Public Register() + new Configure()

[dahlbyk]

Populate() isn't meant to be called more than once because it adds IServiceProvider and such, but for migration purposes it would be useful to be able to register additional collections of services.

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After more tinkering, I also propose a new Configure() method to support the common IServiceCollection AddWhatever(this IServiceCollection services) pattern.

A Registry migration could look like this:

 public class MyRegistry : Registry
 {
-    public MyRegistry()
+    public MyRegistry() => this.Configure(Register);
+
+    public static IServiceCollection Register(IServiceCollection services)
     {
-        For<IService>().Singleton().Use<Service>();
+        services.AddSingleton<IService, Service>();
         // …
+        return services;
     }
 }

Include<MyRegistry>() will continue to work, but parent registries can migrate incrementally:

 public class MyParentRegistry : Registry
 {
     public MyParentRegistry()
     {
-        IncludeRegistry<MyRegistry>();
+        this.Configure(MyRegistry.Register);
         IncludeRegistry<MyOtherRegistry>();
     }
 }

I also included a Configure(this IServiceCollection) to allow these non-extension methods to play nicely until Registry support is no longer required and the class can be made static, e.g.

services
    .AddStuff()
    .Configure(MyRegistry.Register)
    .AddMoreStuff();