Code & Sundry

Jon G Stødle

2.5 angles on singletons in C#

425 words, 3 minutes to read

This is not a thorough walk through of the technical pros and cons of different singleton patterns in C#. This is just me philosophizing over the three approaches I’ve used in my code.

If you want a proper, thorough, technical, accurate and intelligent walk through of singleton patterns in C#, check this out. That’s done properly and by a guy who knows his stuff: Jon Skeet, the C# god of Stack Overflow.

Anyway. Here are my three implementations and thoughts:

public static MySingleton Current { get; } = new MySingleton();

This is one approach I’ve used a couple of times recently. It’s very succinct, but it has one flaw which I didn’t consider until it got me into trouble; it instantiates the class at first access to the MySingleton class.

If your only static property on MySingleton is Current, this is not a problem. However, I wanted to access another static property on the class before using Current: MySingleton.IsAvailable. If IsAvailable is false, I can’t instantiate MySingleton, but that happens automatically when I first access MySingleton.

public static MySingleton Current { get; }

static MySingleton()
    Current = Current ?? new MySingleton();

This approach is more or less the same as the one above. It tries to assign Current to itself, unless it’s null, in which case it sets it to a new instance of MySingleton. This still suffers from the problem mentioned above; Current is instantiated at first access to the class.

One additional trap is that a static constructor might be invoked before the class is even used. If the CLR (which runs the code on the computer) decides it has some spare time, it might start invoking static constructors in case they’re going to be used at a later time. This is done to prevent delays when accessing the class for the first time later.

private static MySingleton mySingleton;
 public static MySingleton Current
         if(mySingleton == null) mySingleton = new MySingleton();
         return mySingleton;

What you want is this. A lazy instantiation of the Current property, i.e. it’s not instantiated before the property itself is used. Even if you use the rest of the class, Current will not instantiate a new MySingleton instance until truly needed.

Which one of the three you end up using might depend on your needs. Sometimes it might be beneficial if the CLR instantiates the singleton before it’s truly needed (if it has some CPU time to spare anyways). Other times however, it might be more desirable with a lazy property.

Choose wisely and happy coding 🙂