Difference Between Static Class, Sealed Class, and Abstract Class in C#

We come across this question very often that what is the difference between Static, Sealed and Abstract class in C#.

Static Class: Declared with Static keyword, methods in Static Class are also static along with variables of the class.

This class cannot be instantiated, i.e we cannot have objects of this class. To access methods of this class, you can directly use classname.method. Also this class cannot be inherited.

Sealed Class: Declared with Sealed keyword, which enables this class to seal all its variables, methods and properties. No other class can inherit anything from this class or in other words, this class cannot be inherited. But we can instantiate this class, i.e we can have any number of objects of a sealed class.

One important point to remember is a non-static class can have static methods. But Static classes must have all members as Static.

Abstract Class: Declared with abstract keyword, this class is primarily created as a Inheritable class. An abstract class enables other classes to inherit from this class, but forbids to instantiate. One can inherit from an abstract class but we cannot create objects of an abstract class. Abstract class can have abstract as well as non abstract methods. Abstract methods are those which are not having method definition.
     An abstract class is a special kind of class that cannot be instantiated. So the question is why we need a class that cannot be instantiated? An abstract class is only to be sub-classed (inherited from). In other words, it only allows other classes to inherit from it but cannot be instantiated. The advantage is that it enforces certain hierarchies for all the subclasses. In simple words, it is a kind of contract that forces all the subclasses to carry on the same hierarchies or standards.

Interface: An interface is not a class. It is an entity that is defined by the word Interface. An interface has no implementation; it only has the signature or in other words, just the definition of the methods without the body. As one of the similarities to Abstract class, it is a contract that is used to define hierarchies for all subclasses or it defines specific set of methods and their arguments. The main difference between them is that a class can implement more than one interface but can only inherit from one abstract class. Since C# doesn’t support multiple inheritance, interfaces are used to implement multiple inheritance.