FactoryFunction.cs

// See https://aka.ms/new-console-template for more information

using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;

public static class Program
{
    public static void Main(string[] args)
    {
        var build = CreateHostBuilder(args).Build();

        var thing = build.Services.GetRequiredService<Thing>();
        var thing2 = build.Services.GetRequiredService<Thing>();

        Console.WriteLine(thing.Equals(thing2));
        build.Run();
    }

    public static IHostBuilder CreateHostBuilder(string[] args) =>
        Host.CreateDefaultBuilder(args)
            .ConfigureServices(builder =>
            {
                builder.AddSingleton(sp =>
                {
                    Console.WriteLine("In in the factory");
                    return new Thing();
                });
                
            });
}

public class Thing
{
    public Thing()
    {
        Console.WriteLine("I'm being created");
    }
}



// Output
// In in the factory
// I'm being created
// True

testingRaceCondition.cs

// See https://aka.ms/new-console-template for more information

using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;

public static class Program
{
    public static async Task Main(string[] args)
    {
        var build = CreateHostBuilder(args).Build();

        var thing = Task.Run(() => build.Services.GetRequiredService<Thing>());
        var thing2 = Task.Run(() => build.Services.GetRequiredService<Thing>());

        var whenAll = await Task.WhenAll(thing, thing2);
        Console.WriteLine(whenAll[0].Equals(whenAll[1]));
        build.Run();
    }

    public static IHostBuilder CreateHostBuilder(string[] args) =>
        Host.CreateDefaultBuilder(args)
            .ConfigureServices(builder =>
            {
                builder.AddSingleton(sp =>
                {
                    Console.WriteLine("In in the factory");
                    Thread.Sleep(3000);
                    return new Thing();
                });
                
            });
}

public class Thing
{
    public Thing()
    {
        Console.WriteLine("I'm being created");
    }
}