How to use Func in built-in dependency injection

Question

Using asp.net 5 I\'d like my controller to be injected with a Funcinstead of T

For example:

Answer 1

You can register a Func or a delegate with a ServiceCollection. I recommend a delegate because it allows you to distinguish between different methods with identical signatures.

Here's an example.

public interface IThingINeed {}
public class ThingINeed : IThingINeed { }

public delegate IThingINeed ThingINeedFactory();

public class DelegateRegistrationTests
{
    [Test]
    public void RegisterDelegateFromDependency()
    {
        var serviceCollection = new ServiceCollection();
        serviceCollection.AddTransient();
        serviceCollection.AddTransient(
            provider => provider.GetService);
        var serviceProvider = serviceCollection.BuildServiceProvider();
        var factoryMethod = serviceProvider.GetService();
        var output = factoryMethod();
        Assert.IsInstanceOf(output);
    }
}

This almost looks like a service locator because the function we're resolving is actually IServiceCollection.GetService(). But that's hidden in the composition root. A class that injects this delegate depends on the delegate, not on the implementation.

You can use the same approach if the method you want to return belongs to a class that the container must resolve.

public interface IThingINeed
{
    string SayHello();
}

public class ThingINeed : IThingINeed
{
    private readonly string _greeting;

    public ThingINeed(string greeting)
    {
        _greeting = greeting;
    }

    public string SayHello() => _greeting;
}

public class ThingINeedFactory
{
    public IThingINeed Create(string input) => new ThingINeed(input);
}

public delegate IThingINeed ThingINeedFactoryMethod(string input);

public class DelegateRegistrationTests
{
    [Test]
    public void RegisterDelegateFromDependency()
    {
        var serviceCollection = new ServiceCollection();
        serviceCollection.AddSingleton();
        serviceCollection.AddSingleton();
        serviceCollection.AddSingleton(provider => 
            provider.GetService().Create);
        var serviceProvider = serviceCollection.BuildServiceProvider();
        var factoryMethod = serviceProvider.GetService();
        var created = factoryMethod("abc");
        var greeting = created.SayHello();
        Assert.AreEqual("abc", greeting);
    }
}

Here's an extension method to (maybe) make it a little bit easier:

public static class ServiceCollectionExtensions
{
    public static IServiceCollection RegisterDelegate(
        this IServiceCollection serviceCollection,
        Func getDelegateFromSource) 
            where TDelegate : class 
    {
        return serviceCollection.AddSingleton(provider =>
            getDelegateFromSource(provider.GetService()));
    }
}

serviceCollection
    .RegisterDelegate(
        factory => factory.Create);