Top 50 C# Interview Questions and Answers

• Good when you are writing small scripts or console applications like Azure functions or GitHub Actions.
• Not changing the existing way of programming, they are just an alternative where the compiler generates the main entry point for your application.

• You want to pass them to the base constructor.
• To pass parameters for Dependency Injection.
• You want to reference it in an instance member.
• To initialize property or field.

• At project level by adding the property enableto your .csproj file
• By adding #nullable enable at the top of your file.
• By adding #nullable enable around the code section.

• Encapsulation is considered as the first principle of OOPs. As per this principle, A Class or Struct can specify the accessibility of its members to code outside of this class. Members not intended to be accessed outside of this class or struct can be hidden. For more visit Object oriented - Encapsulation.
• Inheritance is one of the three characteristics of OOPs. It allows you to create new classes that can re-use, modify and extend the behavior defined in other classes. In Inheritance, the class which inherits or re-use the other class members is known as derived class and the class whose members are inherited is known as Base class.
  C# does not allow multiple inheritance - means a derived class can have only one direct base class. Inheritance is transitive in nature means If class X inherits from class Y and class Y inherits from class Z then class X inherits from both Y and Z. For more visit Inheritance.
• Polymorphism - It is referred to as the third pillar of Object oriented programming. Polymorphism means "Many shapes" - more than one form of an object. It allows you to implement inherited properties and methods in different ways across the abstractions. C# implement polymorphism at compile time and run time. For more visit Polymorphism.

• in keyword is like ref or out keywords but 'in' arguments can not be modified by the called method. 'in' arguments must be initialized before passing to the called method. If you try to modify the value of 'in' argument in the calling method it will throw an error. So it's like a readonly variable. For more visit in keyword.
• ref keyword causes an argument to be passed by reference and may be read or written by the called method. To pass an argument as reference means any change to the parameter in the called method is reflected in the calling method argument. ref argument may be modified by the called method. For more visit ref keyword.
• out keyword causes arguments to be passed by reference and is written by a called method. It is similar to ref keyword, except that ref requires that the variable must be initialized before it is passed. To pass variables as out arguments, both the method definition and the calling method must explicitly use the out keyword. For more visit out keyword.

        class BC
        {
          public void Display()
          {
             System.Console.WriteLine("BC::Display");
          }
        }

        class DC : BC
        {
          public void Display()
          {
             System.Console.WriteLine("DC::Display");
          }
        }

        class Demo
        {
          public static void Main()
          {
             DC obj = new DC();
             obj.Display();
          }
        }

        Output : DC:Display
        

        class DC : BC
        {
          new public void Display()
          {
             System.Console.WriteLine("DC::Display");
          }
        }
        

    class Program
    {
    static void Main(string[] args)
    {
    var x = 10;
    object y = 20;
    dynamic z = 30;

    x = "test"; // Error -  Cannot implicitly convert type 'string' to 'int'

    y = "test";
    y = y - 10;  //Error Operator '-' cannot be applied to operands of type 'object' and 'int'

    z = "test";
    z = z - 10;
    Console.WriteLine("{0},{1},{2}",x,y,z);
    Console.ReadLine();
    }

    void display(var x){} // Error CS0825 - The contextual keyword 'var' may only appear within a in local variable declaration or script code.

    void display(object x){}

    void disp1(dynamic x){}
    }

• var was introduced with C# 3.0, Variables declared at method scope level can have implicit type 'var'. var types are initialized at the time of declaration only and can store any value. Compiler determines the type of var type so it is similar to strongly typed. You can use a var type in the same scope where it is defined - refer example. Type casting is not required as the compiler has all information to perform operations. You can use a var type for constructor invocation expressions, anonymous type etc. For more visit var.
• object is the part of C# 1.0, It is the base class of all types in .NET framework so it can store any type. Compiler has little information about the type. You can pass object type as method argument and can return object type from a method as in the above example. object types need type casting to perform operations and can cause the problem at run time if type casting is not performed. For more visit object.
• dynamic was introduced with C# 4.0, Use of dynamic keyword with variable and it's members bypass the compile time type checking instead it resolves all operations at the runtime. You can pass dynamic type as method argument and method also can return dynamic type - refer example. Casting is not required as operations are resolved at runtime but you should have correct member information to use dynamic types. For more visit dynamic.

