是否应将此C＃代码重构为使用Lazy <T> 课吗？

Question

I have the following code which could be called via multiple web-requests at the same second. As such, I don't want the second+ request hitting the database, but waiting until the first one does.

Should I refactor this to use the Lazy<T> keyword class instead? If 10 calls to a Lazy<T> piece of code occur at the same time, do 9 of those calls wait for the first one to complete?

public class ThemeService : IThemeService
{
    private static readonly object SyncLock = new object();
    private static IList<Theme> _themes;
    private readonly IRepository<Theme> _themeRepository;

    <snip snip snip>

    #region Implementation of IThemeService

    public IList<Theme> Find()
    {
        if (_themes == null)
        {
            lock (SyncLock)
            {
                if (_themes == null)
                {
                    // Load all the themes from the Db.
                    _themes = _themeRepository.Find().ToList();
                }
            }
        }

        return _themes;
    }

    <sip snip snip>

    #endregion
}

Answer 1

Yes you can use Lazy<T>

From MSDN :

By default, Lazy objects are thread-safe. That is, if the constructor does not specify the kind of thread safety, the Lazy objects it creates are thread-safe. In multithreaded scenarios, the first thread to access the Value property of a thread-safe Lazy object initializes it for all subsequent accesses on all threads , and all threads share the same data. Therefore, it does not matter which thread initializes the object, and race conditions are benign.

And yes, it's not a keyword - its a .NET framework class that formalizes the often required use case for lazy initialization and offers this out of the box so you don't have to do it "manually".

Answer 2

As @BrokenGlass pointed out it is safe. But I couldn't resist and had to make a test...

Only one thread id is printed...

private static Lazy<int> lazyInt;

// make it slow
private int fib()
{
    Thread.Sleep(1000);
    return 0;
}

public void Test()
{
    // when run prints the thread id
    lazyInt = new Lazy<int>(
        () =>
        {
            Debug.WriteLine("ID: {0} ", Thread.CurrentThread.ManagedThreadId);
            return fib();
        });

    var t1 = new Thread(() => { var x = lazyInt.Value; });
    var t2 = new Thread(() => { var x = lazyInt.Value; });
    var t3 = new Thread(() => { var x = lazyInt.Value; });

    t1.Start();
    t2.Start();
    t3.Start();

    t1.Join();
    t2.Join();
    t3.Join();
}

But, which one is faster? From the results I got...

Executing the code 100 times

[   Lazy: 00:00:01.003   ]
[  Field: 00:00:01.000   ]

Executing the code 100000000 times

[   Lazy: 00:00:10.516   ]
[  Field: 00:00:17.969   ]

Test code:

Performance.Test("Lazy", TestAmount, false,
    () =>
    {
        var laz = lazyInt.Value;
    });

Performance.Test("Field", TestAmount, false,
    () =>
    {
        var laz = FieldInt;
    });

Test method:

public static void Test(string name, decimal times, bool precompile, Action fn)
{
    if (precompile)
    {
        fn();
    }

    GC.Collect();
    Thread.Sleep(2000);

    var sw = new Stopwatch();

    sw.Start();

    for (decimal i = 0; i < times; ++i)
    {
        fn();
    }

    sw.Stop();

    Console.WriteLine("[{0,15}: {1,-15}]", name, new DateTime(sw.Elapsed.Ticks).ToString("HH:mm:ss.fff"));
}