C# Interview Questions and Answers

C# Interview Questions for Experienced

Below are some hard-level interview questions and answers. 

31. What are generics in C#.NET?

Ans: Generics in C#.NET is a feature that allows the creation of reusable code by creating classes, methods, and data structures that work with various data types without the need for duplicating code. Generics can parameterize a type or method by one or more data types, making the code more flexible and efficient. 

Generics provide type safety and enable better performance by avoiding the need for boxing and unboxing of value types. Generics are commonly used in collections, such as lists and dictionaries, as well as in algorithms and other areas of C# programming.

32. Mention different ways in which a method can be overloaded.

Ans: Different ways by which a method can be overloaded are:

• Change the order of parameters in a method.
   
• Change the number of parameters in a method.
   
• Use different data types for parameters.
   

Example

using System;
namespace Overloading {
    public class Addition {
        public double sum(double a, double b) {
            double sum = a + b;
            return sum;
        }

        public double sum(double a, double b, double c) {
            double sum = a + b + c;
            return sum;
        }
    }

    class Example {
        public static void Main(string[] args) {
            Addition obj = new Addition();
            double a = 3.3;
            double b = 4.9;
            double s = 1.2;
            double sum = obj.sum(a, b, s);
            Console.WriteLine("Total sum: " + sum);

            double c = 3.3;
            double d = 6.3;
            double add = obj.sum(c, d);
            Console.WriteLine("Sum: " + add);
        }
    }
}

Output

Total sum: 9.4

Sum: 9.6

Explanation:

In the above example, the method sum is defined two times. First it contains two arguments to find the sum of two numbers, and second, it is defined with three arguments to find the sum of the three numbers.

33. Differentiate between early and late binding in C#.

Ans: Both early and late binding are examples of polymorphism concepts. Polymorphism enables a language to use the same name in many ways. For instance, the Add method can combine integers, doubles, and decimals.

The two types of polymorphism are:

Compile time polymorphism - Overloading or Early Binding.

Runtime polymorphism - Overriding or Late Binding.

Early Binding

In this, there are multiple methods with the same name but different parameter types(data types).

Example

using System;

namespace Overloading {
    public class Addition {
        public double sum(double a, double b) {
            return a + b;
        }

        public string sum(string a, string b) {
            return a + " " + c;
        }
    }

Late Binding

In this, there will be the same method name with the same number of parameters but not in the same class. Therefore we bind at the runtime in the child class. That’s the reason it is called late binding.

Example

using System;

namespace Overriding {
   class Addition {
      

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

   }

   class ChildClass: Addition {
     
      public sum( int a, int b) {
       return a + b;
      }
   }

34. What are partial classes, and what is their use in C#?

Ans: Partial classes are a special feature in C# that implements the functionality of a single class into multiple physical files. When the application is compiled, all these multiple files are combined into a single class file. The partial keyword is used to split the class definition. 

Syntax

public partial Class_Name  
{
       // code
}

Partial classes in C# provide several advantages, including:

• Organizing code.
• Code reuse.
• Separation of concerns.
• Generated code.

Example

There are two files “Ninjas1” and “Ninjas2” that contain the “Ninjas” class.

Ninjas1.cs

public partial class Ninjas {
    private string name;
    private int age;
 
    public Geeks(string name, int age)
    {
        this.name = name;
        this.age = age;
    }
}

Ninjas2.cs

public partial class Ninjas {
    public void print()
    {
        Console.WriteLine("Name is : " + name);
        Console.WriteLine("Age is : " + age);
    }
}

Below is the main class “Ninjas”, here the main method is not included.

public partial class Ninjas {
    private string name;
    private int age;
 
    public Geeks(string name, int age)
    {
        this.name = name;
        this.age = age;
    }

    public void print()
    {
        Console.WriteLine("Name is : " + name);
        Console.WriteLine("Age is : " + age);
    }
}

35. How would you force Garbage Collection?

Ans: In C# Garbage Collection is a process that is managed by the .NET runtime, which automatically frees up memory that is no longer being used by the application. However, in certain cases, manually forcing Garbage Collection may be necessary.

In C#, you can use the GC.Collect() method to manually force Garbage Collection. The GC.Collect() method triggers the Garbage Collection process immediately rather than waiting for the .NET runtime to perform it automatically.

36. How would you create a responsive UI without impacting users in C#?

Ans: To create a responsive UI without impacting users in C#, there are a few techniques that can be used:

1. Use asynchronous programming: Asynchronous programming enables the UI to remain responsive while background tasks are running. For example, you can use the async and await keywords to run long-running tasks in the background without blocking the UI thread.
    
