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Design patterns in C# – The composite pattern

by Carmel Eve

So, this week I’m moving on from the creational patterns (though I might return at some point) and moving on to the structural design patterns!

Structural design patterns are ones that deal with the relationships between different elements. Essentially, they define how different parts of the code interact with and relate to each other.

This week I’ll be focusing on the composite pattern. This is a fairly simple pattern, which specifically defines the behaviour of objects which are made up of other objects – composite objects. The critical idea is that the group of objects should be treated the same as the individual objects it is composed of. This means that whether a group is made up of individual objects or more groups of objects, the same logic can be applied. There are many examples of this in the real world; One that sticks in my mind is around email groups. When you have a list of recipients, that list can be made up of both individual email addresses and of groups which contain many email addresses, and these groups can also contain child groups containing still more addresses. When you press send, these groups are treated the same as the individuals and everyone is sent the email.

But, in-keeping with the theme, I’ve also produced a dinosaur-related(ish) example!

So say we have an interface, IPlant, which has one method: Eat()

The simplest implementation of IPlant is a Leaf:

We then have our composite object, a Branch. A branch is made up of IPlants.

This means that each Branch can be composed of a mixture of child branches and leaves. When Eat is called on the Branch, each of the IPlants it is made up of will then be eaten in turn. In this way each branch is treated the same as a leaf, with the “eat” action propagating down from branch to leaf, and eventually the whole structure being consumed!

Therefore, if we run the following code:

We have a list which consists of:

  • A big branch, which is made up of 2 smaller branches, one with 2 leaves and one with 4.
  • A branch with 5 leaves
  • 2 individual leaves.

That’s a total of 13 leaves.

So, when we Eat everything in the list, we will get the following output:

That’s 13 leaves eaten! (Trust me I counted, twice.)

Thanks for reading, here’s a link to the GitHub repository which contains all the code for this blog series. And watch of for my next blog on design patterns in C#!

About the author

Carmel is a 3rd year apprentice software engineer focusing on Azure based solutions for data handling. She has a masters degree in physics from the University of Manchester which has given her a keen interest in problem solving in new and imaginative ways. You can follow Carmel on Twitter.