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Enumerable Interleave Extension Method

A recent stackoverflow question, which I didn’t bookmark and now I’m unable to find, inspired me to implement an extension method for Enumerable that allows to insert a constant element between each pair of elements in a sequence. Kind of what String.Join does for strings, but maintaining an enumerable as the return value. Having done the single element part I got a bit carried away and ended up expanding it adding overloads to support interleaving elements of another sequence and support for a predicate to control when interleaving takes place. I have to confess that I did this for fun and now I can’t think of any real usage scenario, nonetheless, it may prove useful for someone. First a simple example: var target = new string[] { "(", ")", "(", ")" }; var result = target.Interleave(".", (f, s) => f == "("); // Prints: (.)(.) Console.WriteLine(String.Join(string.Empty, result)); And now the untested but documented implementation: using System; using System.Collections; using System.Collections.Generic; using System.Linq; public static class EnumerableExtensions { /// <summary> /// Iterates infinitely over a constant element. /// </summary> /// <typeparam name="T"> /// The type of element in the sequence. /// </typeparam> private class InfiniteSequence<T> : IEnumerable<T>, IEnumerator<T> { public InfiniteSequence(T element) { this.Element = element; } public T Element { get; private set; } public IEnumerator<T> GetEnumerator() { return this; } IEnumerator IEnumerable.GetEnumerator() { return this; } T IEnumerator<T>.Current { get { return this.Element; } } void IDisposable.Dispose() { } object IEnumerator.Current { get { return this.Element; } } bool IEnumerator.MoveNext() { return true; } void IEnumerator.Reset() { } } /// <summary> /// Interleaves the specified <paramref name="element"/> between each pair of elements in the <paramref name="target"/> sequence. /// </summary> /// <typeparam name="T"> /// The type of elements in the sequence. /// </typeparam> /// <param name="target"> /// The target sequence to be interleaved. /// </param> /// <param name="element"> /// The element used to perform the interleave operation. /// </param> /// <exception cref="ArgumentNullException"> /// <paramref name="target"/> or <paramref name="element"/> is a null reference. /// </exception> /// <returns> /// The <paramref name="target"/> sequence interleaved with the specified <paramref name="element"/>. /// </returns> public static IEnumerable<T> Interleave<T>( this IEnumerable<T> target, T element) { if (target == null) throw new ArgumentNullException("target"); if (element == null) throw new ArgumentNullException("element"); return InterleaveInternal(target, new InfiniteSequence<T>(element), (f, s) => true); } /// <summary> /// Interleaves the specified <paramref name="element"/> between each pair of elements in the <paramref name="target"/> sequence. /// </summary> /// <remarks> /// The interleave operation is interrupted as soon as the <paramref name="target"/> sequence is exhausted; If the number of <paramref name="elements"/> to be interleaved are not enough to completely interleave the <paramref name="target"/> sequence then the remainder of the sequence is returned without being interleaved. /// </remarks> /// <typeparam name="T"> /// The type of elements in the sequence. /// </typeparam> /// <param name="target"> /// The target sequence to be interleaved. /// </param> /// <param name="elements"> /// The elements used to perform the interleave operation. /// </param> /// <exception cref="ArgumentNullException"> /// <paramref name="target"/> or <paramref name="element"/> is a null reference. /// </exception> /// <returns> /// The <paramref name="target"/> sequence interleaved with the specified <paramref name="elements"/>. /// </returns> public static IEnumerable<T> Interleave<T>( this IEnumerable<T> target, IEnumerable<T> elements) { if (target == null) throw new ArgumentNullException("target"); if (elements == null) throw new ArgumentNullException("elements"); return InterleaveInternal(target, elements, (f, s) => true); } /// <summary> /// Interleaves the specified <paramref name="element"/> between each pair of elements in the <paramref name="target"/> sequence that satisfy <paramref name="predicate"/>. /// </summary> /// <typeparam name="T"> /// The type of elements in the sequence. /// </typeparam> /// <param name="target"> /// The target sequence to be interleaved. /// </param> /// <param name="element"> /// The element used to perform the interleave operation. /// </param> /// <param name="predicate"> /// A predicate used to assert if interleaving should occur between two target elements. /// </param> /// <exception cref="ArgumentNullException"> /// <paramref name="target"/> or <paramref name="element"/> or <paramref name="predicate"/> is a null reference. /// </exception> /// <returns> /// The <paramref name="target"/> sequence interleaved with the specified <paramref name="element"/>. /// </returns> public static IEnumerable<T> Interleave<T>( this IEnumerable<T> target, T element, Func<T, T, bool> predicate) { if (target == null) throw new ArgumentNullException("target"); if (element == null) throw new ArgumentNullException("element"); if (predicate == null) throw new ArgumentNullException("predicate"); return InterleaveInternal(target, new InfiniteSequence<T>(element), predicate); } /// <summary> /// Interleaves the specified <paramref name="element"/> between each pair of elements in the <paramref name="target"/> sequence that satisfy <paramref name="predicate"/>. /// </summary> /// <remarks> /// The interleave operation is interrupted as soon as the <paramref name="target"/> sequence is exhausted; If the number of <paramref name="elements"/> to be interleaved are not enough to completely interleave the <paramref name="target"/> sequence then the remainder of the sequence is returned without being interleaved. /// </remarks> /// <typeparam name="T"> /// The type of elements in the sequence. /// </typeparam> /// <param name="target"> /// The target sequence to be interleaved. /// </param> /// <param name="elements"> /// The elements used to perform the interleave operation. /// </param> /// <param name="predicate"> /// A predicate used to assert if interleaving should occur between two target elements. /// </param> /// <exception cref="ArgumentNullException"> /// <paramref name="target"/> or <paramref name="element"/> or <paramref name="predicate"/> is a null reference. /// </exception> /// <returns> /// The <paramref name="target"/> sequence interleaved with the specified <paramref name="elements"/>. /// </returns> public static IEnumerable<T> Interleave<T>( this IEnumerable<T> target, IEnumerable<T> elements, Func<T, T, bool> predicate) { if (target == null) throw new ArgumentNullException("target"); if (elements == null) throw new ArgumentNullException("elements"); if (predicate == null) throw new ArgumentNullException("predicate"); return InterleaveInternal(target, elements, predicate); } private static IEnumerable<T> InterleaveInternal<T>( this IEnumerable<T> target, IEnumerable<T> elements, Func<T, T, bool> predicate) { var targetEnumerator = target.GetEnumerator(); if (targetEnumerator.MoveNext()) { var elementsEnumerator = elements.GetEnumerator(); while (true) { T first = targetEnumerator.Current; yield return first; if (!targetEnumerator.MoveNext()) yield break; T second = targetEnumerator.Current; bool interleave = true && predicate(first, second) && elementsEnumerator.MoveNext(); if (interleave) yield return elementsEnumerator.Current; } } } }

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