using System;

using System.Linq;

using System.Collections.Generic;

​

public static class Program

{

    public static void Main()

    {

        string source = "'<lookup><entity>a</entity><filter>1,2,3</filter></lookup>'Please select the type of U.S person you ar<lookup><entity>b</entity><filter>4,5,6</filter></lookup> nds with your Federal income t<lookup><entity>c</entity><filter>7,8</filter></lookup> ,{'";

        var ServicesExecutionDic = ExtractLookUpAsDictionary(source);

        foreach(var a in ServicesExecutionDic){

        Console.WriteLine(a);

        }

    }

      public static Dictionary<Tuple<string, string>, List<int>> ExtractLookUpAsDictionary(string source)

        {

            var result = new Dictionary<Tuple<string, string>, List<int>>();

            int[] indexes = source.AllIndexesOf("<lookup>").ToArray();

            foreach (int index in indexes)

            {

                var tag = source.Substring(index).GetBetween("<lookup>", "</lookup>");

                var entity = tag.GetBetween("<entity>", "</entity>");

                var filter = tag.GetBetween("<filter>", "</filter>");

​

                var TupleKey = Tuple.Create(entity, filter);

                if (!result.TryAdd(TupleKey, new List<int> { index }))

                {

                    result[TupleKey].Add(index);

                }

            }

            return result;

        }

​

        public static IEnumerable<int> AllIndexesOf(this string source, string searchstring)

        {

            int minIndex = source.IndexOf(searchstring);

            while (minIndex != -1)

            {

                yield return minIndex;

                minIndex = source.IndexOf(searchstring, minIndex + searchstring.Length);

            }

        }

​

        public static string GetBetween(this string source, string before, string after)

        {

            int beforeStartIndex = source.IndexOf(before);

            int startIndex = beforeStartIndex + before.Length;

            int afterStartIndex = source.IndexOf(after, startIndex);

​

            if (beforeStartIndex == -1 || afterStartIndex == -1)

            {

                return "";

            }

​

            return source.Substring(startIndex, afterStartIndex - startIndex);

        }

}