Tag Archives: double-checked locking

Alternative to Double-Checked Locking: Lazy<T>

A common pattern for lazy-initialization of a possibly-expensive type is double-checked locking.

The pattern is essentially this:

// member field
private volatile MyType myObject = null;
private object myLock = new object();

public MyType MyObject
{
    get
    {
        if (myObject == null)
        {
            lock(myLock)
            {
                if (myObject == null)
                {
                    myObject = new MyType();
                }
            }
        }
    }   
}

It’s quite common, and it’s also common to get it subtly wrong (not using volatile, for example).

However, in .Net 4, there’s a simpler way to do this: use Lazy<T>. This type uses a delegate to initialize the value with double-checked locking (there are other possible thread safety modes as well).

private Lazy<MyType> myObject = new Lazy<MyType>( () => new MyType );

public MyType MyObject
{
    get
    {
        return myObject.Value;
    }   
}

With Lazy<T>, the delegate you pass in will not be called until the Value property is accessed. Depending on the options you set, only a single thread will call your delegate and create the real type. I’d much rather do this than have to look at double-checked locking patterns scattered throughout a code base.

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