[Route("working")]
public ActionResult Working()
{
    using (var connection = new SqlConnection(_connectionString))
    {
        try
        {
            connection.Open();
        }
        catch (SqlException)
        {
            return new HttpStatusCodeResult(503, "Generic error");
        }
    }
   return new EmptyResult();
}

namespace MvcHealthCheckTest
{
    public class Startup
    {
        public void ConfigureServices(IServiceCollection services)
        {
            services.AddHealthChecks()
                    .AddCheck("sql", () =>
                        {
                            using (var connection = new SqlConnection(Configuration["connectionString"]))
                            {
                                try
                                {
                                    connection.Open();
                                }
                                catch (SqlException)
                                {
                                    return HealthCheckResult.Unhealthy();
                                }
                            }
                            return HealthCheckResult.Healthy();
                        });
        }

        public void Configure(IApplicationBuilder app, IHostingEnvironment env)
        {
            app.UseHealthChecks("/working");

    public class SqlServerHealthCheck : IHealthCheck
    {
        private readonly IConfiguration _configuration;

        public SqlServerHealthCheck(IConfiguration configuration)
        {
            _configuration = configuration;
        }

        public Task<HealthCheckResult> CheckHealthAsync(
            HealthCheckContext context, CancellationToken cancellationToken = default(CancellationToken))
        {
            using (var connection = new SqlConnection(_configuration["connectionString"]))
            {
                try
                {
                    connection.Open();
                }
                catch (SqlException)
                {
                    return Task.FromResult(HealthCheckResult.Unhealthy());
                }
            }
            return Task.FromResult(HealthCheckResult.Healthy());
        }
    }

namespace MvcHealthCheckTest
{
    public class Startup
    {
        public void ConfigureServices(IServiceCollection services)
        {
            services.AddHealthChecks()
                    .AddCheck<SqlServerHealthCheck>("sql");

public Task<HealthCheckResult> CheckHealthAsync(
            HealthCheckContext context,
            CancellationToken cancellationToken = default(CancellationToken))
        {
            using (var connection = new SqlConnection(_configuration["connectionString"]))
            {
                try
                {
                    connection.Open();
                }
                catch (SqlException)
                {
                    return Task.FromResult(new HealthCheckResult(
                                                   status: context.Registration.FailureStatus,
                                                   description: "It is dead!"));
                }
            }
            return Task.FromResult(HealthCheckResult.Healthy("Healthy as a horse"));
        }

namespace MvcHealthCheckTest
{
    public class Startup
    {
        public void Configure(IApplicationBuilder app, IHostingEnvironment env)
        {
            app.UseHealthChecks("/working", new HealthCheckOptions
            {
                ResponseWriter = async (context, report) =>
                {
                    var result = JsonConvert.SerializeObject(new
                    {
                        status = report.Status.ToString(),
                        errors = report.Entries.Select(e =>
                        new
                        {
                            key = e.Key,
                            value = Enum.GetName(typeof(HealthStatus), e.Value.Status)
                        })
                    });
                    context.Response.ContentType = MediaTypeNames.Application.Json;
                    await context.Response.WriteAsync(result);
                }
            });

public class TravelAgent
{
    public TravelAgent()
    {
        this.Bookings = new HashSet<Booking>();
    }

    public int AgentId { get; set; }
    public string Name { get; set; }
    public virtual ICollection<Booking> Bookings { get; set; }
}

public class Booking
{
    public int BookingId { get; set; }
    public int AgentId { get; set; }
    public string Customer { get; set; }
    public DateTime BookingDate { get; set; }
    public bool Paid { get; set; }
    public virtual TravelAgent TravelAgent { get; set; }
}

<connectionStrings>
  <add name="Recipe3ConnectionString"
    connectionString="Data Source=.;
    Initial Catalog=EFRecipes;
    Integrated Security=True;
    MultipleActiveResultSets=True"
    providerName="System.Data.SqlClient" />
</connectionStrings>

protected void Application_Start()
{
    // Disable Entity Framework Model Compatibilty
    Database.SetInitializer<Recipe1Context>(null);

