So What’s an Abstract Class Anyway?

Think of an abstract class as a half-finished blueprint. You can’t build directly from it, but it gives any derived classes a head start with some pre-defined structure and behavior. It’s like a parent saying, “Here’s how our family does some things, but you’ll need to figure out these other parts yourself.”

Abstract classes sit right in the sweet spot between interfaces (all talk, no action) and concrete classes (fully ready to use). They’re perfect when you want to share code but still enforce certain implementation requirements.

The Main Things to Know

Here’s what makes abstract classes special:

  1. You can’t create one directly, Try new AbstractClass() and the compiler will stop you

  2. They can include both ready-to-use methods AND placeholder methods:

    • Some methods say “you must implement this yourself” (abstract methods)
    • Others say “here’s a default way to do this, but you can change it” (virtual methods)
    • And some just say “use this as is” (regular methods)

  3. They can do things interfaces can’t, like have constructors, private fields, and protected methods

What They Look Like in Code

Here’s a simple abstract class in action:

public abstract class Shape
{
    // "You must implement these yourself"
    public abstract double CalculateArea();
    public abstract double CalculatePerimeter();

    // "Here's a default, but you can change it"
    public virtual void Display()
    {
        Console.WriteLine($"Area: {CalculateArea()}, Perimeter: {CalculatePerimeter()}");
    }

    // "Just use this as is"
    public string GetShapeType()
    {
        return this.GetType().Name;
    }
}

Using Your Abstract Class

When you create a class that inherits from an abstract class, you need to fill in all the blanks:

public class Rectangle : Shape
{
    public double Width { get; set; }
    public double Height { get; set; }

    public Rectangle(double width, double height)
    {
        Width = width;
        Height = height;
    }

    // Filling in the required blanks
    public override double CalculateArea()
    {
        return Width * Height;
    }

    public override double CalculatePerimeter()
    {
        return 2 * (Width + Height);
    }

    // Customizing the default behavior
    public override void Display()
    {
        Console.WriteLine($"Rectangle: {Width} × {Height}");
        base.Display();
    }
}

When Should You Use Them?

Abstract classes shine when:

  1. You have a family of related classes that share a bunch of common code
  2. You want to force child classes to implement certain methods
  3. You need to provide some default behavior but customize other parts
  4. You want to share private or protected stuff between related classes

Abstract Classes vs. Interfaces: What’s the Difference?

People often confuse these two, but they serve different purposes:

  • Abstract classes can give you actual working code; interfaces just define what methods you need
  • You can only inherit from one abstract class, but you can implement many interfaces
  • Abstract classes let you control access with private/protected members
  • Use interfaces when you want to say “can do this,” use abstract classes when you want to say “is a type of this”

The Bottom Line

Abstract classes are super useful when you’re designing a family of related types. They help you avoid duplicating code while still making sure that specific functionality gets implemented properly in each derived class.

They’re one of those features that mark the transition from beginner to intermediate C# developer. Once you get comfortable with them, you’ll find yourself reaching for them whenever you need to create flexible but consistent class hierarchies.