tiny-named-service-resolver

Demo

using var host = Host.CreateDefaultBuilder(args)
  .UseServiceProviderFactory(new TinyNamedServiceProviderFactory(provider => "foo"))
  .ConfigureServices(services =>
  {
      services.AddTransient<ISomeService, DefaultService>();
      services.AddTransient<ISomeService, FooService>("foo");
      services.AddTransient<ISomeService, BarService>("bar");
  })
  .Build();

Debug.Assert(host.Services.GetRequiredService<ISomeService>() is FooService);

Features

1. Provide the name used for resolving using a custom Func<IServiceProvider, string>:

// just an example of how to select the implementation type 
// based on a request header value
.UseServiceProviderFactory(new TinyNamedServiceProviderFactory(provider =>
{
    return provider.GetRequiredService<IHttpContextAccessor>()
                   .HttpContext.Request.Headers["header_name"][0];
}))

2. Each named service needs to be registered together with the default implementation, otherwise you won't be able to create the IServiceProvider:

// Registration of ISomeService without a name, 
// represent the default implementation
services.AddTransient<ISomeService, DefaultService>();
services.AddTransient<ISomeService, FooService>("foo");
services.AddTransient<ISomeService, BarService>("bar");

3. Arbitrary order of registration:

It doesn't matter in what order you register services. TinyNamedServiceProviderFactory makes sure the last service descriptor for a type contains the necessary logic.

4. Named services of the same service type need to be registered with the same ServiceLifetime. The following will fail when building IServiceProvider:

// Needs to be transient, or the last two scoped.
services.AddScoped<ISomeService, DefaultService>();
services.AddTransient<ISomeService, FooService>("foo");
services.AddTransient<ISomeService, BarService>("bar");

How it works

Just before IHostBuilder initializes the IServiceProvider a slight modification is made to the last NamedServiceDescriptor for every named service type inside IServiceCollection. It introduces (or replaces) the Func<IServiceProvider, TService> implementationFactory with one that knows which implementation type to pick (it keeps the original though, in case it picks itself).