Tag: ioc

ProjectXyz: Why I Started a Side Project (Part 1)

ProjectXyz

Alright, I’ll admit it… Even for a placeholder name on a side project it’s pretty terrible, right? Well, my apologies. So, if you made it to this post you might be wondering what ProjectXyz is and why I started it up. From a high level, I started working on ProjectXyz so that I could have a hobby programming project to tinker with and I figured I’d blog about my adventures in bringing it all together. I plan on making this a mini-series documenting some of the things I’m learning or experimenting with, so this will serve as the intro to the series.

Before we get too far, here’s the link to the GitHub site: https://github.com/ncosentino/ProjectXyz

Why Have a Side Project?

Here’s the main thing I want to talk about in part 1 of this series: Why should you have a side project?

From my experiences in high school and university, I found that having a side project that I could code in was probably the best way that I could continue to learn. It’s a low stress activity that encourages you to be highly creative. You can involve as many other people as you want, or work on it completely solo so that you can minimize external stress and maximize your own creativity. It’s your project, so you have the freedom to do what you want.

I leveraged various small side projects to learn things like how sockets work or how I’d try and structure a multi-tier application. I was able to work on refactoring large amounts of code and learn how to use source control to my advantage. Was there a design pattern I wanted to learn about implementing? Great! Then I could try and find a way to incorporate it into my projects. The goal was always about trying to implement new things and not about cutting corners to try and deliver something to an end user.

Once I started work full-time, I gave up on my programming side projects. I started up this blog which has been fun and I try to take on as much work for my career as I can because I actually enjoy my job… But I stopped coding my own hobby projects. I’ve found that I’m missing out on two major things as a result of that:

  • The ability to experiment with patterns, technologies, and frameworks
  • The ability to get really creative and try out completely new things

Work has been a great opportunity to learn, but it’s learning out of necessity. Myself or my team will hit blocks and we have to work together to try and overcome them. It doesn’t give me the opportunity to go completely into left field to try something new out. Having a side project gives me a bit of freedom to try and learn all sorts of really neat things.

So… Why ProjectXyz?

Okay, well, if you mean why the name… Then I don’t have a great answer. I wanted to start coding but I didn’t want to waste time thinking about a name. I’ll think of something better later, I promise.

Otherwise, you might be wondering why I decided to build ProjectXyz to be what it is (or, what it will be). ProjectXyz is the back end (i.e. not the pretty graphical part) for a role playing game, and I’ve played around with this kind of thing before. I like playing games like this, and I’ve had a lot of fun trying to create a game like this in the past. As a result, I can really focus on what I’m building and not trying to figure out what to build. I’m not pouring energy into wondering “how do I solve this great big popular problem with this piece of software?”, but instead I can just get as creative as I’d like. It’s not about “What does the customer want?”, but instead, I can ask “What do I want to make?”.

The things I’m looking to try out with ProjectXyz (to start with, at least) are:

  • More LINQ usage
  • Coding by interfaces
  • API design
  • Dependency injection and IoC
  • TDD and coded test designs
  • Moq for mocking my classes
  • … GitHub! To make some publicly visible code.

As I work through ProjectXyz, I’ll write more posts on the various things I’m learning as I go! Check out the GitHub page and drop some comments!

https://github.com/ncosentino/ProjectXyz
(The second post in this series is here)


Dependency Injected Singletons… What?

Background

Previously I wrote a bit about singletons. I’m not afraid to say that I think singletons have a time and a place… Okay, I’m afraid to say it, but it’s actually how I feel 🙂 After learning more and more about inversion of control design patterns and programming by interfaces, I started to notice just how inflexible singletons are. When you consider using a singleton, you should be considering both the pros and cons without jumping right into it. Here’s an example of my approach for mixing singletons, programming by interfaces, and a bit of inversion of control.

The Setup

I’m actually surprised you got this far. I’m sure you’re probably just sticking around to see how messed up this could possibly be. I’m actually proud that this little pattern has worked out so well when I’ve used it, so it can’t possibly be that bad.

One major drawback to singletons is that they are often implemented as concrete classes. That is, you ask some class X for it’s instance (via a static property/method) and it provides you it. This doesn’t mix very well with programming by interfaces. So, the first step is to have your singleton return an interface instead of a concrete class. Unfortunately, this on it’s own only provides a few benefits, and it’s really only a minor abstraction layer. You can change your singleton class around all you want, but the only dependencies people are going to see are what’s exposed on the interface. As long as your interface isn’t changing, you’re in a good position. With this change alone, we’ve managed to tackle a bit on programming by interfaces, which makes me just a bit happier.

So, what’s the problem now then? Well, now that I have my singleton returning an interface, the unfortunate truth is it’s always going to be the same class backing that interface. It’s incredibly risky if I start putting some sort of logic in my singleton’s property to get the instance that will return different references that could implement the interface. I mean, once your code is running, you want to make sure you’re always getting the same reference back… or else you’re not using a singleton! The drawback to this is that it completely ruins inversion of control and dependency injection!

 

An Example

Consider that I have a singleton that has information about my application. (If you don’t like my example, then you’ll surely hate using WinForms, because Microsoft does it with their Application class. So, it’s a fair example!) Okay, so if we pretend that we need something just like the Application class in our project, we might create some interface IApplication and create a singleton that returns an instance of an IApplication.

