using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        List<EipBlock> list = new List<EipBlock>() { new EipBlock(1, "foo_bar"),  new EipBlock(2, "marco_polo"), new EipBlock(3, "foo_bar") };

        var dict = new Dictionary<string, List<EipBlock>>();

        foreach(var entry in list.Select(block => new KeyValuePair<string, EipBlock>(block.SearchString, block))) {

            if(!dict.ContainsKey(entry.Key)) {

                dict[entry.Key] = new List<EipBlock>();

            }

            dict[entry.Key].Add(entry.Value);

        }

        Console.WriteLine(dict["foo_bar"][1].Id);

    }

}

​

public class EipBlock {

    public int Id { get; set; }

    public string SearchString { get; set; }

    public EipBlock(int id, string searchString) { Id = id; SearchString = searchString; }

}