What is the difference between Interface and Abstract Class?

In C#, both interfaces and abstract classes provide mechanisms for defining contracts and abstraction in object-oriented programming. However, there are several key differences between interfaces and abstract classes in terms of their purpose, behavior, and usage. Here are the main differences:

  1. Purpose:
    • Interfaces: An interface defines a contract of methods, properties, events, or indexers that a class must implement. It specifies the behavior that implementing classes must adhere to but does not provide any implementation details.
    • Abstract Classes: An abstract class is a class that cannot be instantiated and serves as a blueprint for derived classes. It can contain both abstract and non-abstract members. Abstract classes can provide some default implementation while also defining abstract members that must be implemented by derived classes.
  2. Multiple Inheritance:
    • Interfaces: A class can implement multiple interfaces, allowing it to inherit and implement the contracts of multiple interfaces.
    • Abstract Classes: A class can inherit from only one abstract class. C# does not support multiple inheritance of classes.
  3. Constructor:
    • Interfaces: An interface cannot have constructors because interfaces are not instantiable.
    • Abstract Classes: An abstract class can have constructors that are called when an instance of a derived class is created. The constructor logic can be shared among the derived classes.
  4. Access Modifiers:
    • Interfaces: All members of an interface are implicitly public and cannot have access modifiers (e.g., private, protected).
    • Abstract Classes: Abstract class members can have various access modifiers, such as public, private, protected, etc.
  5. Default Implementation:
    • Interfaces: Interfaces cannot provide any default implementation for their members. Implementing classes must provide their own implementation for each member.
    • Abstract Classes: Abstract classes can provide default implementations for some or all of their members. Derived classes can choose to override these implementations or use them as-is.
  6. Inheritance vs. Implementation:
    • Interfaces: Implementing an interface means a class agrees to adhere to the contract defined by the interface. It provides a way to achieve polymorphism and share common behavior across unrelated classes.
    • Abstract Classes: Inheriting from an abstract class means a class derives from the abstract class and extends its functionality. It provides a way to define a base class with common behavior and allow derived classes to specialize or override certain members.

In general, interfaces are used to define contracts and establish a common interface among unrelated classes, while abstract classes are used to define common behavior and provide a base for derived classes. Interfaces focus on what a class can do, while abstract classes focus on what a class is.

The choice between using an interface or an abstract class depends on the specific requirements of the design and the relationship between the classes involved.

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