    // The bidirectional navigation properties between related entities
    // create a self-referencing loop that breaks Web API's effort to
    // serialize the objects as JSON. By default, Json.NET is configured
    // to error when a reference loop is detected. To resolve problem,
    // simply configure JSON serializer to ignore self-referencing loops.
    GlobalConfiguration.Configuration.Formatters.JsonFormatter
        .SerializerSettings.ReferenceLoopHandling =
        Newtonsoft.Json.ReferenceLoopHandling.Ignore;
    ...
}

public static void Register(HttpConfiguration config)
{
    config.Routes.MapHttpRoute(
      name: "ActionMethodSave",
      routeTemplate: "api/{controller}/{action}/{id}",
      defaults: new { id = RouteParameter.Optional });
}

internal class Program
{
    private HttpClient _client;
    private TravelAgent _agent1, _agent2;
    private Booking _booking1, _booking2, _booking3;
    private HttpResponseMessage _response;

    private static void Main()
    {
        Task t = Run();
        t.Wait();
        Console.WriteLine("\nPress <enter> to continue...");
        Console.ReadLine();
    }

    private static async Task Run()
    {
        var program = new Program();
        program.ServiceSetup();
        // do not proceed until clean-up is completed
        await program.CleanupAsync();
        program.CreateFirstAgent();
        // do not proceed until agent is created
        await program.AddAgentAsync();
        program.CreateSecondAgent();
        // do not proceed until agent is created
        await program.AddSecondAgentAsync();
        program.ModifyAgent();
        // do not proceed until agent is updated
        await program.UpdateAgentAsync();
        // do not proceed until agents are fetched
        await program.FetchAgentsAsync();
    }

    private void ServiceSetup()
    {
        // set up infrastructure for Web API call
        _client = new HttpClient {BaseAddress = new Uri("http://localhost:6687/")};
        // add Accept Header to request Web API content negotiation to return resource in JSON format
        _client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
    }

    private async Task CleanupAsync()
    {
        // call cleanup method in service
        _response = await _client.DeleteAsync("api/travelagent/cleanup/");
    }

    private void CreateFirstAgent()
    {
        // create new Travel Agent and booking
        _agent1 = new TravelAgent {Name = "John Tate"};
        _booking1 = new Booking
        {
            Customer = "Karen Stevens",
            Paid = false,
            BookingDate = DateTime.Parse("2/2/2010")
        };

        _booking2 = new Booking
        {
            Customer = "Dolly Parton",
            Paid = true,
            BookingDate = DateTime.Parse("3/10/2010")
        };
  
        _agent1.Bookings.Add(_booking1);
        _agent1.Bookings.Add(_booking2);
    }

    private async Task AddAgentAsync()
    {
        // call generic update method in Web API service to add agent and bookings
        _response = await _client.PostAsync("api/travelagent/update/",
            _agent1, new JsonMediaTypeFormatter());

        if (_response.IsSuccessStatusCode)
        {
            // capture newly created travel agent from service, which will include
            // database-generated Ids for each entity
            _agent1 = await _response.Content.ReadAsAsync<TravelAgent>();
            _booking1 = _agent1.Bookings.FirstOrDefault(x => x.Customer == "Karen Stevens");
            _booking2 = _agent1.Bookings.FirstOrDefault(x => x.Customer == "Dolly Parton");

            Console.WriteLine("Successfully created Travel Agent {0} and {1} Booking(s)",
            _agent1.Name, _agent1.Bookings.Count);
        }
        else
            Console.WriteLine("{0} ({1})", (int) _response.StatusCode, _response.ReasonPhrase);
    }

    private void CreateSecondAgent()
    {
        // add new agent and booking
        _agent2 = new TravelAgent {Name = "Perry Como"};
        _booking3 = new Booking {
            Customer = "Loretta Lynn",
            Paid = true,
            BookingDate = DateTime.Parse("3/15/2010")};
        _agent2.Bookings.Add(_booking3);
    }

    private async Task AddSecondAgentAsync()
    {
        // call generic update method in Web API service to add agent and booking
        _response = await _client.PostAsync("api/travelagent/update/", _agent2, new JsonMediaTypeFormatter());

