Find each index within string & store the indexes separately

Use Regex to match all the strings and then loop through the matches and find the indexes of each match.

This logic must work. This has been Tested

        List<int> indexes = new List<int>();
        string XMLToAnalyze = "<XML><TAG1>somecontent</TAG1><address source>content</address source><address source>content</address source><TAG2>morecontent</TAG2></XML>";
        var regex = new Regex(@"<address source>");

        foreach (Match match in regex.Matches(XMLToAnalyze))
        {
            indexes.Add(match.Index);
        }

The indexes will have all the indexes of the matched string.

OutPut : 29, 69

What you need is a IDictionary<[string], IList<[int]>>. As you search for an opening tags, you can store them in the dictionary in the following way: If the tag exists, add the new found index to the added list, otherwise add a new element into the dictionary with a new list with a single item in it - the first occurance of the index. After you'll go over all the string - your dictionary will have the map you are looking for.

public static class XmlTagMapBuilder
{
    public static IDictionary<string, IList<int>> GetOpenTagIndexMap(string inputXml)
    {
        // Argument validation goes here

        IDictionary<string, IList<int>> result = new Dictionary<string, IList<int>>();

        int currentIndex = -1;
        string lastOpenTag = null;
        while (true)
        {
            string nextOpenTagName;
            int nextOpenTagIndex;
            if (TryGetNextOpenTagIndex(inputXml, currentIndex, out nextOpenTagName, out nextOpenTagIndex))
            {
                lastOpenTag = nextOpenTagName;
                currentIndex = nextOpenTagIndex;

                IList<int> tagIndicies;
                if (!result.TryGetValue(nextOpenTagName.ToUpperInvariant(), out tagIndicies))
                {
                    tagIndicies = new List<int>();
                    result.Add(nextOpenTagName, tagIndicies);
                }

                tagIndicies.Add(nextOpenTagIndex);
            }
            else
            {
                break;
            }
        }

        return result;
    }

    /// <summary>
    /// Tries to get next open tag in the given <see cref="inputXml"/> string after the specified startIndex.
    /// </summary>
    /// <param name="inputXml">The string which contains the xml tags.</param>
    /// <param name="startIndex">The index after which to look for the open tag.</param>
    /// <param name="nextOpenTagName">If a tag was found, contains its name.</param>
    /// <param name="nextOpenTagIndex">If a tag was found, contains the start index of it.</param>
    /// <returns>true - if the tag was found. false - otherwise.</returns>
    private static bool TryGetNextOpenTagIndex(string inputXml, int startIndex, out string nextOpenTagName, out int nextOpenTagIndex)
    {
        // Need to add implementaiton here
    }
}