• Boxing - When a value type is converted to the 'object' type or to any interface type implemented by this value type is known as Boxing. When CLR performs boxing on a value type it wraps the value inside a System.Object instance and that value is stored on a managed heap.
• UnBoxing - When a value type is extracted from the 'Object' is called Unboxing.

        int a = 13;
        // The below line assigns the value of int a to object o, it's boxing.
        object o = a;
        Now you can assign the value of o to integer a, it's unboxing.
        o = 13;
        a = (int)o;  // unboxing
        

 
                void check(int x)
                {
                // Implementation
                }
                void check(float x)
                {
                    // Implementation
                }
                void check(int x, int y)
                {
                    // Implementation
                } 

• Value types - A Value type variable holds an instance of the type and when you perform any assignment only an instance of the type is copied, not the reference of that instance. By default, when you pass an argument to a method, only variable value is copied. Value types include the following:
  • All numeric data types
  • Boolean, Char, and Date
  • All structures, even if the members of structure are reference types
  • An Enum type, since the underlying type of enum is always SByte, Short, Integer, Long, Byte, UShort, UInteger, or ULong.
  For more Value Types in C#.
• Reference types - Variables of reference types contain the references (memory address) to an instance of the types.
  A reference type holds the address of a memory location that holds the actual data of the variable. So operations you perform on one variable can affect the object referenced by the another variable.
  C# offers some keywords to declare reference types including: class, interface, delegate, record and provides some built-in reference types like dynamic, object, string.
  For more Reference Types in C# and Value Types and Reference Types in C#.

• Abstraction is to create the prototype of relevant attributes and interaction of an entity in the form of classes. This Prototype defines an abstraction representation of a concept. For example, A BankAccount base class creates an abstraction or defines the behavior for the Bank Account concept. Now you can create any type of bank account like saving, just by deriving your new class 'SavingAccount' from this 'BankAccount' base class.
• Encapsulation is the hiding of the functionality and internal state of an object from the code outside of this class and allows access through a public set of properties or functions. For Example, the BankAccount class can contain some private members to hide the data from the code outside of this 'BankAccount' class.
• Abstraction can be implemented using Abstract classes and Interface whereas Encapsulation can be implemented using access modifiers such as public, private, protected etc.
• Abstraction helps to solve the problems at design level using Abstract classes or Interfaces while encapsulation helps to solve problems at implementation level using access modifiers.

• Abstract class provides both declaration and implementation but Interfaces have only declaration.
• When you want to achieve multiple inheritance then you should use Interface not Abstract class because in C#, a class can inherit from one class only but from more than one interface.
• Abstract class contains constructors while Interface does not.
• Interfaces have only public members whereas abstract classes can contain public, private protected modifiers.
• Abstract class can provide complete, partial or no implementation whereas Interface provides only declaration.
• Performance wise, Abstract class is fast as Interface needs to identify the actual method implementation which takes some time.
• Interface provides you the ability of a class whereas An abstract class gives you the identity of class.
• Abstract class allows you to define and implement some common functionality which can be used by many child classes but Interface gives you common method signatures that can be implemented differently as per requirement.
• Abstract class can provide default implementation for any newly added method and all code will work properly but in case of Interface you need to implement that new method in all the classes using that interface.
• Members of Interface can not be static but complete members of Abstract class can be static.
• Interfaces allow you to create Mocks easily in unit testing while Abstract class needs extra coding.

15. What is Polymorphism? Explain the static and Dynamic Polymorphism in C#?

16. Can Abstract class have a constructor in C#? Below code will compile or not, or it will give any error in Error line 1 or 2.

class A
    {
       public virtual void display()
        {
            Console.WriteLine("A - display");
        }
        public virtual void disp()
        {
            Console.WriteLine("A - disp");
        }
    }
abstract class ab :A
    {
        public ab()
        { }
        public abstract override void display(); Error Line 1

        public sealed override void disp()  Error Line 2
        {
            Console.WriteLine("disp is sealed now.");
        }
    }

Solution: Abstract class can contain the constructor. Constructor is used to instantiate the class instance members in C#.
The above code will compile successfully. In C#, we can add Abstract and Sealed keywords with the override keyword.

17. What is the use of the sealed keyword in C#?

    class C {}
    sealed class D : C {} // other classes will not be able to inherit from class D.

18. What is the difference between abstract and virtual keywords ?

Both virtual and abstract are the modifiers in C#. They have different use cases.

• virtual keyword allows you to override any method, property, event declaration or indexer in derived class. It gives the capability to modify the implementation of the base class virtual member in derived class. Virtual members provide default implementation that you can modify in a derived class. When you invoke a virtual member, the run-time type of the object is considered for an overriding member.
  For more visit virtual C#.
• abstract keyword indicates that the base class has incomplete implementation or something missing that you need to complete in derived class. You can use the abstract modifier with class, methods, properties, events and indexers.
  It is necessary to complete the implementation of abstract members of base class. Only an abstract class can contain abstract members and non-abstract members as well.
  For more visit abstract C#.