        if (_response.IsSuccessStatusCode)
        {
            // capture newly created travel agent from service
            _agent2 = await _response.Content.ReadAsAsync<TravelAgent>();
            _booking3 = _agent2.Bookings.FirstOrDefault(x => x.Customer == "Loretta Lynn");
            Console.WriteLine("Successfully created Travel Agent {0} and {1} Booking(s)",
                _agent2.Name, _agent2.Bookings.Count);
        }
        else
            Console.WriteLine("{0} ({1})", (int) _response.StatusCode, _response.ReasonPhrase);
    }

    private void ModifyAgent()
    {
        // modify agent 2 by changing agent name and assigning booking 1 to him from agent 1
        _agent2.Name = "Perry Como, Jr.";
        _agent2.Bookings.Add(_booking1);
    }

    private async Task UpdateAgentAsync()
    {
        // call generic update method in Web API service to update agent 2
        _response = await _client.PostAsync("api/travelagent/update/", _agent2, new JsonMediaTypeFormatter());
        if (_response.IsSuccessStatusCode)
        {
            // capture newly created travel agent from service, which will include Ids
            _agent1 = _response.Content.ReadAsAsync<TravelAgent>().Result;
            Console.WriteLine("Successfully updated Travel Agent {0} and {1} Booking(s)", _agent1.Name, _agent1.Bookings.Count);
        }
        else
            Console.WriteLine("{0} ({1})", (int) _response.StatusCode, _response.ReasonPhrase);
    }

    private async Task FetchAgentsAsync()
    {
        // call Get method on service to fetch all Travel Agents and Bookings
        _response = _client.GetAsync("api/travelagent/retrieve").Result;
        if (_response.IsSuccessStatusCode)
        {
            // capture newly created travel agent from service, which will include Ids
            var agents = await _response.Content.ReadAsAsync<IEnumerable<TravelAgent>>();

            foreach (var agent in agents)
            {
                Console.WriteLine("Travel Agent {0} has {1} Booking(s)", agent.Name, agent.Bookings.Count());
            }
        }
        else
            Console.WriteLine("{0} ({1})", (int) _response.StatusCode, _response.ReasonPhrase);
    }
}

var list = ctx.ProjectStatus.Select(...);

foreach (var item in list)
{...}

foreach (var item in list)
{...}

static void PassByValueSample()
{
    int a = 1;
    PassByValue(a);
    Console.WriteLine($"after the invocation of {nameof(PassByValue)}, {nameof(a)} = {a}");
}
 
static void PassByValue(int x)
{
    x = 2;
}

static void PassByReferenceSample()
{
    int a = 1;
    PassByReference(ref a);
    Console.WriteLine($"after the invocation of {nameof(PassByReference)}, {nameof(a)} = {a}");
}
 
static void PassByReference(ref int x)
{
    x = 2;
}

static void OutSample()
{
    Out(out int a);
    Console.WriteLine($"after the invocation of {nameof(Out)}, {nameof(a)} = {a}");
}
 
static void Out(out int x)
{
    x = 2;
}

if (int.TryParse("42", out var result))
{
   Console.WriteLine($"the result is {result}");
}

 int x = 3;
ref int x1 = ref x;
x1 = 2;
Console.WriteLine($"local variable {nameof(x)} after the change: {x}");

 ref int i = sequence.Count();

 ref int number1 = null; // ERROR
ref int number2 = 42; // ERROR

 static ref int ReturnByReference()
{
    int[] arr = { 1 };
    ref int x = ref arr[0];
    return ref x;
}

static ref int ReturnByReference2()
{
   int[] arr = { 1 };
   return ref arr[0];
}

static ref int ReturnByReference3(ref int x)
{
    x = 2;
    return ref x;
}

class Thing1
{
    ref string _Text1; /* Error */
 
    ref string Text2 { get; set; } /* Error */
 
 
    string _text = "Text";
    ref string Text3 { get { return ref _text; } } // Properties that return a reference are allowed
}

private MyBigStruct[] array = new MyBigStruct[10];
private int current;
 
public ref MyBigStruct GetCurrentItem()
{
   return ref array[current];
}

using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
using System.Data.Entity;
using System.Data.Entity.Migrations;
using System.Linq;

namespace EFGeneral
{
    public class BlogComment
    {
        public int Id { set; get; }

