using System;

using System.Linq;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

        var AList = new List<string>{"1", "2", "3", "4", "5"};

        var BList = new List<string>{"4", "5"};     

        ModifyList(BList, ref AList);

        foreach(var ae in AList)

        {

            Console.WriteLine(ae);

        }

    }

    public static void ModifyList(List<string> BList, ref List<string> AList)

    {

        foreach(var be in BList)

        {

            AList.RemoveAll(ae => ae == be);

        }                   

    }

}

​