Singleton Class which requires some async call

Question

I have a Singleton Class which loads some data on its construction. The problem is that loading this data requires calling async methods, but the constructor cannot be async.

In other words, my class has following structure:

public class Singleton
{
   private static Singleton instance;

   private Singleton() 
   {
       LoadData();
   }

   public static Singleton Instance
   {
      get 
      {
         if (instance == null)
         {
            instance = new Singleton();
         }
         return instance;
       }
    }
}

LoadData() is an async function which calls lots of async functions as well as initialization. How can I call LoadData() properly so everything initialize correctly?

Answer 1

The solution for a thread-safe, async singleton is actually super simple, if we only let the inner mechanisms of the Task class work for us!

So, how does a Task work? Let’s say you have an instance of a Task<T> and you await it once. Now the task is executed, and a value of T is produced and returned to you. What if you await the same task instance again? In this case the task just returns the previously produced value immediately in a completely synchronous manner.

And what if you await the same task instance simultaneously from multiple threads (where you would normally get a race condition)? Well, the first one (since there will be one that gets there first) will execute the task code while the others will wait for the result to be processed. Then when the result has been produced, all the await’s will finish (virtually) simultaneously and return the value.

So the solution for an async singleton that is thread-safe is actually super simple:

public class Singleton
{
    private static readonly Task<Singleton> _getInstanceTask = CreateSingleton();

    public static Task<Singleton> Instance
    {
        get { return _getInstanceTask; }
    }

    private Singleton(SomeData someData)
    {
        SomeData = someData;
    }

    public SomeData SomeData { get; private set; }

    private static async Task<Singleton> CreateSingleton()
    {
        SomeData someData = await LoadData();
        return new Singleton(someData);
    }
}

Now you can access the singleton this way:

Singleton mySingleton = await Singleton.Instance;

or

Singleton mySingleton = Singleton.Instance.Result;

or

SomeData mySingletonData = (await Singleton.Instance).SomeData;

or

SomeData mySingletonData = Singleton.Instance.Result.SomeData;

Read more here: Async singleton initialization