What is the difference between abstract classes and interface classes in C#?

Abstract classes and interface classes are both used to define specifications and constraints on the structure and behavior of classes, but they have some important distinctions.

1. Abstract class:

1. Abstract classes can include both abstract methods and concrete methods.
2. Abstract classes can contain fields, properties, and constructors.
3. A class can only inherit from one abstract class.
4. Abstract classes cannot be instantiated and can only be inherited.

1. Interface class:

1. Interface can only contain abstract methods, properties, events, and indexers.
2. A class can implement multiple interfaces.
3. Interface cannot contain fields, constructors, or concrete methods.
4. Interface can only be used to define the behavior of a class, without containing any implementation.

In general, abstract classes are more suitable for defining the structure and behavior of a class, while interfaces are more suitable for implementing polymorphism and decoupling. In design, you can choose to use abstract classes or interfaces based on actual needs.