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Double check pattern

, 10 Jun 2012
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Double check pattern in lazy loading.

I just answered a question at SO about lazy loading which involved the double check pattern. It’s a really useful pattern for lazy loading since it hurt performance a lot less than always locking.

I thought that I should share explain why by using some comments:

public sealed class Lazy<T> where T : class
{
    private readonly object _syncRoot = new object();
    private readonly Func<T> _factory;
    private T _value;

    public Lazy(Func<T> factory)
    {
        if (factory == null) throw new ArgumentNullException("factory");

        _factory = factory;
    }

    public T Value
    {
        get
        {
            // here is the first check. It only returns true
            // if the instance have not been created, right?
            if (_value == null)
            {
                // so when we enter here, we are within a very small time frame:
                // That is from the above check until the new instance is
                // assigned to the variable.

                // Which is a <strong>very</strong> small time frame
                // (unless you rely on external resources).

                // So let's lock to make sure that no other thread
                // have already started to create the object.
                lock (_syncRoot)
                {
                    // We enter here as soon as any other thread (if there were one)
                    // have stopped working, which means that the value could
                    // have been assigned.

                    // So let's check if another thread have already done the work for us.
                    if (_value == null)
                    {
                        //no, so let's create the object.
                        _value = _factory();
                    }
                }
            }
            return _value;
        }
    }

    public override string ToString()
    {
        return _value == null ? "Not created" : _value.ToString();
    }
}

The double check pattern allows us to have lock free code (other than when the instance is created) which is a huge performance gain compared to using a single lock.

Feel three to use the code in .NET versions earlier than 4.

License

This article, along with any associated source code and files, is licensed under The GNU Lesser General Public License (LGPLv3)

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About the Author

jgauffin
Founder Gauffin Interactive AB
Sweden Sweden
Founder of OneTrueError, a .NET service which captures, analyzes and provide possible solutions for exceptions.
 
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Comments and Discussions

 
SuggestionSuggestion? PinmemberProoskalia10-Jun-12 9:52 
GeneralRe: Suggestion? PinmemberWilliam E. Kempf15-Jun-12 5:53 

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