Dependency Inject a Service from Startup back to Program in .Net Core 3.1

Full source code available here.

Over the past couple of years I wrote a few posts about Dependency Injection in .Net Core 2.1, and this week I received comments from a reader telling me that some of the changes in .Net Core 3.1 mean that some of the approaches no longer work. There have been breaking changes

I wanted to see what would still work so I tried a few things.

You can no longer DI from Program in Startup.

But you can add a transient service and/or a singleton service to the ServiceCollection and use it within Program, and the rest of the application. You can also add a scoped service to the ServiceCollection and use it within the Program, it’s a little different from using transient and singleton so I’ll cover it in the next post.
Here’s how to use transient and singletons inside Program.
Create two services, creatively named ServiceOne and ServiceTwo and have them implement interfaces.

public class ServiceOne : IServiceOne
	public static int staticCounter;

	public ServiceOne()

public class ServiceTwo : IServiceTwo
	public static int staticCounter;

	public ServiceTwo()

I added a static counter to make it easy to see how many times the constructor is called.
For the transient one I expect it to increment every time the service is injected, for the singleton I expect it to remain at 1 for the lifetime of the application.

In Program.cs I split up CreateHostBuilder(args).Build call from the subsequent .Run().

public static void Main(string[] args)
	IHost host = CreateHostBuilder(args).Build();

The CreateHostBuilder() method is not changed –

public static IHostBuilder CreateHostBuilder(string[] args) =>
	.ConfigureWebHostDefaults(webBuilder =>

And I have two methods that use the ServiceCollection to access the registered services –

private static void DoSomethingWithTheTransientService(IServiceProvider serviceProvider)
	Console.WriteLine("Calling the transient service");

	var serviceOne = serviceProvider.GetService<IServiceOne>();

private static void DoSomethingWithTheSingletonService(IServiceProvider serviceProvider)
	Console.WriteLine("Calling the singleton service");

	var serviceTwo = serviceProvider.GetService<IServiceTwo>();

To make what’s happening more obvious I added the services to the constructor call of the WeatherForecastController() and added the counter and hash codes to the data returned by the action method.

public class WeatherForecastController : ControllerBase
	private IServiceOne _serviceOne;
	private IServiceTwo _serviceTwo;
	public WeatherForecastController(IServiceOne serviceOne, IServiceTwo serviceTwo)
		_serviceOne = serviceOne;
		_serviceTwo = serviceTwo;

For completeness, here is the ConfigureServices method in Startup.cs

public void ConfigureServices(IServiceCollection services)
    services.AddTransient<IServiceOne, ServiceOne>();
    services.AddSingleton<IServiceTwo, ServiceTwo>();

Put some breakpoints in Program Main(), DoSomethingWithTheTransientService(), DoSomethingWithTheSingletonService() and in the WeatherForecastController.Get().

Start the application and browse to http://localhost:5000/weatherforecast to see what happens.

I’m going to follow up on this post with a version that shows how to use scoped dependencies in Startup .NET 5.

Full source code available here.

Executing a Method on All Implementations of an Interface

Full source code available here.

Have you ever wanted to execute a method on all implementations of an interface? Say you have an interface called IGenerator and four implementations of it – GeneratorA, GeneratorB, GeneratorC, and GeneratorD.
IGenerator is very simple, it has a single Generate(string details) method.

What I don’t want to do is –

var generatorA = new GeneratorA();
var generatorB = new GeneratorB();
var generatorC = new GeneratorC();
var generatorD = new GeneratorD();

I also don’t want to do this –

List<IGenerator> generators = new List<IGenerator>() { new GeneratorA(), new GeneratorB(), new GeneratorC(), new GeneratorD() } ;
foreach (var generator in generators) 

What I want to do is find all the implementations of the interface without explicitly naming them, create instances of the implementations and call the Generate(..) method. This way I can add a GeneratorE and GeneratorF and I’ll be sure their Generate(..) methods will be called.

Finding the Implementations

This code will find all the implementations of the IGenerator interface –

var iGenerator = typeof(IGenerator);
var generatorsTypes = AppDomain.CurrentDomain.GetAssemblies() 
    .SelectMany(assembly => assembly.GetTypes())
    .Where(type => iGenerator.IsAssignableFrom(type) && type.IsClass);

On line 1 I specify the type I am going to look for.

Line 2 gets all the assemblies, line 3 gets all the types in those assemblies. Line 4 limits the type to those that implement IGenerator and are a class, checking that the type is a class is important, because without it the interface itself would be included in the result.

Activating the Implementations

At this point all we have done is find the types, we have not instantiated them, so we cannot yet call the Generate(..) methods.
But of course there is a way to do just that.

List<IGenerator> generators = new List<IGenerator>();

foreach (var generator in generatorsTypes)
    generators.Add(Activator.CreateInstance(generator) as IGenerator);

Now I have a list with instances of all the implementations of IGeneraotor.

Calling the methods

Easy –

generators.ForEach(g => g.Generate("Hi from a generator."));

That’s it, find the implementations, activate them, call the method. The end.

Full source code available here.