Top 20 C# interview questions and answers

By | May 5, 2016
In this article I will discuss top 20 frequently asked C# interview questions and answers with code examples.Though there can be whole sea of questions which interviewer can ask from the C# .

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C# Interview Questions and Answers

1. What is type safety in .NET? 
Ans. Type safety in .NET has been introduced to prevent the objects of one type from peeking into the memory assigned for the other object. Type safety helps the compiler and CLR in .NET to execute the code in the memory space defined for the objects and instances. It further helps to build the robust and error free applications which are definitely error free at run time.
An excellent example of type safety is the following example –

If .NET would have allowed this code to execute, type safety could have easily been compromised here resulting in some unpredictable results and hence in turn reducing the credibility of the code. You can read more about type safety here in myarticle

2. What is the base class from which all the classes are derived from?

Ans.     System.Object is the base class from which all the reference type as well value type are  derived from. In C# we cannot create a class which doesn’t derive from these two classes.

3. What are the functions which System.Object class have.

Ans.  Following are the functions which are present in the System.Object class

·         Equals(Object) – Determines whether the specified object is equal to the current object.
·         Equals(Object, Object) – Determines whether the specified object instances are considered equal.
·         Finalize – Allows an object to try to free resources and perform other clean up operations before it is reclaimed by garbage collection.
·         GetHashCode – Serves as the default hash function.
·         GetType – Gets the Type of the current instance.
·         MemberwiseClone – Creates a shallow copy of the current Object.
·         ReferenceEquals – Determines whether the specified Object instances are the same instance.
·         ToString – Returns a string that represents the current object.

4. Why do we override Equals() method of System.Object.

Ans. The default implementation of the Equals look like following.

From the above code snippet we can check that the default implementation only checks the references of the current object and the obj parameter.

But suppose if we want to override this default implementation in that case we can override this function and develop our own equality compare logic as shown below.

5. What are the functions which we can override in base class derived from System.object?

Ans .Following are the methods which we can override in the user defined type
Equals – Supports comparisons between objects.
Finalize – Performs cleanup operations before an object is automatically reclaimed.
GetHashCode – Generates a number corresponding to the value of the object to support the use of a hash table.
ToString – Manufactures a human-readable text string that describes an instance of the class

6. What is difference between compile time polymorphism and run time polymorphism 

Compile time polymorphism or the static polymorphism is the type of polymorphism which helps to identify the called methos at the compile time. We use different signature of the same method in the defining class as shown below

Run time polymorphism – Run time polymorphism or dynamic polymorphism is the type of polymorphism which helps us to determine at the run time which function should be called. Example of the same is as below .

This type of polymorphism can be achieved by using the abstract or virtual keyword with the functions in the base class, which is turn need to be overridden in the derived class. You can read more here.

7.    Does C# supports multiple inheritance.

Ans –  C# does not support multiple inheritance with classes. It means that we cannot have a class derived from multiple classes in C#. To overcome this restriction Interface have been introduced in C#.

8.    Why multiple inheritance is not supported in C#

Ans – To understand this question first we need to know the diamond problem.

The Diamond problem is an ambiguity that arises when two classes B and C inherit from Class A and class D inherits from both B and C. If a method in D calls a method defined in A(and does not override the method), and B and C have overridden that method differently, then from which class does it inherit: B or C? You can read more about it here.

9.     How is multiple inheritance achieved in C#?

Ans. Multiple inheritance in C# can be achieved using interface as shown in the following code snippet.  .

10. What are interfaces in C# and how they are different from abstract class.

Ans . This is one of the common interview questions.An interface contains only signatures or declarations for a number of related functions that a class or a struct can implement. As we know that multiple inheritance is not supported in C#, it can be achieved using the interfaces as shown in the previous question. Moreover interfaces only contains the method declarations and not the implementation. Apart from methods interfaces can also contain Property and events.

We cannot create an instance of the abstract class. These classes are meant to be derived by the other class. These classes can contain a method which could be abstract in nature means that it will have only its declaration and not the implementation in the abstract class. It need to be implemented in the derived class. Moreover if we want our derived class to prevent the implementation of the abstract function in the derived class in that case we need to declare the derived class as abstract as well.

