Adapter Design Pattern - Illustration and Code

Let's learn about Adapters - a structural design pattern - with an intuitive diagram and matching C# code sample

The adapter pattern is a structural design pattern that is often used when integrating new code with a legacy library where you wish to avoid updating the old code base, or for example, converting an external XML feed to JSON.

Why C#

For the design patterns series I'll be using C# as I find it an elegant language that makes understanding traditional OOP concepts easier than a language such as Python, which I prefer for algorithmic concepts.

Adapter pattern illustrated

The following illustration is a contrived example of attempting to insert a square into a legacy sorter that can only handle circles, and how the adapter pattern could be used to overcome this.

Adapter design pattern illustration

Code example

Let's look at example code for adapting our Square to be compatible with a LegacySorter that can only sort circles.

The adaptee

First, we create our adaptee (Square) that is incompatible with the iCircle interface:

class Square
    public int Width { get; private set; }
    public Square(int width)
        Width = width;


Now we need to create our adapter which implements the original iCircle interface and overrides the iCircle's Radius property with the square's width / 2 value:

// Adapter that can adapt our square to the iCircle interface
class SquareAdapter : iCircle
    protected Square _square;
    public int Radius => _square.Width / 2;

    public SquareAdapter(Square square)
        _square = square;

Using our new adapter

And finally, we can use our new SquareAdapter to seamlessly add our square to the legacy sorter:

// Let's add a couple circles to our Legacy Sorter.
LegacySorter.AddSortedItem(new Circle(5));
LegacySorter.AddSortedItem(new Circle(2));

// Create our adaptee object.
Square square = new Square(6);

// Adapt our square to the iCircle interface.
iCircle adaptedSquare = new SquareAdapter(square);

// And finally, we can now add our square to the legacy item sorter.

The end - plus full project code

The adapter pattern is one of the easier patterns to implement and understand, however (as with all design patterns), it can be abused so make sure there is a valid reason to build an adapter rather than simply refactoring a problematic interface.

I've uploaded a full runnable C# code sample to Github here.


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