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;
}
}
