hello, world!";

Can you explain the difference between the “var” keyword and explicit typing in C#?

The “var” keyword in C# is used to declare a variable whose type is inferred from the expression used to initialize it. This means that you don’t need to explicitly specify the data type of the variable, and the compiler will automatically determine the appropriate type based on the value assigned to the variable. In contrast, explicit typing involves specifying the data type of a variable when you declare it, using a syntax such as “int number = 10;”.

Here is an example of the “var” keyword and explicit typing in C#:

// This uses the "var" keyword.
var number = 10;

// This uses explicit typing.
int number = 10;

What is the purpose of the “using” statement in C#?

The “using” statement in C# is used to specify a block of code in which a particular resource, such as a file or network connection, will be used. The “using” statement ensures that the resource is properly disposed of when the block of code is finished executing. This helps to prevent resource leaks and improve the performance of your code.

Here is an example of the “using” statement in C#:

using (FileStream stream = new FileStream("file.txt", FileMode.Open))
{
    // Use the stream here.
}

Can you describe the role of the “static” keyword in C#?

The “static” keyword in C# is used to declare a member of a class that belongs to the class itself, rather than to a specific instance of the class. This means that a static member can be accessed without creating an instance of the class. This can be useful for members that are shared by all instances of a class, such as constants or utility methods.

Here is an example of the “static” keyword in C#:

public class MyClass
{
    // This is a static field.
    public static int Count = 0;

    // This is a static method.
    public static int Increment()
    {
        return ++Count;
    }
}

Can you explain the difference between a class and a struct in C#?

In C#, a class is a reference type that defines a blueprint for an object, while a struct is a value type that defines a lightweight object. This means that a class is allocated on the heap and accessed through a reference, while a struct is allocated on the stack and accessed directly. Classes support inheritance, while structs do not.

Here is an example of a class and a struct in C#:

// This is a class.
public class MyClass
{
    // Class members go here.
}

// This is a struct.
public struct MyStruct
{
    // Struct members go here.
}

Can you describe the difference between an interface and an abstract class in C#?

In C#, an interface is a contract that defines a set of methods and properties that a class must implement. An interface does not provide an implementation for these members, and a class can implement multiple interfaces. In contrast, an abstract class is a class that cannot be instantiated and provides an implementation for some or all of its members. An abstract class can also define abstract members, which must be implemented by derived classes.

Here is an example of an interface and an abstract class in C#:

// This is an interface.
public interface IMyInterface
{
    // Interface members go here.
}

// This is an abstract class.
public abstract class MyAbstractClass
{
    // Abstract class members go here.
}

Can you explain the difference between a method and a property in C#?

In C#, a method is a code block that contains a series of statements and defines an action that can be performed by an object. A method has a return type and can accept zero or more parameters. In contrast, a property is a member of a class that provides a way to get or set the value of a private field. A property has a type and does not have a return type or parameters.

Here is an example of a method and a property in C#:

public class MyClass
{
    // This is a method.
    public void MyMethod(int parameter)
    {
        // Method body goes here.
    }

    // This is a property.
    public int MyProperty { get; set; }
}

Can you describe the purpose of the “async” and “await” keywords in C#?

The purpose of the “async” and “await” keywords in C# is to simplify the process of writing asynchronous code. The “async” keyword is used to mark a method as asynchronous, and the “await” keyword is used to suspend the execution of the method until a specified task is complete. This allows the method to run in the background while other code continues to execute. Here is an example of how these keywords are used:

public async Task DoWorkAsync()
{
    // Do some work...
    await Task.Delay(1000); // Suspend execution until the specified task is complete.
    // Continue doing work...
}

Can you explain the difference between a “continue” statement and a “break” statement in C#?

The “continue” statement in C# is used to skip the current iteration of a loop and continue with the next iteration. This is useful if you want to skip certain values or conditions in a loop. Here is an example of how the “continue” statement is used:

for (int i = 0; i < 10; i++)
{
    if (i % 2 == 0)
    {
        continue; // Skip the current iteration if i is even.
    }

    Console.WriteLine(i); // Only odd values will be printed.
}

On the other hand, the “break” statement is used to exit a loop immediately. This is useful if you want to stop looping under certain conditions. Here is an example of how the “break” statement is used:

for (int i = 0; i < 10; i++)
{
    if (i == 5)
    {
        break; // Exit the loop if i is equal to 5.
    }

    Console.WriteLine(i); // Only values from 0 to 4 will be printed.
}

Can you describe the difference between a “for” loop and a “foreach” loop in C#?

