Caching IEnumerable

2019-03-11 01:06发布

站内文章 / C#
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问题:

public IEnumerable<ModuleData> ListModules()
{
    foreach (XElement m in Source.Descendants("Module"))
    {
        yield return new ModuleData(m.Element("ModuleID").Value);
    }
}

Initially the above code is great since there is no need to evaluate the entire collection if it is not needed.

However, once all the Modules have been enumerated once, it becomes more expensive to repeatedly query the XDocument when there is no change.

So, as a performance improvement:

public IEnumerable<ModuleData> ListModules()
{
    if (Modules == null)
    {
        Modules = new List<ModuleData>();
        foreach (XElement m in Source.Descendants("Module"))
        {
            Modules.Add(new ModuleData(m.Element("ModuleID").Value, 1, 1));
        }
    }
    return Modules;
}

Which is great if I am repeatedly using the entire list but not so great otherwise.

Is there a middle ground where I can yield return until the entire list has been iterated, then cache it and serve the cache to subsequent requests?

回答1:

You can look at Saving the State of Enumerators which describes how to create lazy list (which caches once iterated items).



回答2:

Check out MemoizeAll() in the Reactive Extensions for .NET library (Rx). As it is evaluated lazily you can safely set it up during construction and just return Modules from ListModules():

Modules = Source.
    Descendants("Module").
    Select(m => new ModuleData(m.Element("ModuleID").Value, 1, 1)).
    MemoizeAll();

There's a good explanation of MemoizeAll() (and some of the other less obvious Rx extensions) here.



回答3:

I like @tsemer's answer. But I would like to propose my solutions, which has nothing to do with FP. It's naive approach, but it generates a lot less of allocations. And it is not thread safe.

public class CachedEnumerable<T> : IEnumerable<T>, IDisposable
{
    IEnumerator<T> _enumerator;
    readonly List<T> _cache = new List<T>();

    public CachedEnumerable(IEnumerable<T> enumerable) 
        : this(enumerable.GetEnumerator())
    {
    }

    public CachedEnumerable(IEnumerator<T> enumerator)
    {
        _enumerator = enumerator;
    }

    public IEnumerator<T> GetEnumerator()
    {
        // The index of the current item in the cache.
        int index = 0;

        // Enumerate the _cache first
        for (; index < _cache.Count; index++)
        {
            yield return _cache[index];
        }

        // Continue enumeration of the original _enumerator, 
        // until it is finished. 
        // This adds items to the cache and increment 
        for (; _enumerator != null && _enumerator.MoveNext(); index++)
        {
            var current = _enumerator.Current;
            _cache.Add(current);
            yield return current;
        }

        if (_enumerator != null)
        {
            _enumerator.Dispose();
            _enumerator = null;
        }

        // Some other users of the same instance of CachedEnumerable
        // can add more items to the cache, 
        // so we need to enumerate them as well
        for (; index < _cache.Count; index++)
        {
            yield return _cache[index];
        }
    }

    public void Dispose()
    {
        if (_enumerator != null)
        {
            _enumerator.Dispose();
            _enumerator = null;
        }
    }

    IEnumerator IEnumerable.GetEnumerator()
    {
        return GetEnumerator();
    }
}

This is how the matrix test from @tsemer's answer will work:

var ints = new [] { 1, 2, 3, 4, 5 };
var cachedEnumerable = new CachedEnumerable<int>(ints); 
foreach (var x in cachedEnumerable)
{
    foreach (var y in cachedEnumerable)
    {
        //Do something
    }
}
  1. The outer loop (x) skips first for, because _cache is empty;
  2. x fetches one item from the _enumerator to the _cache;
  3. x pauses before second for loop;
  4. The inner loop (y) enumerates one element from the _cache;
  5. y fetches all elements from the _enumerator to the _cache;
  6. y skips the third for loop, because its index variable equals 5;
  7. x resumes, its index equals 1. It skips the second for loop because _enumerator is finished;
  8. x enumerates one element from the _cache using the third for loop;
  9. x pauses before the third for;
  10. y enumerates 5 elements from the _cache using first for loop;
  11. y skips the second for loop, because _enumerator is finished;
  12. y skips the third for loop, because index of y equals 5;
  13. x resumes, increments index. It fetches one element from the _cache using the third for loop.
  14. x pauses.
  15. if index variable of x is less than 5 then go to 10;
  16. end.


