Pair-wise iteration in C# or sliding window enumerator

Question

In .NET 4 this becomes even easier:-

var input = new[] { "a", "b", "c", "d", "e", "f" };
var result = input.Zip(input.Skip(1), (a, b) => Tuple.Create(a, b));

Rather than require a tuple (pair) type, why not just accept a selector:

public static IEnumerable<TResult> Pairwise<TSource, TResult>(this IEnumerable<TSource> source, Func<TSource, TSource, TResult> resultSelector)
{
    TSource previous = default(TSource);

    using (var it = source.GetEnumerator())
    {
        if (it.MoveNext())
            previous = it.Current;

        while (it.MoveNext())
            yield return resultSelector(previous, previous = it.Current);
    }
}

Which allows you to skip the intermediate object if you want:

string[] items = new string[] { "a", "b", "c", "d" };
var pairs = items.Pairwise((x, y) => string.Format("{0},{1}", x, y));

foreach(var pair in pairs)
    Console.WriteLine(pair);

Or you can use an anonymous type:

var pairs = items.Pairwise((x, y) => new { First = x, Second = y });

Update: I just implemented this on a real project and used C# 7.0 ValueTuple instead:

public static IEnumerable<(T, T)> Pairwise<T>(this IEnumerable<T> source)
{
    var previous = default(T);
    using (var it = source.GetEnumerator())
    {
        if (it.MoveNext())
            previous = it.Current;

        while (it.MoveNext())
            yield return (previous, previous = it.Current);
    }
}

The easiest way is to use ReactiveExtensions

using System.Reactive;
using System.Reactive.Linq;

and make yourself an extension method to kit bash this together

public static IEnumerable<IList<T>> Buffer<T>(this IEnumerable<T> seq, int bufferSize, int stepSize)
{
    return seq.ToObservable().Buffer(bufferSize, stepSize).ToEnumerable();
}

Just for convenience, here is a selector-less version of @dahlbyk's answer.

public static IEnumerable<Tuple<T, T>> Pairwise<T>(this IEnumerable<T> enumerable)
{
    var previous = default(T);

    using (var e = enumerable.GetEnumerator())
    {
        if (e.MoveNext())
            previous = e.Current;

        while (e.MoveNext())
            yield return Tuple.Create(previous, previous = e.Current);
    }
}