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Design Patterns: Strategy

Russ Hammett
Dad, programmer, faux-chef, gamer, book reader
Originally published at blog.kritner.com on ・2 min read

The strategy pattern is one of the first patterns I learned, and it leads into at least one other pattern that I used constantly! Let’s do this thing!

I’ve not been having as much time to focus on writing as I’d like, so I figured I’d try to knock out some pattern review for myself while hopefully working on more significant posts in the background. The first post in this potential series of posts - The strategy pattern!

The strategy pattern

In computer programming, the strategy pattern (also known as the policy pattern) is a behavioral software design pattern that enables selecting an algorithm at runtime. Instead of implementing a single algorithm directly, code receives run-time instructions as to which in a family of algorithms to use.

from Wikipedia

What does that mean? Well to me, it’s just having multiple implementations of a single interface. That’s it. One of the first things I think about when it comes to the strategy pattern, is having multiple “Provider” implementations for something like logging.

Implementation

public interface ILogger
{
    void Log(string message);
}

That’s our interface. There’s not much to the contract. We have a method called Log which expects a string message.

So how does this get utilized as a “strategy”? Multiple implementations!

public class ConsoleLogger : ILogger
{
    public void Log(string message)
    {
        Console.WriteLine(message);
    }
}

public class FileLogger : ILogger
{
    public void Log(string message)
    {
        // Gonna have to use our imagination here...
        var currentColor = Console.ForegroundColor;
        Console.ForegroundColor = ConsoleColor.Blue;
        Console.WriteLine(message);
        Console.ForegroundColor = currentColor;        
    }
}

(Note, don’t use that file logger, it’s terrible… In fact! don’t use any of it, it’s just an example!)

static void Main(string[] args)
{
    ILogger logger = null;

    logger = new ConsoleLogger();
    logger.Log($"Doot doot, this should be a {nameof(ConsoleLogger)}.  {logger.GetType()}");

    logger = new FileLogger();
    logger.Log($"Doot doot, this should be a {nameof(FileLogger)}.  {logger.GetType()}");
}

Reasons to use this pattern

What does the use of this pattern buy us?

  • Writing code to the abstraction that is ILogger rather than a concrete implementation - making Unit testing easier.
  • Select an algorithm implementation at run time, perhaps based on a user’s input, or configuration.
  • Ties into other patterns, one of my favorites being the “Factory” (TODO write about a factory).

Note that I got some pushback in a programming slack group that the above is not in fact an example of the strategy pattern, due to not having selected the implementation at run time (since I did it in code at compile time). I disagree, but wanted to throw that out there cuz I’ve been wrong before! :D

The code for this pattern can be found on my pattern repo: https://github.com/Kritner-Blogs/Kritner.PatternExamples

Resources:

Discussion (2)

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thejoezack profile image
Joe Zack

Strategy may be my favorite design pattern. I usually feel pretty good about the places where I used it, as compared to...like, "Builder" which is the one I probably misuse the most.

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Russ Hammett Author

OOOOoooooh man I love me some builder pattern. There are some pretty fancy things you can do with them!

Maybe that'll be next after I finish the TODO factory ;)