2. Use multithreading: By creating and managing multiple threads, you can offload heavy processing from the UI thread and keep the UI responsive. However, it's important to synchronize access to shared data across threads to avoid race conditions and other threading issues. 
    
3. Use background workers: The BackgroundWorker class in C# provides a simple way to run long-running tasks in the background while keeping the UI responsive. It uses a separate thread to execute the task and provides events to report progress and completion.

37. What is the difference between a string and a StringBuilder in C#?

Ans: A string is an immutable data type, meaning you cannot change its value once you create a string. On the other hand, a StringBuilder is a mutable data type, allowing you to change its value and append new characters to it. 

StringBuilder is more efficient for concatenating strings or making multiple modifications to a string, as it does not create a new string instance for each change.

So if you want a string object not to change anywhere in the code then use string class otherwise StringBuilder.

Example of string object

using System;
namespace Application{
   class Example {
      public static void Main(String[] args){
          string s = "Coding ";
          s += "Ninjas";
          Console.WriteLine(s);
          
      }

     }
   }

Output

Coding Ninjas

Example of StringBuilder

using System;
using System.Text;


   class Example {
      public static void Main(){
          StringBuilder s = new StringBuilder("");
          s.Append("Coding ");
          s.Append("Ninjas");
          Console.WriteLine(s);
          
      }
   }

Output

Coding Ninjas

38. Why is the singleton pattern referred to as an anti-pattern in C#?

Ans: The Singleton pattern is often referred to as an anti-pattern in C# because it can introduce several issues and limitations that make code difficult to maintain and test. Some of the reasons why the Singleton pattern is considered an anti-pattern are:

1. Global state: The Singleton pattern creates a global instance that is shared across the application. This can make it difficult to maintain and test since changes to the Singleton can affect the behavior of other parts of the application.
    
2. Tight coupling: The Singleton pattern can create tight coupling between different parts of the application since the Singleton instance is often accessed directly from other classes. 
    
3. Inflexibility: The Singleton pattern can be inflexible since it provides a single instance of an object that cannot be easily substituted or extended. This can make it difficult to adapt to changing requirements or to implement different behaviors for testing or other purposes.
    
4. Testing challenges: The Singleton pattern can be difficult to test since it introduces a global state that can affect the behavior of other tests. It can also be difficult to substitute or mock the Singleton instance for testing purposes.

39. What is a virtual method in C#?

One of the features of object-oriented programming is Polymorphism. It allows you to use the same function to perform different actions according to your needs. 

In C#, you can use the virtual keyword to declare a method as a virtual method in a class. 

Now, if you inherit from this base class having a virtual method, then in the child class, you can override the virtual method using the override keyword, i.e., you can add the other implementation of the same method in the child class different from the parent class.

Example of virtual method in the base class:

public class Base
{
	public virtual void virtualMethod() {
		//Method implementation
	}
}

public class Derived : Base
{
	public override void virtualMethod() {
		//New implementation of virtualMethod
	}
}

40. List the features of read-only variables in C#.

The features of read-only variables in C# are as follows:

• They are initialized at runtime.
   
• They can be declared only at the class level.
   
• They can be used with static modifiers.
   
• The value of a readonly variable cannot be changed or reassigned.

41. What are stream reader and stream writer classes in C#?

The stream reader and stream writer classes inherit from the abstract base class “stream.” 

The abstract class Stream is used for writing and reading bytes from a text file.

Stream writer is used for writing to a file, while the stream reader is used for reading from a file. 

Some of the methods present in the:

• Stream Reader Class: Close(), Peek(), and Read(). 
   
• Stream Writer Class: Close(), Flush(), Write(bool value), WriteLine(), etc. 

42. What do you mean by boxing and unboxing in C#?

Boxing and unboxing are used for type conversion in C#. 

C# treats all the value types as objects. This feature of C# is emphasized by boxing and unboxing.

• Boxing
  It converts the value type to an object or the interface’s data type implemented by the current value type.CLR performs the boxing of the value type and wraps the value in the System.object, and saves it in the heap area of the application domain. 
  It may use implicit code.
   
• Unboxing
  Unboxing does the opposite of boxing, i.e., extracts the value type from the object or interface. It uses explicit code.

43. Differentiate between Task and Thread in C#.

The thread represents a separate program execution path, while a task represents an asynchronous operation. 

The task is used in the TPL(Task Parallel Library) and manages the parallel execution of code. It is easier to manage and use tasks in comparison to threads. 

