using System;

using System.Linq;

using System.Collections.Generic;

using System.Collections;

public class Program

{

    public static void Main()

    {

        var cardDetails = new List<Card>

        {

            new Card {CardID = "1", TransactionRef = "20150824|Guid1"},

            new Card {CardID = "2", TransactionRef = "20150824|Guid2"},

            new Card {CardID = "3", TransactionRef = "20150824|Guid3"}  

        };

        var transDetails = new List<Transaction>

        {

            new Transaction {TxnID = "23", TxnDetails = "Guid1"},

            new Transaction {TxnID = "24", TxnDetails = "Guid2"}    

        };

        var allTxnDetails = new HashSet<string>(transDetails.Select(t => t.TxnDetails));

        var cards = 

            from card in cardDetails

            let txnDetails = GetTxnDetails(card)

            where ! allTxnDetails.Contains(txnDetails)

            select card;

​

​

        foreach(var c in cards)

            Console.WriteLine(c.CardID);

    }

    static string GetTxnDetails(Card card)

    {

        return card.TransactionRef.Split('|')[1];

    }

    public class Card 

    {

        public string CardID {get; set;}

        public string TransactionRef {get; set;}    

    }

    public class Transaction

    {

        public string TxnID {get; set;}

        public string TxnDetails {get; set;}    

    }

}