        [MaxLength]
        public string Body { set; get; }

        public virtual BlogComment Reply { set; get; }
        public int? ReplyId { get; set; }
        public ICollection<BlogComment> Children { get; set; }
    }

    public class MyContext : DbContext
    {
        public DbSet<BlogComment> BlogComments { get; set; }

        protected override void OnModelCreating(DbModelBuilder modelBuilder)
        {
            // Self Referencing Entity
            modelBuilder.Entity<BlogComment>()
                        .HasOptional(x => x.Reply)
                        .WithMany(x => x.Children)
                        .HasForeignKey(x => x.ReplyId)
                        .WillCascadeOnDelete(false);

            base.OnModelCreating(modelBuilder);
        }
    }

    public class Configuration : DbMigrationsConfiguration<MyContext>
    {
        public Configuration()
        {
            AutomaticMigrationsEnabled = true;
            AutomaticMigrationDataLossAllowed = true;
        }

        protected override void Seed(MyContext context)
        {
            var comment1 = new BlogComment { Body = "نظر من این است که" };
            var comment12 = new BlogComment { Body = "پاسخی به نظر اول", Reply = comment1 };
            var comment121 = new BlogComment { Body = "پاسخی به پاسخ به نظر اول", Reply = comment12 };

            context.BlogComments.Add(comment121);
            base.Seed(context);
        }
    }

    public static class Test
    {
        public static void RunTests()
        {
            Database.SetInitializer(new MigrateDatabaseToLatestVersion<MyContext, Configuration>());

            using (var ctx = new MyContext())
            {
                var list = ctx.BlogComments
                          //.where ...
                          .ToList() // fills the childs list too
                          .Where(x => x.Reply == null) // for TreeViewHelper                        
                          .ToList();

                if (list.Any())
                {

                }
            }
        }
    }
}

    public class BlogComment
    {
        // ... 
        public ICollection<BlogComment> Children { get; set; }
    }

private void AppendChildren(TagBuilder parentTag, T parentItem, Func<T, IEnumerable<T>> childrenProperty)
        {
            var children = childrenProperty(parentItem);
            if (children == null || !children.Any())
            {
                return;
            }
//...

facid: 9, Name: 'Spa', membercost: 20, guestcost: 30, initialoutlay: 100000, monthlymaintenance: 800.

insert into facilities
    (facid, name, membercost, guestcost, initialoutlay, monthlymaintenance)
    values (9, 'Spa', 20, 30, 100000, 800);
-- OR
insert into facilities values (9, 'Spa', 20, 30, 100000, 800);

context.Facilities.Add(new Facility
                {
                    Name = "Spa",
                    MemberCost = 20,
                    GuestCost = 30,
                    InitialOutlay = 100000,
                    MonthlyMaintenance = 800
                });
context.SaveChanges();

facid: 9, Name: 'Spa', membercost: 20, guestcost: 30, initialoutlay: 100000, monthlymaintenance: 800.
facid: 10, Name: 'Squash Court 2', membercost: 3.5, guestcost: 17.5, initialoutlay: 5000, monthlymaintenance: 80.

insert into facilities
    (facid, name, membercost, guestcost, initialoutlay, monthlymaintenance)
    values
        (9, 'Spa', 20, 30, 100000, 800),
        (10, 'Squash Court 2', 3.5, 17.5, 5000, 80);

context.Facilities.Add(new Facility
                {
                    Name = "Spa",
                    MemberCost = 20,
                    GuestCost = 30,
                    InitialOutlay = 100000,
                    MonthlyMaintenance = 800
                });

context.Facilities.Add(new Facility
                {
                    Name = "Squash Court 2",
                    MemberCost = 3.5M,
                    GuestCost = 17.5M,
                    InitialOutlay = 5000,
                    MonthlyMaintenance = 80
                });
context.SaveChanges();

Name: 'Spa', membercost: 20, guestcost: 30, initialoutlay: 100000, monthlymaintenance: 800.

