using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var masterlist = new List<Customer>()

        {

            new Customer() { ID=1, Cash=25 },

                new Customer() { ID=2, Card_ID="card2" },

                new Customer() { ID=3, Card_ID="card33" }

        };

        var existinglist = new List<Customer>()

        {

            new Customer() { ID=1, Card_ID="card1", Cash=6 },

                new Customer() { ID=2, Card_ID="card12222222222", Cash=27 },

                new Customer() { ID=4, Cash=99 }

        };

        var mergedlist = from a in masterlist

            join b in existinglist 

            on a.ID equals b.ID

            select Merge(a, b);

        foreach (Customer c in mergedlist)

        {

            c.Dump();

        }

    }

    public static T Merge<T>(T master, T other) where T : new()

    {

        var properties = typeof(T)

            .GetProperties()

            .Where(p => p.CanRead && p.CanWrite && !p.GetIndexParameters().Any());

        T result = new T();

        foreach (var prop in properties)

        {

            object value = prop.GetValue(master) ?? prop.GetValue(other);

            prop.SetValue(result, value);

        }

        return result;

    }

}

​

public class Customer

{

    public int ID { get; set; }

    public string Card_ID { get; set; }

    public decimal? Cash { get; set; }

}

​