How can I pass a runtime parameter as part of the dependency resolution?

Question

I need to be able to pass a connection string into some of my service implementations. I am doing this in the constructor. The connection string is configurable by user will be

Answer 1

Simple configuration

public void ConfigureServices(IServiceCollection services)
{
    // Choose Scope, Singleton or Transient method
    services.AddSingleton();
    services.AddSingleton(serviceProvider=>
    {
         return new NestedService("someConnectionString");
    });
}

With appSettings.json

If you decide to hide your connection string inside appSettings.json, e.g:

"Data": {
  "ConnectionString": "someConnectionString"
}

Then provided that you've loaded your appSettings.json in the ConfigurationBuilder (usually located in the constructor of the Startup class), then your ConfigureServices would look like this:

public void ConfigureServices(IServiceCollection services)
{
    // Choose Scope, Singleton or Transient method
    services.AddSingleton();
    services.AddSingleton(serviceProvider=>
    {
         var connectionString = Configuration["Data:ConnectionString"];
         return new NestedService(connectionString);
    });
}

With extension methods

namespace Microsoft.Extensions.DependencyInjection
{
    public static class RootServiceExtensions //you can pick a better name
    {
        //again pick a better name
        public static IServiceCollection AddRootServices(this IServiceCollection services, string connectionString) 
        {
            // Choose Scope, Singleton or Transient method
            services.AddSingleton();
            services.AddSingleton(_ => 
              new NestedService(connectionString));
        }
    }
}

Then your ConfigureServices method would look like this

public void ConfigureServices(IServiceCollection services)
{
    var connectionString = Configuration["Data:ConnectionString"];
    services.AddRootServices(connectionString);
}

With options builder

Should you need more parameters, you can go a step further and create an options class which you pass to RootService's constructor. If it becomes complex, you can use the Builder pattern.