Suppose in one implementation we have, we really want to be using reflection and the assembly information to figure out our application’s information. Our concrete class would implement the IApplication interface but the methods would revolve around using reflection and querying the assembly for what it wants. If we built up a whole framework that used this singleton… We’d be tied to that implementation!

What happens if we go to make another application that wants to use this same framework, but it needs to be able to provide the application information through a configuration file? Well… We’d be pooched! Our singleton that’s used everywhere (and yes, please continue crying about the use of the singleton… Just remember Microsoft’s Application class) and we can’t really do anything about it!

Or can we?

The “Solution”

Okay, so here we are. I’ve demonstrated the problem with a scenario that isn’t too far fetched. How do we fix this crappy situation we’ve got ourselves in by using the dreaded singleton pattern? We use my other best friend: events. Except we use them in a way that makes me cringe. Static events.

Here’s my solution the the above problem:

The Interface:

    public interface IApplication
    {
        string Name { get; }

        string Version { get; }
    }

The Singleton:

    public static class Application
    {
        private static readonly object _instanceLock = new object();
        private static IApplication _instance;

        public static event EventHandler<QueryTypeEventArgs> QueryType;

        public static IApplication Instance
        {
            get
            {
                if (_instance == null)
                {
                    lock (_instanceLock)
                    {
                        if (_instance == null)
                        {
                            _instance = CreateInstance();
                        }
                    }
                }

                return _instance;
            }
        }

        private static IApplication CreateInstance()
        {
            var handler = QueryType;
            if (handler == null)
            {
                throw new InvalidOperationException(
                    "Cannot create an instance because the QueryType event " +
                    "handler was never set.");
            }

            var args = new QueryTypeEventArgs();
            handler.Invoke(null, args);

            if (args.Type == null)
            {
                throw new InvalidOperationException(
                    "Cannot create an instance because the type has not been " +
                    "provided.");
            }

            // NOTE: here's where things get weird. you need to define your own
            // sort of "contract" for what type of constructor you will allow.
            // you might not even use a constructor here... but you need to
            // define what mechanism the provided type must have to provide
            // you with a singleton instance. i'm a fan of providing a type
            // with a private parameterless constructor, so i'll demonstrate
            // with that. your requirements will change what this section of
            // code looks like.
            if (!typeof(IApplication).IsAssignableFrom(args.Type))
            {
                throw new InvalidOperationException(
                    "Cannot create an instance because the provided type does " +
                    "not implement the IApplication interface.");
            }

            const BindingFlags FLAGS = 
                BindingFlags.CreateInstance | 
                BindingFlags.Instance | 
                BindingFlags.NonPublic;

            var constructors = args.Type.GetConstructors(FLAGS);
            if (constructors.Length != 1)
            {
                throw new InvalidOperationException(
                    "Cannot create an instance because a single private " +
                    "parameterless constructor was expected.");
            }

            return (IApplication)constructors[0].Invoke(null);
        }
    }

The Program (With two types to inject!)

    internal class Program
    {
        private static void Main(string[] args)
        {
            Application.QueryType += (sender, e) =>
            {
                e.Type = typeof(ApplicationB);
            };

            Console.WriteLine(string.Format(
                "Application Name: {0}rnVersion: {1}",
                Application.Instance.Name,
                Application.Instance.Version));

            Console.WriteLine("Press enter to exit.");
            Console.ReadLine();
        }
    }

    internal class ApplicationA : IApplication
    {
        private ApplicationA()
        {
        }

        public string Name
        {
            get
            {
                return "Application A (Pretend this was from the assembly info)";
            }
        }

        public string Version
        {
            get { return "1234"; }
        }
    }

    internal class ApplicationB : IApplication
    {
        private ApplicationB()
        {
        }

        public string Name
        {
            get
            {
                return "Application B (Pretend this was from an XML file)";
            }
        }

        public string Version
        {
            get { return "9876"; }
        }
    }

So, if you were to run the program, what outputs would you expect in either case? What happens when you forget to set your event handler? What happens when you set your event handler and don’t provide a type? What if it’s a bad type?

 

Summary


I’m not claiming this is the best approach to solve this problem, and I’m not even encouraging that everyone go ahead and use it. Some of the pros of this are:

  • Advantages of programming by interfaces! You’re only exposing interface definitions to others.
  • Dependency injection capabilities. Inject your “singleton” into other applications.
  • All the goodies related to singletons
  • Easy to use. Just hook up your type to a event handler when your program initializes.

Some of the cons are:

  • All the things people hate about singletons. All of them.
  • Static events are absolutely hideous.
  • You’re giving some other class control of creating your singleton instance.
  • There is no compile time checking or contracts for how your singleton needs to be created.

And now that you know some of the good and some of the bad, can you leverage a design like this? You can check out a working solution I put together for the code and example I described above over at Google Code. Hope you were able to learn something (either good or bad)!


  • Nick Cosentino

    Nick Cosentino

    I work as a team lead of software engineering at Magnet Forensics (http://www.magnetforensics.com). I'm into powerlifting, bodybuilding, and blogging about leadership/development topics over at http://www.devleader.ca.

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