int[] indexes = input.Select((item, index) => new { item, index })
                     .Where(x => x.item == "A")
                     .Select(x => x.index)
                     .ToArray();

public static int FindIndex<T>(
    T[] array,
    int startIndex,
    Predicate<T> match)

public static List<Int32> IndicesOf<T>(this T[] array, T value)
{
    var indices = new List<Int32>();

    Int32 startIndex = 0;
    while (true)
    {
        startIndex = Array.IndexOf<T>(array, value, startIndex);
        if (startIndex != -1)
        {
            indices.Add(startIndex);
            startIndex++;
        }
        else
        {
            break;
        }
    }

    return indices;
}

var symbols = new Char[] { 'a', 'b', 'c', 'a' };
    var indices = symbols.IndicesOf('a');
    indices.ForEach(index => Console.WriteLine(index));