namespace EFCorePgExercises.Entities
{
    public class FacilityConfiguration : IEntityTypeConfiguration<Facility>
    {
        public void Configure(EntityTypeBuilder<Facility> builder)
        {
            builder.HasKey(facility => facility.FacId);
            builder.Property(facility => facility.FacId).IsRequired().UseIdentityColumn(seed: 0, increment: 1);

CREATE TABLE [dbo].[Facilities](
[FacId] [int] IDENTITY(0,1) NOT NULL,
--- ...
 CONSTRAINT [PK_Facilities] PRIMARY KEY CLUSTERED 
(
[FacId] ASC
);

update facilities
    set initialoutlay = 10000
    where facid = 1;

var facility1 = context.Facilities.Find(1);
facility1.InitialOutlay = 10000;
context.SaveChanges();

update cd.facilities
    set
        membercost = 6,
        guestcost = 30
    where facid in (0,1);

int[] facIds = { 0, 1 };
var tennisCourts = context.Facilities.Where(x => facIds.Contains(x.FacId)).ToList();
foreach (var tennisCourt in tennisCourts)
{
     tennisCourt.MemberCost = 6;
     tennisCourt.GuestCost = 30;
}

context.SaveChanges();

update cd.facilities facs
    set
        membercost = (select membercost * 1.1 from cd.facilities where facid = 0),
        guestcost = (select guestcost * 1.1 from cd.facilities where facid = 0)
    where facs.facid = 1;

var fac0 = context.Facilities.Where(x => x.FacId == 0).First();
var fac1 = context.Facilities.Where(x => x.FacId == 1).First();
fac1.MemberCost = fac0.MemberCost * 1.1M;
fac1.GuestCost = fac0.GuestCost * 1.1M;

context.SaveChanges();

delete from bookings

context.Bookings.RemoveRange(context.Bookings.ToList());
context.SaveChanges();

delete from members where memid = 37;

var mem37 = context.Members.Where(x => x.MemId == 37).First();
context.Members.Remove(mem37);

context.SaveChanges();

var entry = context.Entry(new Member { MemId = 37 });
entry.State = EntityState.Deleted;
context.SaveChanges();

SET NOCOUNT ON;
DELETE FROM [Members]
WHERE [MemId] = @p0;
SELECT @@ROWCOUNT;

delete from members where memid not in (select memid from cd.bookings);

var mems = context.Members.Where(x =>
  !context.Bookings.Select(x => x.MemId).Contains(x.MemId)).ToList();
context.Members.RemoveRange(mems);

context.SaveChanges();

class EventSource : System.Progress<int>
{
    public async System.Threading.Tasks.Task<int> PerformExpensiveCalculation()
    {
        var sum = 0;
        for (var i = 0; i < 100; i++)
        {
            await System.Threading.Tasks.Task.Delay(100);
            sum += i;
            this.OnReport(sum);
        }
        return sum;
    }
}

static class Program
{
    static void Main(string[] args)
    {
        var source = new EventSource();
        System.EventHandler<int> handler = (_, progress) => System.Console.WriteLine(progress);
        source.ProgressChanged += handler;
        System.Console.WriteLine(source.PerformExpensiveCalculation().Result);
        source.ProgressChanged -= handler;

        source.ProgressChanged += ProgressChangedMethod;
        System.Console.WriteLine(source.PerformExpensiveCalculation().Result);
        source.ProgressChanged -= ProgressChangedMethod;
    }

    private static void ProgressChangedMethod( object sender, int e )
    {
        System.Console.WriteLine(e);
    }
}

