Split a List<int> into groups of consecutive numbers

Here is an extension method taken from http://bugsquash.blogspot.com/2010/01/grouping-consecutive-integers-in-c.html

public static IEnumerable<IEnumerable<int>> GroupConsecutive(this IEnumerable<int> list) {
    var group = new List<int>();
    foreach (var i in list) {
        if (group.Count == 0 || i - group[group.Count - 1] <= 1)
            group.Add(i);
        else {
            yield return group;
            group = new List<int> {i};
        }
    }
    yield return group;
}

You can use it like this:

var numbers = new[] { 1, 2, 3, 4, 6, 7, 9 };
var groups = numbers.GroupConsecutive();

Once C# 7 is released, this can made even more efficient with the use of Span to avoid creating new lists.

---

This updated version does it without allocating any lists.

public static class EnumerableExtensions
{
    public static IEnumerable<IEnumerable<int>> GroupConsecutive(this IEnumerable<int> list)
    {
        if (list.Any())
        {
            var count = 1;
            var startNumber = list.First();
            int last = startNumber;

            foreach (var i in list.Skip(1))
            {
                if (i < last)
                {
                    throw new ArgumentException($"List is not sorted.", nameof(list));
                }
                if (i - last == 1)
                    count += 1;
                else
                {
                    yield return Enumerable.Range(startNumber, count);
                    startNumber = i;
                    count = 1;
                }
                last = i;
            }
            yield return Enumerable.Range(startNumber, count);
        }
    }
}

Here is my suggestion for an extension method using iterators:

public static IEnumerable<IEnumerable<int>> GroupConsecutive(this IEnumerable<int> src) {
    var more = false; // compiler can't figure out more is assigned before use
    IEnumerable<int> ConsecutiveSequence(IEnumerator<int> csi) {
        int prevCurrent;
        do
            yield return (prevCurrent = csi.Current);
        while ((more = csi.MoveNext()) && csi.Current-prevCurrent == 1);
    }

    var si = src.GetEnumerator();
    if (si.MoveNext()) {
        do
            // have to process to compute outside level  
            yield return ConsecutiveSequence(si).ToList();
        while (more);
    }
}

I must say the Python algorithm is very impressive, here is a C# implementation of it:

public static IEnumerable<IEnumerable<int>> GroupConsecutive(this IEnumerable<int> iterable, Func<int,int> ordering = null) {
    ordering = ordering ?? (n => n);
    foreach (var tg in iterable
                         .Select((e, i) => (e, i))
                         .GroupBy(t => t.i - ordering(t.e)))
        yield return tg.Select(t => t.e);
}

Here is a C# one-line implementation of the Python algorithm:

public static IEnumerable<IEnumerable<int>> GroupConsecutive(this IEnumerable<int> iterable, Func<int,int> ordering = null) => 
    iterable
      .Select((e, i) => (e, i))
      .GroupBy(
        t => t.i - (ordering ?? (n => n))(t.e),
        (k,tg) => tg.Select(t => t.e));

NOTE: C# 8 with nullable annotation context enabled should use Func<int,int>? in both Python methods. You could also use ??= to assign ordering.

The correct implementation of @Bradley Uffner and @NetMage non allocating iterator method is like this:

public static IEnumerable<IEnumerable<int>> GroupConsecutive(this IEnumerable<int> source)
{
    using (var e = source.GetEnumerator())
    {
        for (bool more = e.MoveNext(); more; )
        {
            int first = e.Current, last = first, next;
            while ((more = e.MoveNext()) && (next = e.Current) > last && next - last == 1)
                last = next;
            yield return Enumerable.Range(first, last - first + 1);
        }
    }
}

It works correctly even for unordered input, iterates the source sequence only once and handles correctly all corner cases and integer over/underflow. The only case it fails is for consecutive range count bigger than int.MaxValue.

But looking at your follow up question, probably the following implementation will better suit your needs:

public static IEnumerable<(int First, int Last)> ConsecutiveRanges(this IEnumerable<int> source)
{
    using (var e = source.GetEnumerator())
    {
        for (bool more = e.MoveNext(); more;)
        {
            int first = e.Current, last = first, next;
            while ((more = e.MoveNext()) && (next = e.Current) > last && next - last == 1)
                last = next;
            yield return (first, last);
        }
    }
}