回答4:

I've seen a handful of implementations out there, some older and not taking advantage of newest .Net classes, some too elaborate for my needs. I ended up with the most concise and declarative code I could muster, which added up to a class with roughly 15 lines of (actual) code. It seems to align well with OP's needs:

Edit: Second revision, better support for empty enumerables

/// <summary>
/// A <see cref="IEnumerable{T}"/> that caches every item upon first enumeration.
/// </summary>
/// <seealso cref="http://blogs.msdn.com/b/matt/archive/2008/03/14/digging-deeper-into-lazy-and-functional-c.aspx"/>
/// <seealso cref="http://blogs.msdn.com/b/wesdyer/archive/2007/02/13/the-virtues-of-laziness.aspx"/>
public class CachedEnumerable<T> : IEnumerable<T> {
  private readonly bool _hasItem; // Needed so an empty enumerable will not return null but an actual empty enumerable.
  private readonly T _item;
  private readonly Lazy<CachedEnumerable<T>> _nextItems;

  /// <summary>
  /// Initialises a new instance of <see cref="CachedEnumerable{T}"/> using <paramref name="item"/> as the current item
  /// and <paramref name="nextItems"/> as a value factory for the <see cref="CachedEnumerable{T}"/> containing the next items.
  /// </summary>
  protected internal CachedEnumerable(T item, Func<CachedEnumerable<T>> nextItems) {
    _hasItem = true;
    _item = item;
    _nextItems = new Lazy<CachedEnumerable<T>>(nextItems);
  }

  /// <summary>
  /// Initialises a new instance of <see cref="CachedEnumerable{T}"/> with no current item and no next items.
  /// </summary>
  protected internal CachedEnumerable() {
    _hasItem = false;
  }

  /// <summary>
  /// Instantiates and returns a <see cref="CachedEnumerable{T}"/> for a given <paramref name="enumerable"/>.
  /// Notice: The first item is always iterated through.
  /// </summary>
  public static CachedEnumerable<T> Create(IEnumerable<T> enumerable) {
    return Create(enumerable.GetEnumerator());
  }

  /// <summary>
  /// Instantiates and returns a <see cref="CachedEnumerable{T}"/> for a given <paramref name="enumerator"/>.
  /// Notice: The first item is always iterated through.
  /// </summary>
  private static CachedEnumerable<T> Create(IEnumerator<T> enumerator) {
    return enumerator.MoveNext() ? new CachedEnumerable<T>(enumerator.Current, () => Create(enumerator)) : new CachedEnumerable<T>();
  }

  /// <summary>
  /// Returns an enumerator that iterates through the collection.
  /// </summary>
  public IEnumerator<T> GetEnumerator() {
    if (_hasItem) {
      yield return _item;

      var nextItems = _nextItems.Value;
      if (nextItems != null) {
        foreach (var nextItem in nextItems) {
          yield return nextItem;
        }
      }
    }
  }

  /// <summary>
  /// Returns an enumerator that iterates through a collection.
  /// </summary>
  IEnumerator IEnumerable.GetEnumerator() {
    return GetEnumerator();
  }
}

A useful extension method could be:

public static class IEnumerableExtensions {
  /// <summary>
  /// Instantiates and returns a <see cref="CachedEnumerable{T}"/> for a given <paramref name="enumerable"/>.
  /// Notice: The first item is always iterated through.
  /// </summary>
  public static CachedEnumerable<T> ToCachedEnumerable<T>(this IEnumerable<T> enumerable) {
    return CachedEnumerable<T>.Create(enumerable);
  }
}

And for the unit testers amongst you: (if you don't use resharper just take out the [SuppressMessage] attributes)

/// <summary>
/// Tests the <see cref="CachedEnumerable{T}"/> class.
/// </summary>
[TestFixture]
public class CachedEnumerableTest {
  private int _count;

  /// <remarks>
  /// This test case is only here to emphasise the problem with <see cref="IEnumerable{T}"/> which <see cref="CachedEnumerable{T}"/> attempts to solve.
  /// </remarks>
  [Test]
  [SuppressMessage("ReSharper", "PossibleMultipleEnumeration")]
  [SuppressMessage("ReSharper", "ReturnValueOfPureMethodIsNotUsed")]
  public void MultipleEnumerationAreNotCachedForOriginalIEnumerable() {
    _count = 0;

    var enumerable = Enumerable.Range(1, 40).Select(IncrementCount);

    enumerable.Take(3).ToArray();
    enumerable.Take(10).ToArray();
    enumerable.Take(4).ToArray();