Tasks provide a higher level of abstraction, while thread provides a lower level of abstraction.

Tasks help in the efficient utilization of resources than threads.

There can be multiple processes running simultaneously for a task. While a thread can have only one task at a time.

It is faster to create a task over a thread.

You can cancel a task in progress using cancellation tokens, but the thread does not support cancellation.

44. What is finally block in C#?

The finally block is used while handling exceptions in the code.

As we know, we use a try-and-catch block to handle exceptions. If an exception occurs while executing the try block, control reaches the catch block, and after that, control is transferred to the finally block. So, in the end, the finally block gets executed. 

Some of the most common uses of the finally block include releasing file handlers,  closing connections to the database, etc. 

Syntax of a general try-catch-block having a finally block is as follows:

try {
   // code block to be cheked for exception
} 
catch( ExceptionName e ) {
   // error handling code
} 
finally {
   // statements to be executed always
}

45. How do you implement exception handling in C#?

We use the following elements while implementing exception handling in C#:

• try block
  It consists of code for which we need to check the exception.
   
• catch block
  It is used for catching an exception and contains the code which will be executed once the exception occurs.
   
• finally block
  It consists of code that is always executed irrespective of whether the exception occurs or not.
   
• throw
  It is responsible for throwing an exception whenever it occurs instead of handling it.

46. What are dynamic type variables in C#?

As the name itself suggests, dynamic type variables can store data of any type. 

Type checking for such variables takes place at run time. 

You can define a dynamic type variable using a dynamic keyword in C#. 

Example:

dynamic newDynamicVariable = 2;

In most cases, the compiler compiles the dynamic variables to object types. But the final type is resolved only at runtime on the basis of their assigned value. 

If, after assigning one type of value, you reassign it to some other value of different type, then this type conversion takes place implicitly.

Example:

dynamic newDynamic1 = “hi”;
newDynamic1 = 10;
int s = newDynamic1;

47. Explain Anonymous types in C#.

You can create a new type without defining them beforehand using anonymous types in C#. 

It is a reference type, and you can define it using the var keyword. 

Anonymous types in C# can have multiple properties, all of which are read only. Basically, you can use it to bundle some read only properties in a single unit.

The difference between a C# class and anonymous types is that the classes can have fields and methods also, but anonymous types can only have properties.

You can access the anonymous type and its properties inside the method where it is defined. Therefore, its accessibility scope is limited.

Example:

var newAnonymousType {
property1 = “pr1”,
property2 = “pr2”
}

It is also possible to use nested anonymous types in C#. 

48. How is encapsulation implemented in C#?

Encapsulation is one of the most important and widely used feature of object oriented programming in C#. 

You can implements encapsulation in C# using various access modifiers like:

• Public
  The public access modifier is used to expose a member of a class like class variables or functions to other classes or objects. This implies that you can access the public members of a class from outside the class.
   
• Private
  The private access modifier conceals the class members like variables and functions from external world like other classes or objects. The private members of a class can only be accessed by the functions of same class.
   
• Protected
  If you declare a class member as protected, then the children classes can access it. Thus, it is useful while implementing inheritance in object oriented programming as it allows to reuse the protected members like functions or variables.

49. Differentiate between constant and readonly in C#.

The values of the constant and readonly variables cannot be changed once assigned. 

But the only difference between them is that the const variables need to be initialized when defined, whereas the readonly variables can be defined at the runtime.

The following table summarizes the differences between the constant and readonly variables in C#. 

50. What is the use of the yield keyword in C#?

Following are the uses of the yield keyword in C#:

• It is used for performing stateful iteration.
• It helps to perform a custom iteration over a collection like lists, arrays, etc.
• Using the yield keyword moves the control back and forth between the caller function and source and vice versa.
   

You can use the yield keyword in two forms:

• yield return
  It is used for returning expressions at each iteration.
   
• yield break
  It is used for terminating the iteration.

