Read this one when done with Design Patterns book

 Amazingly simple book on quiet complex things. Recommended. The person who was preaching about it was JP Boodhoo - damn he was right about it.

Definition of a singleton is simple. So is the usage, but when it comes to testing/mocking/dynamically replacing it - that's a pain. While investigating MVP pattern, and reading materials from Jeremy D. Miller, I ran into on of his blogs about Singleton. It inspired me to adopt the approach and share the thoughts.

Singleton (noun) -  single card of a particular suit (Cards); individual occurrence or item, single instance or thing

I liked the 1st definition - a single card of particular suit. In real project a singleton is one and only, but from time to time, you need to replace it with a different implementation. So this is what I learned:

1. Define an interface ("Code to interface, not a concrete implementation" - that's what they say)
2. Create Singleton class (variations) with interface being implemented
3. During run-time/unit testing decide what to do

  public interface ISingleton
  {
    /// <summary>All implementors of <see cref="ISingleton"/> interface will have the method.</summary>
    string DoThisAndThat();
  }

  public class Singleton : ISingleton
  {
    private static readonly Singleton _instance;
    private static readonly string _testData = "Unique data for this variation of singleton";

    static Singleton()
    {
      _instance = new Singleton();
    }

    private Singleton() {}

    public static Singleton Current
    {
      get
      {
        return _instance;
      }
    }

    #region ISingleton Members

    public string DoThisAndThat()
    {
      return _testData;
    }

    #endregion
  }

Usage:

ISingleton current = Singleton.Current;
Console.WriteLine(current.DoThisAndThat());

Better approach would be to have a factory that would return the Right singleton as a ISingleton, based one what is registered/required. One of the probably the best things about it is the fact that now it can be mocked as well.