In C#, an abstract class is a class that cannot be instantiated directly. It is designed to serve as a base class for other classes. Abstract classes are typically used when you have a base class that should provide common functionality, but some methods need to be defined in derived classes.

Key Features of Abstract Classes:

1. Cannot be instantiated: You cannot create an instance of an abstract class directly.
2. Can contain abstract methods: These methods have no implementation in the abstract class and must be implemented in any derived class.
3. Can contain non-abstract methods: The abstract class can also include methods with full implementations that can be inherited by derived classes.
4. Can contain properties, fields, and constructors: Like a regular class, abstract classes can have these members.

Syntax:

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abstract class Animal
{
    public abstract void MakeSound(); // Abstract method

    public void Sleep() // Non-abstract method
    {
        Console.WriteLine("Sleeping...");
    }
}

Derived Class:

The class that derives from an abstract class must implement all its abstract methods.

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class Dog : Animal
{
    public override void MakeSound() // Implement abstract method
    {
        Console.WriteLine("Woof!");
    }
}

class Cat : Animal
{
    public override void MakeSound() // Implement abstract method
    {
        Console.WriteLine("Meow!");
    }
}

Usage:

You can use the abstract class as a base for polymorphism:

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Animal myDog = new Dog();
myDog.MakeSound(); // Output: Woof!
myDog.Sleep();     // Output: Sleeping...

Animal myCat = new Cat();
myCat.MakeSound(); // Output: Meow!
myCat.Sleep();     // Output: Sleeping...

Key Points:

• Abstract vs. Interface: Abstract classes allow you to define both methods with and without implementation, while interfaces only allow method declarations.
• Inheritance: A class can inherit only one abstract class but can implement multiple interfaces.