The “for” loop in C# is a type of loop that allows you to specify a loop counter and a condition for when the loop should stop executing. This is useful if you know how many iterations you want to execute, or if you have a specific condition that you want to use to control the loop. Here is an example of how the “for” loop is used:

for (int i = 0; i < 10; i++)
{
    Console.WriteLine(i); // Prints the values from 0 to 9.
}

On the other hand, the “foreach” loop is a type of loop that allows you to iterate over the elements in a collection. This is useful if you want to perform the same action for each element in a collection, without having to worry about loop counters or stop conditions. Here is an example of how the “foreach” loop is used:

var numbers = new List<int> { 1, 2, 3, 4, 5 };
foreach (int number in numbers)
{
    Console.WriteLine(number); // Prints the values from 1 to 5.
}

The main difference between the “for” loop and the “foreach” loop is that the “for” loop gives you more control over the loop, while the “foreach” loop is simpler and easier to use with collections.

Can you explain the difference between a “try” block and a “catch” block in C#?

The “try” block in C# is used to enclose code that might throw an exception. This is useful because it allows you to handle exceptions in a structured way, without having to check for exceptions in every line of code. Here is an example of how the “try” block is used:

try
{
    // Code that might throw an exception goes here.
}
catch (Exception ex)
{
    // Handle the exception here.
}

The “catch” block in C# is used to handle exceptions that are thrown in the “try” block. The “catch” block allows you to specify the type of exception that you want to handle, and provides a variable that contains the exception object. This allows you to access the exception information and take appropriate action. Here is an example of how the “catch” block is used:

try
{
    // Code that might throw an exception goes here.
}
catch (Exception ex)
{
    Console.WriteLine(ex.Message); // Print the exception message.
}

The main difference between the “try” block and the “catch” block is that the “try” block is used to enclose code that might throw an exception, while the “catch” block is used to handle exceptions that are thrown.

Can you describe the purpose of the “lock” keyword in C#?

The “lock” keyword in C# is used to synchronize access to a block of code, so that only one thread can execute the code at a time. This is useful when you are working with shared resources that can be accessed by multiple threads, because it ensures that the shared resource is not modified by two threads simultaneously. Here is an example of how the “lock” keyword is used:

private readonly object _lock = new object();

public void DoWork()
{
    lock (_lock)
    {
        // Code that accesses a shared resource goes here.
    }
}

In this example, the “lock” keyword is used to synchronize access to the code inside the “lock” block, so that only one thread can execute the code at a time. This ensures that the shared resource is not modified by two threads simultaneously, which can cause conflicts or errors.

Can you explain the difference between the “String” class and the “string” data type in C#?

The “String” class in C# is a reference type that represents a sequence of Unicode characters. This class provides various methods and properties for manipulating strings, such as concatenating strings, finding substrings, and comparing strings. Here is an example of how the “String” class is used:

var s1 = new String('a', 5); // Create a string with 5 'a' characters.
var s2 = s1.Substring(1, 3); // Get a substring of s1 starting at index 1 with length 3.
var s3 = s1 + s2; // Concatenate s1 and s2.

Console.WriteLine(s3); // Prints "aaaab".

On the other hand, the “string” data type in C# is a value type that represents a sequence of Unicode characters. This data type is similar to the “String” class, but it is more efficient and has some limitations. Here is an example of how the “string” data type is used:

string s1 = "hello";
string s2 = s1.Substring(1, 3); // Get a substring of s1 starting at index 1 with length 3.
string s3 = s1 + s2; // Concatenate s1 and s2.

Console.WriteLine(s3); // Prints "helloell".

The main difference between the “String” class and the “string” data type is that the “String” class is a reference type, while the “string” data type is a value type. This means that the “String” class is allocated on the heap, while the “string” data type is allocated on the stack. This has implications for how the two types are used and stored in memory.

Can you describe the role of the “delegate” keyword in C#?

The “delegate” keyword in C# is used to define a type that represents a reference to a method. This is useful because it allows you to pass a method as an argument to another method, or to return a method as the result of another method. Here is an example of how the “delegate” keyword is used:

delegate int MathOperation(int x, int y);

int Add(int x, int y)
{
    return x + y;
}

int Multiply(int x, int y)
{
    return x * y;
}

public void DoWork()
{
    MathOperation operation = Add;
    int result1 = operation(1, 2); // Invokes the Add method.
    Console.WriteLine(result1); // Prints 3.

    operation = Multiply;
    int result2 = operation(1, 2); // Invokes the Multiply method.
    Console.WriteLine(result2); // Prints 2.
}

In this example, the “delegate” keyword is used to define a type called “MathOperation” that represents a reference to a method that takes two “int” arguments and returns an “int” result. The “Add” and “Multiply” methods are defined to match the signature of the “MathOperation” delegate, so they can be assigned to a variable of type “MathOperation”. The “DoWork” method uses the “MathOperation” variable to invoke the “Add” and “Multiply” methods, depending on the value of the “operation” variable. This demonstrates how the “delegate” keyword allows you to pass a method as an argument and invoke it dynamically.

