using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var lstStuff = new List<string>{"Code1","Code2","Code3"};

        List<Codes> lstCode = new List<Codes>() {

                            new Codes() {Code = "Code1", HaveVal=null},

                            new Codes() {Code = "Code31", HaveVal="yes"},

                            new Codes() {Code = "Code41", HaveVal="yes"}

                          };

​

        bool isValIncNulls = lstCode.Any(c => lstStuff.Any(s => s == c.Code));

        bool isValNoNulls = lstCode.Any(c => !string.IsNullOrEmpty(c.HaveVal) 

                                 && lstStuff.Any(s => s == c.Code)); 

        Console.WriteLine($"isValIncNulls: {isValIncNulls}");

        Console.WriteLine($"isValNoNulls: {isValNoNulls}");

    }

}

public class Codes 

{

    public string Code {get;set;}

    public string HaveVal {get;set;}

}