using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        List<MerchantOrderLine> Lines = null;

        List<MerchantOrderLine> LinesNotNull = new List<MerchantOrderLine>()

        {

            new MerchantOrderLine()

            {

                Tags = new List<OrderTag>()

                {

                    new OrderTag()

                    {

                        Tag = "BANANA"

                    }

                }

            }

        };      

        var IsFta1 =

            Lines?.Any(

                x => x.Tags.Any(y =>

                    y.Tag.Equals("FTA", StringComparison.InvariantCultureIgnoreCase) ||

                    y.Tag.Equals("FTAOnlyForInvoice", StringComparison.InvariantCultureIgnoreCase))) ?? false;

        var IsFta2 =

            LinesNotNull?.Any(

                x => x.Tags.Any(y =>

                    y.Tag.Equals("FTA", StringComparison.InvariantCultureIgnoreCase) ||

                    y.Tag.Equals("FTAOnlyForInvoice", StringComparison.InvariantCultureIgnoreCase))) ?? false;

        Console.WriteLine("Lines NULL, IsFTA: " + IsFta1);

        Console.WriteLine("Lines NOT NULL, IsFTA: " + IsFta2);

    }

}

​

public class MerchantOrder

{

    public IEnumerable<MerchantOrderLine> Lines { get; set; }   

}

public class MerchantOrderLine

{

    public IEnumerable<OrderTag> Tags { get; set; } 

}

public class OrderTag

{

    public string Tag { get; set; }

}