How to update a property of an object in LINQ without returning new objects.

If you want to update an property of an object in LINQ without creating new objects, you can use the following code:

using System;
using System.Collections.Generic;
using System.Linq;
using Microsoft.VisualStudio.TestTools.UnitTesting;
using Assert = Xunit.Assert;

namespace Research.Rli.Tests
{
    public class Appointment
    {
        public int Id { get; set; }
        public string Location { get; set; }
    }

    [TestClass]
    public class ResearchTester
    {
        [TestMethod]
        public void Should_update_appointments()
        {
            var appointments1 = new List<Appointment>
            {
                new Appointment { Id = 1, Location = "" },
                new Appointment { Id = 2, Location = "" },
                new Appointment { Id = 3, Location = "" }
            };

            var appointments2 = new List<Appointment>
            {
                new Appointment { Id = 1, Location = "My location 1" },
                new Appointment { Id = 2, Location = "" },
                new Appointment { Id = 3, Location = "My location 3" }
            };

            Func<Appointment, Appointment, Appointment> UpdateLocation 
                = ((a, b) => { a.Location = b.Location; return a; });

            var updatedAppointmets = (from a1 in appointments1
                                     join a2 in appointments2 on a1.Id equals a2.Id
                                      select UpdateLocation(a1, a2)).ToList();

            foreach (Appointment a in updatedAppointmets)
            {
                Console.WriteLine
                (
                    string.Format("Id [{0}] Location [{1}]", a.Id, a.Location)
                );
            }

            // Output:
            // Id [1] Location [My location 1]
            // Id [2] Location []
            // Id [3] Location [My location 3]
        }
    }
}

If you want to use a LEFT OUTER JOIN in the linq query, use:

 

using System;
using System.Collections.Generic;
using System.Linq;
using Microsoft.VisualStudio.TestTools.UnitTesting;
using Assert = Xunit.Assert;

namespace Research.Rli.Tests
{
    public class Appointment
    {
        public int Id { get; set; }
        public string Location { get; set; }
    }

    [TestClass]
    public class ResearchTester
    {
        [TestMethod]
        public void Should_update_appointments()
        {
            var appointments1 = new List<Appointment>
            {
                new Appointment { Id = 1, Location = "" },
                new Appointment { Id = 2, Location = "" },
                new Appointment { Id = 3, Location = "" }
            };

            var appointments2 = new List<Appointment>
            {
                new Appointment { Id = 1, Location = "My location 1" },
                new Appointment { Id = 3, Location = "My location 3" }
            };

            Func<Appointment, Appointment, Appointment> UpdateLocation
                = ((a, b) => { if (b != null) { a.Location = b.Location; } return a; });

            var updatedAppointmets =
            (
                from a1 in appointments1
                join a2 in appointments2 on a1.Id equals a2.Id into g
                from g1 in g.DefaultIfEmpty(null)
                select UpdateLocation(a1, g1)
            ).ToList();

            foreach (Appointment a in updatedAppointmets)
            {
                Console.WriteLine
                (
                    string.Format("Id [{0}] Location [{1}]", a.Id, a.Location)
                );
            }

            // Output:
            // Id [1] Location [My location 1]
            // Id [2] Location []
            // Id [3] Location [My location 3]
        }
    }
}