Apart from that abstract class need to have some default implementation which derived classes needs to adhere with. We need to implement the abstract method in the derived class and it should be implemented with the override keyword. Below is the example of the abstract class.

11. What are static classes.

Ans Static classes are the classes which cannot be instantiated. These classes cannot be derived from and the method parameters cannot be of type static class. These classes are loaded once per app domain and they are loaded as soon as any member variable or function is called for that class. One class is shared across the whole application.

12. When are static classes initialized.

Ans. The static classes are loaded in the memory as shoon as any of the datamember or the function member is called for the first time.

13. What is difference between static class and singleton class.

Ans.

Static class
Cannot be initialized
Cannot be the base class
Cannot be the parameter to a function
All the member variables and functions should be static
Can have only static constructor.

Singleton Class (You can read about it here)
Can have only one instance per app domain
Can be passed as parameter to a function.
Singleton class usually have a single private constructor

14. Is it possible to have a static constructor in class. If yes why do we need to have a static constructor.

Ans. Static constructors are used to initialize the static member variables of the class. Hence a static constructor executed once per type instead of once per instance. A type can define only one static constructor and it must be parameterless. The runtime automatically invokes a static constructor just prior to type being used. Two things trigger this-

  •  Instantiating the type
  •  Accessing a static member in the type.

 

15. What are delegates.

Ans. Delegates are reference types which are used to contain the function pointers. Delegates are type safe and they adhere to compile type safety. A delegate can declared as following –

public delegate void MyDelegate(int number);

This is a delegate which would contain a reference to a function which should return void and take one parameter of type int. You can learn more about delegates in my article.

16. What is a multicast delegate.

Ans. Each delegate type is derived from Multicast delegate, which would help to have a invocation list for each and every delegate. Having an invocation list means that we can attach multiple methods attached to a delegate which would be called one after the other in the order which they are added to the delegate. Since multiple methods can be called using delegates that is why it is generally advise to have void as return type of multicast delegate.

17. How can we achieve asynchronous programming using delegates.

Ans. We can call a method asynchronously using the delegate by using the BeginInvoke() function of the delegate.

18. Are delegates type safe. 

Ans. Yes delegates are type safe and at compile time itself we can get to know the parameter type and return type of a delegate. More about type safety in my article

19. What is the difference between Convert.ToSting() and Object.ToSting() instance method of a class.

Ans. Convert.ToString() can handle the NULL values but ToString() cannot handle null, it will throw object reference null exception and we can override Object.ToString() to provide custom implementation for the class.

20. What are is and as keywords used for?
Ans. The is keyword is used to check the casting at runtime. If the cast is valid it will return true else it will return false as shown in the below code.

As keyword is used to cast the variable of one type to other type(base type). If the casting is not valid it will not throw InvalidCasting exception which we will get if we want to so same explicitly. As keyword can be used as shown below.

You can find more about these keyword in my article about type conversion here.

Conclusion: These were some of the important C# interview questions and answers for the entry level and experienced level software professional. I hope you have liked this article.

 

  • Dmitry Bytchenko

    4th answer flaws:

    – if(obj is person) // typo, should be Person

    – if (obj is Person) {

    p = obj as Person // two redundant casts “is” and “as”

    – p.Name.Equals(this.Name) // error: what if p.Name == null?

  • Dmitry Bytchenko

    11th answer: it worth mentioned, IMHO, that internally (this can be checked via Reflection) static class is sealed abstract class and so static class can’t be inherited (sealed) as well as instantiated (abstract)

  • Dmitry Bytchenko

    4th answer flaw (edited version): wrong “if” condition

    what if p is not null, but both this and p have null Name?

    if(p != null && !string.IsNullOrWhiteSpace(p.Name))

    Counter example

    // Please, note that test.Name == null;
    Person test = new Person();

    // return false when true expected
    test.Equals(test);

    Suggested implementation:

    public override Boolean Equals(Object obj) {
    if (Object.ReferenceEquals(this, obj))
    return true;

    Person other = obj as Person;

    if (null == other)
    return false;

    return String.Equals(Name, other.Name);
    }