IL_0007:  ldsfld     class [mscorlib]System.EventHandler`1<int32> LambdaPerformance.Program/'<>c'::'<>9__0_0'
IL_000c:  dup
IL_000d:  brtrue.s   IL_0026
IL_000f:  pop
IL_0010:  ldsfld     class LambdaPerformance.Program/'<>c' LambdaPerformance.Program/'<>c'::'<>9'
IL_0015:  ldftn      instance void LambdaPerformance.Program/'<>c'::'<Main>b__0_0'(object, int32)
IL_001b:  newobj     instance void class [mscorlib]System.EventHandler`1<int32>::.ctor(object, native int)
IL_0020:  dup
IL_0021:  stsfld     class [mscorlib]System.EventHandler`1<int32> LambdaPerformance.Program/'<>c'::'<>9__0_0'
IL_0026:  stloc.1
IL_0027:  ldloc.0
IL_0028:  ldloc.1
IL_0029:  callvirt   instance void class [mscorlib]System.Progress`1<int32>::add_ProgressChanged(class [mscorlib]System.EventHandler`1<!0>)
IL_002e:  nop
IL_002f:  ldloc.0
IL_0030:  callvirt   instance class [mscorlib]System.Threading.Tasks.Task`1<int32> LambdaPerformance.EventSource::PerformExpensiveCalculation()
IL_0035:  callvirt   instance !0 class [mscorlib]System.Threading.Tasks.Task`1<int32>::get_Result()
IL_003a:  call       void [mscorlib]System.Console::WriteLine(int32)
IL_003f:  nop
IL_0040:  ldloc.0
IL_0041:  ldloc.1
IL_0042:  callvirt   instance void class [mscorlib]System.Progress`1<int32>::remove_ProgressChanged(class [mscorlib]System.EventHandler`1<!0>)

IL_004a:  ldftn      void LambdaPerformance.Program::ProgressChangedMethod(object, int32)
IL_0050:  newobj     instance void class [mscorlib]System.EventHandler`1<int32>::.ctor(object, native int)
IL_0055:  callvirt   instance void class [mscorlib]System.Progress`1<int32>::add_ProgressChanged(class [mscorlib]System.EventHandler`1<!0>)
IL_005a:  nop
IL_005b:  ldloc.0
IL_005c:  callvirt   instance class [mscorlib]System.Threading.Tasks.Task`1<int32> LambdaPerformance.EventSource::PerformExpensiveCalculation()
IL_0061:  callvirt   instance !0 class [mscorlib]System.Threading.Tasks.Task`1<int32>::get_Result()
IL_0066:  call       void [mscorlib]System.Console::WriteLine(int32)
IL_006b:  nop
IL_006c:  ldloc.0
IL_006d:  ldnull
IL_006e:  ldftn      void LambdaPerformance.Program::ProgressChangedMethod(object, int32)
IL_0074:  newobj     instance void class [mscorlib]System.EventHandler`1<int32>::.ctor(object, native int)
IL_0079:  callvirt   instance void class [mscorlib]System.Progress`1<int32>::remove_ProgressChanged(class [mscorlib]System.EventHandler`1<!0>)

static class Program
{
    static void Main(string[] args)
    {
        for (var repeats = 10; repeats <= 1000000; repeats *= 10)
        {
            VersionOne(repeats);
            VersionTwo(repeats);
        }
    }

    private static void VersionOne(int repeats)
    {
        var timer = new System.Diagnostics.Stopwatch();
        timer.Start();

        var source = new EventSource();
        for (var i = 0; i < repeats; i++)
        {
            System.EventHandler<int> handler = (_, progress) => System.Console.WriteLine(progress);
            source.ProgressChanged += handler;
            // Console.WriteLine(source.PerformExpensiveCalculation().Result);
            source.ProgressChanged -= handler;
        }

        timer.Stop();

        System.Console.WriteLine($"Version one: {repeats} add/remove takes {timer.ElapsedMilliseconds}ms");
    }

    private static void VersionTwo(int repeats)
    {
        var timer = new System.Diagnostics.Stopwatch();
        timer.Start();

        var source = new EventSource();
        for (var i = 0; i < repeats; i++)
        {
            source.ProgressChanged += ProgressChangedMethod;
            // Console.WriteLine(source.PerformExpensiveCalculation().Result);
            source.ProgressChanged -= ProgressChangedMethod;
        }

        timer.Stop();

        System.Console.WriteLine($"Version two: {repeats} add/remove takes {timer.ElapsedMilliseconds}ms");
    }

    private static void ProgressChangedMethod(object sender, int e)
    {
        System.Console.WriteLine(e);
    }
}

