Convert list to number range string

Question

This question is pretty much the opposite of this question: Does C# have built-in support for parsing page-number strings?

So given

1,3,5,6,7,8,9,10         


        

Answer 1

Here a slightly modifed Version of RedFilter's version.

It returns a String instead of an Array of Strings, it Removes 0 ,if in the list, it avoids the Exception if only one Value is in the List.

 public static string ToRanges(this List ints)
    {
        ints.Remove(0); // Note: Remove this if you like to include the Value 0
        if (ints.Count < 1) return "";
        ints.Sort();
        var lng = ints.Count;
        if (lng == 1)
            return ints[0].ToString();

        var fromnums = new List();
        var tonums = new List();
        for (var i = 0 ; i < lng - 1 ; i++)
        {
            if (i == 0)
                fromnums.Add(ints[0]);
            if (ints[i + 1] > ints[i] + 1)
            {
                tonums.Add(ints[i]);
                fromnums.Add(ints[i + 1]);
            }
        }
        tonums.Add(ints[lng - 1]);


        string[] ranges = Enumerable.Range(0, tonums.Count).Select(
            i => fromnums[i].ToString() +
                (tonums[i] == fromnums[i] ? "" : "-" + tonums[i].ToString())
        ).ToArray();

        if (ranges.Length == 1)
            return ranges[0];
        else
            return String.Join(",", ranges);
    }