Where is Microsoft's useful documentation for IServiceCollection.Add(new ServiceDescriptor(...))?

Question

I am working on an existing WebAPI project and inside

public void ConfigureServices(IServiceCollection services)

the IoC container is setup like so

services.Add(new ServiceDescriptor(typeof(ISQLConnectionFactory), new SQLConnectionFactory(GetConnectionString("DefaultConnection"))));

(there are also lots of services.AddScoped which I'm not asking about)

Questions

Where can I find Microsoft's documentation that explains the concepts behind "IServiceCollection.Add(new ServiceDescriptor ..." (this is too little)

or perhaps someone can provide some insight

What does „services.Add(new ServiceDescriptor ...“ do ?
What is a ServiceDescriptor ?
By debugging the code, I see that SQLConnectionFactory is only instantiated once. Does the call "services.Add" (always) create a singleton object? If so what is the difference to services.AddSingleton?

score 3 · Accepted Answer · answered Jul 26 '19 at 05:33

What does „services.Add(new ServiceDescriptor ...“ do ?

For detail information about services.Add, you could refer the source code Add.

public static IServiceCollection Add(
    this IServiceCollection collection,
    ServiceDescriptor descriptor)
{
    if (collection == null)
    {
        throw new ArgumentNullException(nameof(collection));
    }

    if (descriptor == null)
    {
        throw new ArgumentNullException(nameof(descriptor));
    }

    collection.Add(descriptor);
    return collection;
}

For this code, it add the ServiceDescriptor to the ServiceCollection.

What is a ServiceDescriptor ?

ServiceDescriptor describes a service with its service type, implementation, and lifetime. It will be used to initializes a new instance of ServiceDescriptor with the specified implementationType.

By debugging the code, I see that SQLConnectionFactory is only instantiated once. Does the call "services.Add" (always) create a singleton object? If so what is the difference to services.AddSingleton?

This depends on whehter you pass the scope for services.Add. The default scope for services.add is ServiceLifetime. You could descipe the service with different scope by passing ServiceLifetime like services.Add(new ServiceDescriptor(typeof(ISQLConnectionFactory), new SQLConnectionFactory(GetConnectionString("DefaultConnection")), ServiceLifetime.Scoped));

There is no different between services.Add and AddSingleton. The AddSingleton just call services.Add with passing ServiceLifetime.Singleton

public static IServiceCollection AddSingleton(
    this IServiceCollection services,
    Type serviceType,
    Type implementationType)
{
    if (services == null)
    {
        throw new ArgumentNullException(nameof(services));
    }

    if (serviceType == null)
    {
        throw new ArgumentNullException(nameof(serviceType));
    }

    if (implementationType == null)
    {
        throw new ArgumentNullException(nameof(implementationType));
    }

    return Add(services, serviceType, implementationType, ServiceLifetime.Singleton);
}

score 0 · Answer 2 · answered Jul 25 '19 at 16:21

This link could provide some explanations https://learn.microsoft.com/en-us/aspnet/core/fundamentals/dependency-injection?view=aspnetcore-2.2

services.Add( adds new dependency registration that later can be resolved
new ServiceDescriptor constructor provide explicit registration with the factory that will supply new instances and lifetime description
services.AddSingleton is just a shorthand for doing this with a defined lifetime as Singleton

Where is Microsoft's useful documentation for IServiceCollection.Add(new ServiceDescriptor(...))?

2 Answers2