C# Coding Problems Interview Questions

51. Reverse a String in C#

using System;

class Program
{
    static void Main()
    {
        string str = "Infosys";
        char[] charArray = str.ToCharArray();
        Array.Reverse(charArray);
        string reversedString = new string(charArray);
        Console.WriteLine("Reversed String: " + reversedString);
    }
}

Output

Reversed String: sysofni

52. Find the Factorial of a Number

using System;

class Program
{
    static void Main()
    {
        int num = 5;
        int factorial = 1;

        for (int i = 1; i <= num; i++)
        {
            factorial *= i;
        }

        Console.WriteLine("Factorial of " + num + " is: " + factorial);
    }
}

Output

Factorial of 5 is: 120

53. Check if a Number is Prime

using System;

class Program
{
    static void Main()
    {
        int num = 29;
        bool isPrime = true;

        for (int i = 2; i <= Math.Sqrt(num); i++)
        {
            if (num % i == 0)
            {
                isPrime = false;
                break;
            }
        }

        if (isPrime && num > 1)
        {
            Console.WriteLine(num + " is a prime number.");
        }
        else
        {
            Console.WriteLine(num + " is not a prime number.");
        }
    }
}

Output

29 is a prime number.

54. Find the Largest Element in an Array

using System;

class Program
{
    static void Main()
    {
        int[] numbers = { 5, 8, 12, 19, 3, 7 };
        int largest = numbers[0];

        foreach (int number in numbers)
        {
            if (number > largest)
            {
                largest = number;
            }
        }

        Console.WriteLine("The largest number in the array is: " + largest);
    }
}

Output

The largest number in the array is: 19

55. Check if a String is a Palindrome

using System;

class Program
{
    static void Main()
    {
        string str = "madam";
        string reversedStr = new string(str.ToCharArray().Reverse().ToArray());

        if (str == reversedStr)
        {
            Console.WriteLine(str + " is a palindrome.");
        }
        else
        {
            Console.WriteLine(str + " is not a palindrome.");
        }
    }
}

Output

madam is a palindrome.

C# MCQs

1. What is the default value of an unassigned int variable in C#?

a) 0

b) 1

c) null

d) Undefined

Answer: a) 0

2. Which of the following is the correct way to declare a nullable type in C#?

a) int*

b) int?

c) nullable int

d) int!

Answer: b) int?

3. What is the size of an int in C#?

a) 4 bytes

b) 8 bytes

c) 2 bytes

d) Depends on the system architecture

Answer: a) 4 bytes

4. What is the purpose of the using statement in C#?

a) To include namespaces

b) To release unmanaged resources

c) To define a reference variable

d) To declare an alias for a namespace

Answer: b) To release unmanaged resources

5. Which of the following statements is true about C# interfaces?

a) An interface can contain fields.

b) A class can implement multiple interfaces.

c) Interfaces can have a constructor.

d) A class cannot inherit an interface and another class at the same time.

Answer: b) A class can implement multiple interfaces.

6. Which keyword is used to prevent a class from being inherited in C#?

a) sealed

b) static

c) private

d) readonly

Answer: a) sealed

7. Which of the following correctly declares a delegate in C#?

a) delegate void MyDelegate();

b) void MyDelegate();

c) event void MyDelegate();

d) public delegate event MyDelegate();

Answer: a) delegate void MyDelegate();

8. What does the params keyword allow in a method declaration?

a) Passing an unlimited number of arguments

b) Passing parameters by reference

c) Returning multiple values

d) Creating named parameters

Answer: a) Passing an unlimited number of arguments

9. Which collection in C# is used to store key-value pairs?

a) ArrayList

b) List<T>

c) Dictionary<TKey, TValue>

d) Queue<T>

Answer: c) Dictionary<TKey, TValue>

10. What is the difference between Array and ArrayList in C#?

a) ArrayList can store elements of different types, while Array cannot.

b) Array can store only reference types, while ArrayList stores value types.

c) Array can grow dynamically, while ArrayList has a fixed size.

d) ArrayList is faster than Array.

Answer: a) ArrayList can store elements of different types, while Array cannot.

Frequently Asked Questions

What is C# and why C#?

C# is a modern general-purpose programming language developed by Microsoft. It is an object-oriented programming language. C# is a robust language mainly used to develop high-level software. C# strongly supports object-oriented paradigms with high versatility and community. It supports cross-platform development, which makes it highly usable.  

What is the main function of C#?

The main function of C# is to provide high support to object-oriented concepts and help in building robust, versatile, scalable, and cross-platform independent applications. 

Is C# and .NET same?

No, C# and .NET are not the same, although both are developed by Microsoft. C# is an object-oriented programming language, whereas .NET is a framework with support to multiple programming languages, and C# is one of the languages that can be used with the .NET framework to develop applications. 

Conclusion

In this article, we have discussed C# interview questions. Preparing for a C# interview requires a solid understanding of core programming concepts, coding practices, and problem-solving skills. By familiarizing yourself with common interview questions, coding problems, multiple-choice questions, and theoretical concepts, you can confidently tackle both technical and practical questions during the interview process.

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