asp.net-core Dependency Injection Inner workings of Microsoft.Extensions.DependencyInjection
Example
IServiceCollection
To start building an IOC container with Microsoft's DI nuget package you start with creating an IServiceCollection. You can use the already provided Collection: ServiceCollection:
var services = new ServiceCollection();
This IServiceCollection is nothing more than an implementation of: IList<ServiceDescriptor>, ICollection<ServiceDescriptor>, IEnumerable<ServiceDescriptor>, IEnumerable
All the following methods are only extension methods to add ServiceDescriptor instances to the list:
services.AddTransient<Class>(); //add registration that is always recreated
services.AddSingleton<Class>(); // add registration that is only created once and then re-used
services.AddTransient<Abstract, Implementation>(); //specify implementation for interface
services.AddTransient<Interface>(serviceProvider=> new Class(serviceProvider.GetService<IDependency>())); //specify your own resolve function/ factory method.
services.AddMvc(); //extension method by the MVC nuget package, to add a whole bunch of registrations.
// etc..
//when not using an extension method:
services.Add(new ServiceDescriptor(typeof(Interface), typeof(Class)));
IServiceProvider
The serviceprovider is the one 'Compiling' all the registrations so that they can be used quickly, this can be done with services.BuildServiceProvider() which is basically an extension mehtod for:
var provider = new ServiceProvider( services, false); //false is if it should validate scopes
Behind the scenes every ServiceDescriptor in the IServiceCollection gets compiled to a factory method Func<ServiceProvider, object> where object is the return type and is: the created instance of the Implementation type, the Singleton or your own defined factory method.
These registrations get added to the ServiceTable which is basically a ConcurrentDictionary with the key being the ServiceType and the value the Factory method defined above.
Result
Now we have a ConcurrentDictionary<Type, Func<ServiceProvider, object>> which we can use concurrently to ask to create Services for us. To show a basic example of how this could have looked.
var serviceProvider = new ConcurrentDictionary<Type, Func<ServiceProvider, object>>();
var factoryMethod = serviceProvider[typeof(MyService)];
var myServiceInstance = factoryMethod(serviceProvider)
This is not how it works!
This ConcurrentDictionary is a property of the ServiceTable which is a property of the ServiceProvider
