using System;

using System.Collections.Generic;

using System.Linq;

​

public class Sample {

   public string Letter {get;set;} // "A","B","C","D","#"

   public string Number {get;set;} // not relevant

   public override string ToString() => $"Letter: {Letter} | Number: {Number}";

}

​

public class Program

{

​

    public static void Main()

    {

        var listA = new List<string> { "B", "C", "A" };

        var listB = new List<Sample>

        {

            new Sample { Letter = "E", Number = "1" },

            new Sample { Letter = "C", Number = "2" },

            new Sample { Letter = "A", Number = "3" },

            new Sample { Letter = "B", Number = "4" },

            new Sample { Letter = "D", Number = "5" }

        };

        listB = listA.Union(listB.Select(entry => entry.Letter))

            .Join(listB,

                 letter => letter,

                 sample => sample.Letter,

                 ( _, sample ) => sample)

            .ToList();

        Console.WriteLine(string.Join(Environment.NewLine, listB));

    }

}