Question

I have a project consisting of three projects,

1. WCF service
2. Asp.net MVC 3 application
3. Class library.

The one in the class library is my singleton, which I have made like this;

public sealed class Singleton
{
    public static Singleton Instance { get; set; }

    private Singleton()
    {

    }

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

The thing is, I put a Debug.WriteLinein the constructor, and it gets called twice.
What I am trying to do is use the singleton from the mvc 3 application and from the WCF service, but they make different instances. Why?

EDIT: I tried a treadsafe singleton earlier. It made no difference.

Answer 1

There’s a couple of things that could be going on here.

The most likely is that your MVC application and your WCF service are running in different AppDomains. It will be impossible for the code to ‘share’ the same instance if this is the case.

An alternative and less likely cause, is that because your code is not thread safe multiple instances are created. If the Singleton constructor takes a long time to return then this could be the issue. Since your using MVC3, I’ll assume .Net 4, in which case the Lazy class is your friend:

private static readonly Lazy<Singleton> _singleton = new Lazy<Singleton>(() => new Singleton());
public static Singleton Instance { get { return _singleton.Value; } }