Can you explain the difference between a “nullable” type and a regular data type in C#?

A “nullable” type in C# is a type that can hold either a value or a null reference. This is useful because it allows you to assign a null value to a variable, even if the variable is a value type. Here is an example of how a “nullable” type is used:

int? x = null; // The "?" operator specifies that x is a nullable type.
Console.WriteLine(x.HasValue); // Prints "False", because x does not have a value.

x = 5;
Console.WriteLine(x.Value); // Prints "5", because x has a value.

In this example, the “int?” type is used to define a nullable integer variable called “x”. The “?” operator specifies that the type is nullable, and the “HasValue” and “Value” properties are used to check if the variable has a value and to access the value of the variable. This demonstrates how a “nullable” type allows you to assign a null value to a variable and check for a null value.

On the other hand, a regular data type in C# is a type that cannot hold a null reference. This is the default behavior for value types, such as “int”, “float”, and “bool”, and for reference types, such as “string” and “object”. Here is an example of how a regular data type is used:

int x = 5; // This is a regular data type, so x cannot be assigned a null value.
Console.WriteLine(x); // Prints "5".

// x = null; // This line will cause a compile-time error, because x is a regular data type.

In this example, the “int” type is used to define a regular integer variable called “x”. The variable is assigned the value 5, and it is not possible to assign a null value to the variable. This demonstrates how a regular data type does not allow you to assign a null value to a variable.

Can you describe the difference between an “event” and a “delegate” in C#?

The “event” keyword in C# is used to define an event that can be raised by an object to notify other objects that something has happened. This is useful because it allows you to create a publish-subscribe model, where objects can subscribe to events and be notified when the events are raised. Here is an example of how the “event” keyword is used:

public class Clock
{
    // Define an event of type "EventHandler".
    public event EventHandler Tick;

    public void Start()
    {
        while (true)
        {
            System.Threading.Thread.Sleep(1000); // Wait for 1 second.

            // Raise the "Tick" event.
            Tick?.Invoke(this, EventArgs.Empty);
        }
    }
}

public class Timer
{
    public Timer(Clock clock)
    {
        // Subscribe to the "Tick" event.
        clock.Tick += OnTick;
    }

    private void OnTick(object sender, EventArgs e)
    {
        Console.WriteLine("Tick!"); // Print a message when the event is raised.
    }
}

public void DoWork()
{
    var clock = new Clock();
    var timer = new Timer(clock);

    clock.Start(); // Start the clock, which will raise the "Tick" event every second.
}

In this example, the “Clock” class defines an event called “Tick” of type “EventHandler”. The “Timer” class subscribes to the “Tick” event and defines a method called “OnTick” that is executed when the event is raised. The “DoWork” method creates a “Clock” and a “Timer” object, and starts the clock. This causes the clock to raise the “Tick” event every second, which is handled by the “OnTick” method in the “Timer” class. This demonstrates how the “event” keyword allows you to create and raise events, and how other objects can subscribe to the events and be notified when they are raised.

On the other hand, a “delegate” in C# is a type that represents a reference to a method. This is useful because it allows you to pass a method as an argument to another method, or to return a method as the result of another method. Here is an example of how a “delegate” is used:

delegate int MathOperation(int x, int y);

int Add(int x, int y)
{
    return x + y;
}

int Multiply(int x, int y)
{
    return x * y;
}

public void DoWork()
{
    MathOperation operation = Add;
    int result1 = operation(1, 2); // Invokes the Add method.
    Console.WriteLine(result1); // Prints 3.

    operation = Multiply;
    int result2 = operation(1, 2); // Invokes the Multiply method.
    Console.WriteLine(result2); // Prints 2.
}

Can you explain the difference between the “is” and “as” operators in C#?

The “is” operator in C# is a type-checking operator that is used to determine whether an object is of a certain type. This is useful because it allows you to perform type-specific operations on an object, such as casting the object to the specified type or calling type-specific methods on the object. Here is an example of how the “is” operator is used:

object x = "hello";

if (x is string) // Check if x is of type "string".
{
    string s = (string)x; // Cast x to type "string".
    Console.WriteLine(s.Length); // Print the length of the string.
}

On the other hand, the “as” operator in C# is a type-casting operator that is used to cast an object to a specified type. This is similar to the “is” operator, but it automatically performs the type-cast if the object is of the specified type. This is useful because it allows you to avoid having to check the type of the object before casting it, which can make your code cleaner and more concise. Here is an example of how the “as” operator is used:

object x = "hello";

string s = x as string; // Try to cast x to type "string".

if (s != null) // Check if the cast was successful.
{
    Console.WriteLine(s.Length); // Print the length of the string.
}

Can you describe the purpose of the “params” keyword in C#?