Version one: 10 add/remove takes 0ms
Version two: 10 add/remove takes 0ms
Version one: 100 add/remove takes 0ms
Version two: 100 add/remove takes 0ms
Version one: 1000 add/remove takes 0ms
Version two: 1000 add/remove takes 0ms
Version one: 10000 add/remove takes 0ms
Version two: 10000 add/remove takes 1ms
Version one: 100000 add/remove takes 8ms
Version two: 100000 add/remove takes 13ms
Version one: 1000000 add/remove takes 93ms
Version two: 1000000 add/remove takes 121ms

private static void VersionOne(int repeats)
{
    var timer = new System.Diagnostics.Stopwatch();
    timer.Start();

    var source = new EventSource();
    for (var i = 0; i < repeats; i++)
    {
        // System.EventHandler<int> handler = (_, progress) => System.Console.WriteLine(progress);
        source.ProgressChanged += (_, progress) => System.Console.WriteLine(progress);
        // Console.WriteLine(source.PerformExpensiveCalculation().Result);
        source.ProgressChanged -= (_, progress) => System.Console.WriteLine(progress);
    }

    timer.Stop();

    System.Console.WriteLine($"Version one: {repeats} add/remove takes {timer.ElapsedMilliseconds}ms");
}

Version one: 10 add/remove takes 0ms
Version two: 10 add/remove takes 0ms
Version one: 100 add/remove takes 1ms
Version two: 100 add/remove takes 0ms
Version one: 1000 add/remove takes 102ms
Version two: 1000 add/remove takes 0ms
Version one: 10000 add/remove takes 10509ms
Version two: 10000 add/remove takes 1ms
Version one: 100000 add/remove takes 1039014ms
Version two: 100000 add/remove takes 11ms

  IL_0018:  nop
  IL_0019:  ldloc.1
  IL_001a:  ldsfld     class [mscorlib]System.EventHandler`1<int32> LambdaPerformance.Program/'<>c'::'<>9__1_0'
  IL_001f:  dup
  IL_0020:  brtrue.s   IL_0039
  IL_0022:  pop
  IL_0023:  ldsfld     class LambdaPerformance.Program/'<>c' LambdaPerformance.Program/'<>c'::'<>9'
  IL_0028:  ldftn      instance void LambdaPerformance.Program/'<>c'::'<VersionOne>b__1_0'(object, int32)
  IL_002e:  newobj     instance void class [mscorlib]System.EventHandler`1<int32>::.ctor(object, native int)
  IL_0033:  dup
  IL_0034:  stsfld     class [mscorlib]System.EventHandler`1<int32> LambdaPerformance.Program/'<>c'::'<>9__1_0'
  IL_0039:  callvirt   instance void class [mscorlib]System.Progress`1<int32>::add_ProgressChanged(class [mscorlib]System.EventHandler`1<!0>)
  IL_003e:  nop
  IL_003f:  ldloc.1
  IL_0040:  ldsfld     class [mscorlib]System.EventHandler`1<int32> LambdaPerformance.Program/'<>c'::'<>9__1_1'
  IL_0045:  dup
  IL_0046:  brtrue.s   IL_005f
  IL_0048:  pop
  IL_0049:  ldsfld     class LambdaPerformance.Program/'<>c' LambdaPerformance.Program/'<>c'::'<>9'
  IL_004e:  ldftn      instance void LambdaPerformance.Program/'<>c'::'<VersionOne>b__1_1'(object, int32)
  IL_0054:  newobj     instance void class [mscorlib]System.EventHandler`1<int32>::.ctor(object, native int)
  IL_0059:  dup
  IL_005a:  stsfld     class [mscorlib]System.EventHandler`1<int32> LambdaPerformance.Program/'<>c'::'<>9__1_1'
  IL_005f:  callvirt   instance void class [mscorlib]System.Progress`1<int32>::remove_ProgressChanged(class [mscorlib]System.EventHandler`1<!0>)
  IL_0064:  nop
  IL_0065:  nop
  IL_0066:  ldloc.2
  IL_0067:  stloc.3
  IL_0068:  ldloc.3
  IL_0069:  ldc.i4.1
  IL_006a:  add
  IL_006b:  stloc.2
  IL_006c:  ldloc.2
  IL_006d:  ldarg.0
  IL_006e:  clt
  IL_0070:  stloc.s    V_4
  IL_0072:  ldloc.s    V_4
  IL_0074:  brtrue.s   IL_0018