using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        List<string> codigos = new List<string>(){"2020_123B","2020_123C","2020_123F","2020_123xxx"};

        var filtrar= codigos.Where(x=> x.Contains("2020_123"));

        foreach (var resultado in filtrar)

        {

            Console.WriteLine(resultado);

        }

    }

}