Abstract Classes

Syntax

abstract class ClassName {\n  public abstract returnType MethodName();\n}

Examples

public abstract class Animal
{
    protected string Name;

    protected Animal(string name)
    {
        Name = name;
    }

    public abstract void MakeSound();  // no body - subclasses must implement

    public void Sleep()  // concrete method, shared by all subclasses
    {
        Console.WriteLine($"{Name} is sleeping");
    }
}

public class Dog : Animal
{
    public Dog(string name) : base(name) { }

    public override void MakeSound() => Console.WriteLine($"{Name} says: Woof!");
}

// Animal a = new Animal("Generic");  // Error - can't instantiate abstract class
Dog dog = new Dog("Rex");
dog.MakeSound();
dog.Sleep();

public abstract class Employee
{
    protected string Name;
    protected double BaseSalary;

    protected Employee(string name, double baseSalary)
    {
        Name = name;
        BaseSalary = baseSalary;
    }

    public abstract double CalculateBonus();

    public double TotalPay() => BaseSalary + CalculateBonus();
}

public class Manager : Employee
{
    public Manager(string name, double baseSalary) : base(name, baseSalary) { }
    public override double CalculateBonus() => BaseSalary * 0.2;
}

public class Developer : Employee
{
    public Developer(string name, double baseSalary) : base(name, baseSalary) { }
    public override double CalculateBonus() => BaseSalary * 0.1;
}

Employee m = new Manager("Fola", 60000);
Console.WriteLine(m.TotalPay());  // 72000

Best practices

• Use an abstract class when subclasses share both state (fields) and some common implemented behavior, not just a method signature
• Use an interface instead when unrelated classes just need to share a capability, with no shared fields or implementation
• A class can only extend one abstract class but can implement multiple interfaces - factor that into your design choice
• Make a method abstract only when every subclass genuinely needs its own distinct implementation - otherwise provide a sensible default in the abstract class

At a glance

Purpose

Applications on the .NET platform

File extension

.cs

Runs in

.NET runtime

Usually used with

.NET SDK and libraries

Specifications & further reading

• C# language reference

Related C# documentation