C# Language Tutorial => abstract

Example

A class marked with the keyword abstract cannot be instantiated.

A class must be marked as abstract if it contains abstract members or if it inherits abstract members that it doesn't implement. A class may be marked as abstract even if no abstract members are involved.

Abstract classes are usually used as base classes when some part of the implementation needs to be specified by another component.

abstract class Animal 
{
    string Name { get; set; }
    public abstract void MakeSound();
}

public class Cat : Animal 
{
    public override void MakeSound()
    {
        Console.WriteLine("Meov meov");
    }
}

public class Dog : Animal 
{   
    public override void MakeSound()
    {
        Console.WriteLine("Bark bark");
    }
}

Animal cat = new Cat();       // Allowed due to Cat deriving from Animal
cat.MakeSound();              // will print out "Meov meov"    

Animal dog = new Dog();       // Allowed due to Dog deriving from Animal
dog.MakeSound();              // will print out "Bark bark"

Animal animal = new Animal(); // Not allowed due to being an abstract class

A method, property, or event marked with the keyword abstract indicates that the implementation for that member is expected to be provided in a subclass. As mentioned above, abstract members can only appear in abstract classes.

abstract class Animal 
{
   public abstract string Name { get; set; }
}

public class Cat : Animal 
{
    public override string Name { get; set; }
}

public class Dog : Animal 
{
    public override string Name { get; set; }
}

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