More efficient way to get all indexes of a character in a string

You can use String.IndexOf, see example below:

    string s = "abcabcabcabcabc";
    var foundIndexes = new List<int>();

    long t1 = DateTime.Now.Ticks;
    for (int i = s.IndexOf('a'); i > -1; i = s.IndexOf('a', i + 1))
        {
         // for loop end when i=-1 ('a' not found)
                foundIndexes.Add(i);
        }
    long t2 = DateTime.Now.Ticks - t1; // read this value to see the run time

I use the following extension method to yield all results:

public static IEnumerable<int> AllIndexesOf(this string str, string searchstring)
{
    int minIndex = str.IndexOf(searchstring);
    while (minIndex != -1)
    {
        yield return minIndex;
        minIndex = str.IndexOf(searchstring, minIndex + searchstring.Length);
    }
}

usage:

IEnumerable<int> result = "foobar".AllIndexesOf("o"); // [1,2]

Side note to a edge case: This is a string approach which works for one or more characters. In case of "fooo".AllIndexesOf("oo") the result is just 1 https://dotnetfiddle.net/CPC7D2