    Assert.AreEqual(17, _count);
  }

  /// <remarks>
  /// This test case is only here to emphasise the problem with <see cref="IList{T}"/> which <see cref="CachedEnumerable{T}"/> attempts to solve.
  /// </remarks>
  [Test]
  [SuppressMessage("ReSharper", "PossibleMultipleEnumeration")]
  [SuppressMessage("ReSharper", "ReturnValueOfPureMethodIsNotUsed")]
  public void EntireListIsEnumeratedForOriginalListOrArray() {
    _count = 0;
    Enumerable.Range(1, 40).Select(IncrementCount).ToList();
    Assert.AreEqual(40, _count);

    _count = 0;
    Enumerable.Range(1, 40).Select(IncrementCount).ToArray();
    Assert.AreEqual(40, _count);
  }

  [Test]
  [SuppressMessage("ReSharper", "ReturnValueOfPureMethodIsNotUsed")]
  public void MultipleEnumerationsAreCached() {
    _count = 0;

    var cachedEnumerable = Enumerable.Range(1, 40).Select(IncrementCount).ToCachedEnumerable();

    cachedEnumerable.Take(3).ToArray();
    cachedEnumerable.Take(10).ToArray();
    cachedEnumerable.Take(4).ToArray();

    Assert.AreEqual(10, _count);
  }

  [Test]
  public void FreshCachedEnumerableDoesNotEnumerateExceptFirstItem() {
    _count = 0;

    Enumerable.Range(1, 40).Select(IncrementCount).ToCachedEnumerable();

    Assert.AreEqual(1, _count);
  }

  /// <remarks>
  /// Based on Jon Skeet's test mentioned here: http://www.siepman.nl/blog/post/2013/10/09/LazyList-A-better-LINQ-result-cache-than-List.aspx
  /// </remarks>
  [Test]
  [SuppressMessage("ReSharper", "LoopCanBeConvertedToQuery")]
  public void MatrixEnumerationIteratesAsExpectedWhileStillKeepingEnumeratedValuesCached() {
    _count = 0;

    var cachedEnumerable = Enumerable.Range(1, 5).Select(IncrementCount).ToCachedEnumerable();

    var matrixCount = 0;

    foreach (var x in cachedEnumerable) {
      foreach (var y in cachedEnumerable) {
        matrixCount++;
      }
    }

    Assert.AreEqual(5, _count);
    Assert.AreEqual(25, matrixCount);
  }

  [Test]
  public void OrderingCachedEnumerableWorksAsExpectedWhileStillKeepingEnumeratedValuesCached() {
    _count = 0;

    var cachedEnumerable = Enumerable.Range(1, 5).Select(IncrementCount).ToCachedEnumerable();

    var orderedEnumerated = cachedEnumerable.OrderBy(x => x);
    var orderedEnumeratedArray = orderedEnumerated.ToArray(); // Enumerated first time in ascending order.
    Assert.AreEqual(5, _count);

    for (int i = 0; i < orderedEnumeratedArray.Length; i++) {
      Assert.AreEqual(i + 1, orderedEnumeratedArray[i]);
    }

    var reorderedEnumeratedArray = orderedEnumerated.OrderByDescending(x => x).ToArray(); // Enumerated second time in descending order.
    Assert.AreEqual(5, _count);

    for (int i = 0; i < reorderedEnumeratedArray.Length; i++) {
      Assert.AreEqual(5 - i, reorderedEnumeratedArray[i]);
    }
  }

  private int IncrementCount(int value) {
    _count++;
    return value;
  }
}


回答5:

I quite like hazzik's answer...nice and simple is always the way. BUT there is a bug in GetEnumerator

it sort of realises there is a problem, and thats why there is a strange 3rd loop after the 2nd enumerator loop....but it isnt quite as simple as that. The problem that triggers the need for the 3rd loop is general...so it needs to be recursive.

The answer though looks even simpler.

    public IEnumerator<T> GetEnumerator()
    {
        int index = 0;

        while (true)
        {
            if (index < _cache.Count)
            {
                yield return _cache[index];
                index = index + 1;
            }
            else
            {
                if (_enumerator.MoveNext())
                {
                    _cache.Add(_enumerator.Current);
                }
                else
                {
                    yield break;
                }
            }
        }
    }

yes you can make it a tiny bit more efficient by yielding current...but I'll take the microsecond hit...it only ever happens once per element.

and its not threadsafe...but who cares about that.



回答6:

I don't see any serious problem with the idea to cache results in a list, just like in the above code. Probably, it would be better to construct the list using ToList() method. 

public IEnumerable<ModuleData> ListModules()
{
    if (Modules == null)
    {
        Modules = Source.Descendants("Module")
                      .Select(m => new ModuleData(m.Element("ModuleID").Value, 1, 1)))
                      .ToList();
    }
    return Modules;
}


标签： c# return ienumerable yield
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