var array = new int[] { 1, 2, 3, 4, 7, 8, 11, 15, 16, 17, 18 };
var result = string.Join(",", array
.Distinct()
.OrderBy(x => x)
.GroupAdjacentBy((x, y) => x + 1 == y)
.Select(g => new int[] { g.First(), g.Last() }.Distinct())
.Select(g => string.Join("-", g)));
with
public static class LinqExtensions
{
public static IEnumerable<IEnumerable<T>> GroupAdjacentBy<T>(
this IEnumerable<T> source, Func<T, T, bool> predicate)
{
using (var e = source.GetEnumerator())
{
if (e.MoveNext())
{
var list = new List<T> { e.Current };
var pred = e.Current;
while (e.MoveNext())
{
if (predicate(pred, e.Current))
{
list.Add(e.Current);
}
else
{
yield return list;
list = new List<T> { e.Current };
}
pred = e.Current;
}
yield return list;
}
}
}
}