Here is my C# version:

class Segment {
    public char Key { get; set; }
    public long Count { get; set; }
}

// Complete the SubstrCount function below.
static long SubstrCount(int n, string s) {        
    var count = 0L;
    List<Segment> segments = ToSegments(s);

    for (var i = 0; i < segments.Count; i++)
        count += GetCount(segments, i);

    return count;
}

private static List<Segment> ToSegments(string s) {
    List<Segment> segments = new List<Segment>();
    var str = s + " ";
    var count = 1L;
    var ch = str[0];
    for (int i = 1; i <= s.Length ; i++) {
        if(str[i].Equals(ch)) {
            count++;
        } else {
            segments.Add(new Segment { Key = ch, Count = count });
            count = 1;
            ch = str[i];
        }
    }
    return segments;
}

private static long GetCount(List<Segment> segments, int i) {
    return IsMid(segments, i)
        ? Math.Min(segments[i - 1].Count, segments[i + 1].Count) + 1
        : SegmentToCount(segments[i]);
}

private static bool IsMid(List<Segment> segments, int i) {
    return i > 0 
        && i < segments.Count - 1
        && segments[i].Count == 1 
        && segments[i - 1].Key == segments[i + 1].Key;
}

private static long SegmentToCount(Segment s) {
    return (s.Count * (s.Count + 1)) / 2;
}