19. Explain string interpolation in C#.

        var name = "Microsoft";
        Console.WriteLine($"Hello {name}"); // O/P: Hello Microsoft
        

20. Explain about String Immutability in C#.

Once you create a String object in C#, It can not be modified. When you perform any operation like appending then you get a new instance of a modified String object instead of modifying the actual instance value. So String Objects are immutable in C#. If you are planning to perform many append operations on a string object then use StringBuilder class.
For more visit Immutable String in C#.

21. How to compare two String Objects in C#?

You should use the Compare method to determine the relationship of two strings in sorting order and if you want to test for equality of two strings then you should Instance Equals or static Equals method with a StringComparison type parameter.
For more about string operations visit String in C#.

21. What is nullable value type? How to read value from a nullable value type in C#?

22. What are the new features in C# 7.0?

To know more about C# 7.0 new features visit What's new in C# 7.3

23. What is Covariance and Contravariance in C#?

Fore more Covariance and Contravariance in C#

24. What points (related to try, catch and finally) you should keep in mind while implementing exception handling in C#?

25. What is Generics in C#? What are the benefits of Generics?

For more visit Generics in C#.

Memory Management in C# (26 - 31)

26. Explain Memory management (Heap vs Stack) in a C# Program.

For more visit Heaping vs Stacking in .NET and C# Memory Management.

29. How does Garbage Collection work in C#? Explain Generations.

Garbage Collection (GC) is an automatic memory management mechanism in C# that reclaims the memory occupied by objects that are no longer in use by the application. When an object is created, it is allocated memory on the managed heap. The GC periodically checks the managed heap for objects that are no longer reachable (objects with no active references). These unreachable objects are marked for collection, and their memory is reclaimed for future allocations. GC plays a crucial role in managing memory efficiently and preventing memory leaks in C# applications.
For more visit Garbage Collection in C# and How GC manages memory?

30. What is using statement in C#?

    string message=@"Hello I am good , first line
    How are you, second line
    I am fine third line";

    using var reader = new StringReader(message);
    string? line;
    do {
    line = reader.ReadLine();
    Console.WriteLine(line);
    } while(line != null);

27. What is IDisposable Interface in C#? How to implement the dispose method?

For more visit Finalizer & Dispose.

28. Explain Dispose Pattern in C#.

31. What are the Finalizers in C#? Explain SupressFinalize() method.

For more visit Finalizers in C#

Collections in C# (32 - 40)

32. Explain different kinds of collections in C#.

33. What are the benefits of Generic Collections?

33. What is the difference between IEnumerable and IQueryable in C#? When would you use one over the other?

IEnumerable and IQueryable are both interfaces in C# used for working with collections and querying data, but they have different purposes and characteristics. IEnumerable represents a sequence of elements that can be iterated over, and its implementations (e.g., List, Array) typically store the data in memory. IQueryable, on the other hand, represents a sequence of elements that can be queried using Language Integrated Query (LINQ) expressions, and its implementations (e.g., Entity Framework DbSet) typically work with remote data sources like databases. IQueryable allows for deferred execution and query translation, where the query is not executed until it's enumerated, and the query expression is translated into a data source-specific query language (e.g., SQL). You would use IEnumerable when working with in-memory data collections, while IQueryable is more suitable when querying remote data sources like databases or web services.

34. Explain algorithmic complexity of collections in C#.

37. Explain internal working of Dictionary or Hashtable.

For more visit Dictionary or Hashtable internal working Hashtable internal working.

35. When to use a thread-safe collection in C#?

36. How to perform comparisons and sorts within collections?

37. Explain Iterators in C#.

38. What is the difference between the Equality operator == and Equals() method in C#?

39. What is the use of new keyword apart from creating a new object in C#?

40. What are the extension methods in C#?

public static class StringPowerUps
{
    // this is the signature to create an extension method
    public static string Shout(this string text)
    {
        return text.ToUpper() + "!";
    }
}

// usage of extemsion method
string normalString = "hello world";
string loudString = normalString.Shout();

Console.WriteLine(loudString); // Outputs: HELLO WORLD!

41. Explain the Lambda expressions in C#?

42. What are the anonymous methods in C#?

43. Explain Partial class and methods.

44. What are the different ways to pass parameters in C#?

45. How will you differentiate EXE and DLL files?

46. What is the DLL hell Problem?

47. Explain the concept of Parallel Processing, Concurrency, and Async Programming in .NET?

For more visit Parallel Processing, Concurrency, and Async Programming in C#.

48. How will you differentiate Thread vs Task?

48. What is volatile keyword in C#?

In C#, volatile keyword indicates that multiple executing threads which are executing at the same time can modify the value of a field. The fields which are declared volatile are observed by all the threads for volatile writes by any other thread in the same order in which it was performed. All reads and writes to memory locations of volatile fields may be rearranged by the compiler, the runtime and even hardware for performance reasons. For more visit C# volatile.

