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

In C#, both abstract classes and interfaces are used for defining abstractions and providing a blueprint for derived types, but they have distinct differences in their usage and capabilities.

Abstract Classes:

1. Definition: An abstract class is a class declared with the `abstract` keyword. It can include a combination of abstract (unimplemented) methods and concrete (implemented) methods.

2. Implementation: Abstract classes can provide method implementations, including fields and properties. Derived classes are required to provide implementations for abstract methods using the `override` keyword.

3. Single Inheritance: C# supports single inheritance, meaning a class can inherit from only one abstract class. This limits the flexibility of reusing code across multiple base classes.

4. Common Use Cases: Abstract classes are often used when you want to provide a common base for multiple related classes, allowing you to share implementation details while enforcing specific behaviors in derived classes.

Interfaces:

1. Definition: An interface is a contract specifying a set of methods, properties, events, or indexers without providing any implementation. Interfaces are declared using the `interface` keyword.

2. Implementation: Interfaces cannot include any method implementations; they only define method signatures. Classes that implement an interface must provide concrete implementations for all the interface’s members.

3. Multiple Inheritance: C# supports multiple interface inheritance, allowing a class to implement multiple interfaces. This provides greater flexibility for reusing code across various scenarios.

4. Common Use Cases: Interfaces are used when you want to define a contract that multiple unrelated classes can adhere to. They enable you to achieve polymorphism, allowing different classes to be treated interchangeably based on the shared interface they implement.

Abstract classes are typically used when you want to create a common base class with some shared implementation, while interfaces are used to define contracts that multiple classes can adhere to, regardless of their inheritance hierarchy. The choice between abstract classes and interfaces depends on your design goals and the level of reusability and flexibility you require in your code.