What is the difference between using an abstract class and an interface when defining contracts for classes?

An abstract class and an interface in C# serve to define contracts but have key differences:

• Abstract class can contain method implementations, fields, constructors, and properties. It is used when you need to establish a base implementation and common functionality for subclasses.
• Interface only defines method signatures, properties, and events without implementation (up to C# 8.0; starting from C# 8.0, interfaces can contain default implementations).

Also, a class can inherit from only one abstract class but can implement multiple interfaces.

Example:

abstract class Animal
{
    public abstract void MakeSound();
    public void Sleep() { Console.WriteLine("Sleeping"); }
}

interface IFlyable
{
    void Fly();
}

class Bird : Animal, IFlyable
{
    public override void MakeSound() { Console.WriteLine("Chirp"); }
    public void Fly() { Console.WriteLine("Flying"); }
}

Thus, an abstract class is suitable for common base behavior, while an interface is for defining a set of capabilities that can be implemented by different classes.