Locking the Static object inside the static property in the HTTPModule

Question

We have a custom DLL implemented IHttpModule to handle the httpApplication_EndRequest, what I want to know is

The DLL has a class (not a static class) which has a static property is used to create an instance for a static variables/object reference defined inside the class.

Now, should I need to lock inside static property before creating an instance for the static object/variable?

Eg:-

public class SPEnvironment : IEnvironment
{
    private static SPEnvironment _instance;
    private static object _syncRoot = new object();

    private SPEnvironment()
    {
        try {
        .....
        }
        finally {
        ......
        }
    }

    public static SPEnvironment Instance
    {
        get
        {
            if (_instance == null)
            {
                lock (_syncRoot)
                {
                    if (_instance == null)
                    {
                        _instance = new SPEnvironment();
                    }
                }
            }
            return _instance;
        }
    }
}

I will calling this from an another class, like below

SPEnvironment.Instance;

Is it the right way? or the lock should be removed?

Answer 1

The double null check with a lock in the middle is a good, thread-safe way of instantiating a singleton. However, you could save yourself a lot of code by just saying

public class SPEnvironment : IEnvironment
{
    public static SPEnvironment Instance = new SPEnvironment();

    private SPEnvironment()
    {
        try {
        .....
        }
        finally {
        ......
        }
    }
}

The difference between the two is that this code instantiates the singleton the first time an object of that type is created where your code instantiates the singleton the first time SPEnvironment.Instance is accessed. In almost all cases, those are the same thing; in most of the remaining cases, it doesn't matter; but it is a subtle distinction that is worth understanding for that very rare edge case.