abstract classes and methods

Context: An abstract class cannot be instantiated. It may contain abstract methods (no body) that must be implemented by non‑abstract derived classes.

Abstract Class

public abstract class Vehicle
{
    public abstract void StartEngine(); // no body
    public void Stop() => Console.WriteLine("Stopped");
}

Abstract Method Implementation

public class Car : Vehicle
{
    public override void StartEngine()
    {
        Console.WriteLine("Car engine started");
    }
}

Usage

Vehicle v = new Car(); // OK
v.StartEngine();       // calls Car's implementation
// Vehicle v2 = new Vehicle(); // error: abstract

Key Points

• Abstract members can only appear in abstract classes.
• A class inheriting an abstract class must implement all abstract members (unless it is also abstract).
• Abstract classes can have constructors, fields, and concrete methods.