49. What is async and await in C#?

    public async Task<int> ExampleMethodAsync()
    {
    //...
    string contents = await httpClient.GetStringAsync(requestUrl);
    }

50. How to handle exceptions in Async methods?

For more visit Exceptions in async methods.

50. Explain C# 6.0 new features.

C# 6.0 has many new features. For more visit What's new in C# 6.0

51. What is Deep copy and Shallow copy?

For more visit Cloning;

52. What is the use of yield keywords in C#?

53. How User-defined conversions work in C#?

54. Differentiate Action vs Func in C#.

    
        class Test
        {
        // Func Example
        public string CreateNamefromName1()
        {
        return CreateName(GetName1);
        }
        public string CreateNameFromName2()
        {
        return CreateName(GetName2);
        }
        private string CreateName(Func<string, string, string> createName)
            {
            var name1 = "hello";
            var name2 = "testing";
            return createName(name1, name2);
            }

        private string GetName1(string arg1, string arg2) => $"{arg1}-{arg2}";
        private string GetName2(string arg1, string arg2) => $"{arg1}::{arg2}";

         }
    

55. Explain the use of below interfaces in C#?

C# Coding Interview Questions

49. Print the output of the below program? Error Line will throw any error in below code?

  class A
    {
       public virtual void display()
        { Console.WriteLine("A - display method"); }
    }
    class B:A
    {
        public override void display()
        { Console.WriteLine("B - display method"); }
    }
    class Test
    {
        static void Main()
        {
            A obj = new A();
            obj.display();

            A obj1 = new B();
            obj1.display();

            B o = new B();
            o.display();
            // B ob = new A();  Error Line
            Console.ReadLine();
        }
    }

Output: 'Error Line' will throw error - "Cannot implicitly convert type 'A' to 'B'. An explicit conversion exists (are you missing a cast?)".
Output will be as below.
A - display method
B - display method
B - display method

55. What will be the output here? or will it throw any error?

    public void display()

    {
    int x = 0;
    int y = 10;
    try
    {
    int z = y / x;
    }
    catch (Exception)
    {
    throw;
    }
    catch (DivideByZeroException)
    {
    throw;
    }
    finally
    {
    }

    }

56. Print the output of this program?

    int a = 0;
    try
    {
    int x = 4;
    int y ;
    try
    {
    y = x / a;
    }
    catch (Exception e)
    {
    Console.WriteLine("inner ex");
    //throw;   // Line 1
    //throw e;   // Line 2
    //throw new Exception("divide by 0");  // Line 3
    }

    }
    catch (Exception ex)
    {
    Console.WriteLine("outer ex");
    throw ex;
    }

• Output will be - "inner ex" as we are not throwing anything from the inner catch block, so the outer catch block will not execute.
• If we uncomment 'Line 1' then output will be - "inner ex" and "outer ex" and outer catch will throw an exception.
• If we uncomment 'Line 2' then output will be the same - "inner ex" and "outer ex" and outer catch will throw an exception.
• If we uncomment 'Line 3' then output will be - "inner ex" and "outer ex" but the exception message in outer catch will be different, overridden by a throw message from inner catch block.

Some General Interview Questions for C#

1. How much will you rate yourself in C#?

When you attend an interview, Interviewer may ask you to rate yourself in a specific Technology like C#, So It's depend on your knowledge and work experience in C#. The interviewer expects a realistic self-evaluation aligned with your qualifications.

2. What challenges did you face while working on C#?

The challenges faced while working on C# projects are highly dependent on one's specific work experience and the technology involved. You should explain any relevant challenges you encountered related to C# during your previous projects.

3. What was your role in the last Project related to C#?

This question is commonly asked in interviews to understand your specific responsibilities and the functionalities you implemented using C# in your previous projects. Your answer should highlight your role, the tasks you were assigned, and the C# features or techniques you utilized to accomplish those tasks.

4. How much experience do you have in C#?

Here you can tell about your overall work experience on C#.

5. Have you done any C# Certification or Training?

Whether a candidate has completed any C# certification or training is optional. While certifications and training are not essential requirements, they can be advantageous to have.

Conclusion

We have covered some frequently asked C# Interview Questions and Answers to help you for your Interview. All these Essential C# Interview Questions are targeted for mid level of experienced Professionals and freshers.
While attending any C# Interview if you face any difficulty to answer any question please write to us at info@qfles.com. Our IT Expert team will find the best answer and will update on the portal. In case we find any new C# questions, we will update the same here.