Splits a string by delimiters but retails delimiter

split-retain-splitchar.cs

private IEnumerable<string> GetIdentifierParts(string source)
{
    char[] splitBy = new[] { '.', '#' };
    int start = 0;
    int index = 0;

    string previousCharacter = "";

    while ((index = source.IndexOfAny(splitBy, start)) != -1)
    {
        index++;
        index = Interlocked.Exchange(ref start, index);

        yield return previousCharacter + source.Substring(index, start - index - 1);
        previousCharacter = source.Substring(start - 1, 1);
    }

    if (start < source.Length)
    {
        yield return previousCharacter + source.Substring(start);
    }
}