The “params” keyword in C# is used to define a parameter that takes a variable number of arguments. This is useful because it allows you to call a method with a varying number of arguments, depending on your needs. Here is an example of how the “params” keyword is used:

public void PrintNumbers(params int[] numbers)
{
    foreach (int number in numbers)
    {
        Console.WriteLine(number);
    }
}

public void DoWork()
{
    PrintNumbers(1, 2, 3, 4, 5); // Prints the numbers 1, 2, 3, 4, and 5.
    PrintNumbers(6, 7, 8); // Prints the numbers 6, 7, and 8.
    PrintNumbers(); // Prints nothing.
}

In this example, the “PrintNumbers” method is defined with a “params” parameter of type “int[]”. This means that the method can be called with any number of “int” arguments, and they will be collected into an array of “int” values. The “DoWork” method calls the “PrintNumbers” method with different numbers of arguments, and the “PrintNumbers” method prints the numbers that are passed to it. This demonstrates how the “params” keyword allows you to define a method that takes a variable number of arguments.

Can you explain the difference between “structural” and “nominal” typing in C#?

Structural typing in C# is a way of defining the structure of an object, rather than the name of the object’s type. This means that two objects are considered to be of the same type if they have the same properties and methods, regardless of their actual type names. This is useful because it allows you to write code that is more flexible and adaptable, because it is not tied to specific type names. Here is an example of how structural typing is used in C#:

public interface IShape
{
    double GetArea();
}

public class Circle : IShape
{
    public double Radius { get; set; }

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

public class Rectangle : IShape
{
    public double Width { get; set; }
    public double Height { get; set; }

    public double GetArea()
    {
        return Width * Height;
    }
}

public void DoWork()
{
    IShape circle = new Circle() { Radius = 5 };
    IShape rectangle = new Rectangle() { Width = 10, Height = 20 };

    Console.WriteLine(circle.GetArea()); // Prints 78.5398163397448.
    Console.WriteLine(rectangle.GetArea()); // Prints 200.
}

In this example, the “IShape” interface defines a method called “GetArea” that must be implemented by any class that implements the interface. The “Circle” and “Rectangle” classes both implement the “IShape” interface, and they define their own versions of the “GetArea” method. The “DoWork” method creates a “Circle” and a “Rectangle” object, and assigns them to variables of type “IShape”. This means that the objects are considered to be of the same type, even though they are of different classes.

Nominal typing in C# is a way of defining the type of an object based on its actual type name. This means that two objects are considered to be of the same type only if they have the same type name, regardless of their structure. This is the default behavior in C#, and it is useful because it allows you to enforce strict type-checking and avoid runtime errors. Here is an example of how nominal typing is used in C#:

public class Circle
{
    public double Radius { get; set; }

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

public class Rectangle
{
    public double Width { get; set; }
    public double Height { get; set; }

    public double GetArea()
    {
        return Width * Height;
    }
}

public void DoWork()
{
    Circle circle = new Circle() { Radius = 5 };
    Rectangle rectangle = new Rectangle() { Width = 10, Height = 20 };

    Console.WriteLine(circle.GetArea()); // Prints 78.5398163397448.
    Console.WriteLine(rectangle.GetArea()); // Prints 200.
}

In this example, the “Circle” and “Rectangle” classes are defined without any interfaces or base classes. This means that they are considered to be of different types, because they have different type names. The “DoWork” method creates a “Circle” and a “Rectangle” object, and assigns them to variables of the same type as the objects. This means that the objects are considered to be of different types, and you cannot assign a “Circle” object to a “Rectangle” variable or vice versa. This demonstrates how nominal typing allows you to enforce strict type-checking in your code.

Can you describe the role of the “dynamic” keyword in C#?

The “dynamic” keyword in C# is used to specify that an object should be treated dynamically at runtime, instead of statically at compile time. This means that the type of the object is determined at runtime, rather than at compile time, and that the object can be used to call methods and access properties that are not known at compile time. This is useful because it allows you to write code that is more flexible and adaptable, because it is not tied to specific type names. Here is an example of how the “dynamic” keyword is used in C#:

public void DoWork(dynamic obj)
{
    Console.WriteLine(obj.GetType()); // Prints the type of the object.
    Console.WriteLine(obj.GetArea()); // Prints the area of the object.

    // The "GetArea" method is not known at compile time,
    // but it will be called at runtime if the object has a "GetArea" method.
}

public void DoWork()
{
    Circle circle = new Circle() { Radius = 5 };
    Rectangle rectangle = new Rectangle() { Width = 10, Height = 20 };

    DoWork(circle); // Prints "Circle" and 78.5398163397448.
    DoWork(rectangle); // Prints "Rectangle" and 200.
}

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