.NET Core comes with its own dependency injection system, that you probably know from ASP.NET Core. In this session, we will have a detailed look at all of the specifics of Microsoft's default DI system for .NET Core applications. You will learn how to properly use the Inversion of Control features, such as registering services, scopes, lifetimes, and how to resolve instances. Armed with this DI knowledge, we will revisit ASP.NET Core and investigate bootstrapping and typical scenarios for middleware, background processes and resolving objects from MVC and Razor. At the end there will be a deep dive into topics with service descriptors, implementation factories, do's and don'ts and pitfalls to avoid memory leaks in your implementation.

1. 1
It depends:
Loving .NET Core dependency
injection or not
Alex Thissen
Cloud architect at Xpirit, The Netherlands
@alexthissen

2. 2
1. Unit testing
2. SOLID: Dependency
Inversion Principle
3. Loose coupling
4. Clean architecture Domain
Logic
Presentation

3. 3
Container HomeController
FloppyDiskRepository
IRepository
ILogger
ConsoleLogger

4. 4
Demo: ASP.NET Core DI
Quick look at (not so) obvious
injection

5. 5
Register
How
• Add… and TryAdd… of
type mappings
• Add… extension
methods
Where
• Application root
• Startup class (ASP.NET)
Resolve
Implicit
• Constructor injection
• ASP.NET Core specifics
Explicit
• GetService<T>
• GetRequiredService<T>
• Also for enumerables
Release
Automatic
• Resolved instances and
dependencies
Scopes
• End of scope
• Might need to create
child scopes
IServiceCollection IServiceProvider IDisposable
1 2 3

6. 6
ServiceType ImplementationType

7. 7
Singleton
• Easy way to
implement singleton
pattern
• One instance per
DI container
• Beware of
concurrency and
threading issues
Scoped
• Duration of scope
• ASP.NET uses web request as scope
• Useful for UnitOfWork objects,
e.g. DbContext
• See also ServiceScopeFactory
Transient
• Single use
• Most common and safest

8. 8
public class Startup
{
public Startup(IConfiguration configuration, IHostEnvironment env)
public void ConfigureServices(IServiceCollection services)
public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
}
builder.UseStartup<Startup>();
1
2
3

9. 9
IServiceProvider
ActivatorUtilities
IServiceProvider
Register using
Configure(Services)
injection in Startup
class
Constructor
injection
[FromServices]
attribute
@inject
IApplicationBuilder.
ApplicationServices
HttpContext.
RequestServices
Middleware
Constructor or
Invoke method

10. 10
Demo: Resolving services
in ASP.NET Core
Controller constructors
@inject
FromServicesAttribute
Application- and RequestServices

11. 11
IHostedService
BackgroundService
Host
IHostBuilder
WebHostBuilder
HostBuilder Configure
Microsoft.Extensions.Hosting
Microsoft.Extensions.DependencyInjection

12. 12
Demo: Generic Host
New .NET Core 3.0 hosting

13. 13
ServiceDescriptor

14. 14
IDisposable Dispose
ServiceScopeFactory
AddHostedService<T>
using (var scope = serviceScopeFactory.CreateScope()) {
ISomeRepository scoped =
scope.ServiceProvider.GetRequiredService<ISomeRepository>();
await DoMyWork(scoped);
}

15. 15
UseServiceProviderFactory
ConfigureContainer
IServiceProvider
Some alternative
DI Frameworks
Autofac
Castle Windsor
Lamar
LightInject
Ninject
SimpleInjector
Spring.NET
Unity

16. 16
Demo: Advanced DI
Scopes
Replacing DI container
Custom DI containers

17. 17
Configure{Environment}Services Configure{Environment}
Startup{Environment} StartupProduction
UseStartup<Startup>
Host.CreateDefaultBuilder(args)
.ConfigureWebHostDefaults(webBuilder => {
var assemblyName = typeof(Startup).GetTypeInfo().Assembly.FullName;
webBuilder.UseStartup(assemblyName);
});

18. 18
Host.CreateDefaultBuilder(args)
.UseDefaultServiceProvider((context, options) => {
options.ValidateScopes =
context.HostingEnvironment.IsDevelopment();
options.ValidateOnBuild = true;
})

19. 20
Demo: Tips and tricks
Convention based startup
Scope validation
TestServer services override

20. 21
IServiceProvider
IServiceProvider.CreateScope
IHttpContextAccessor
accessor.HttpContext.RequestServices

21. 22

22. 23

23. 24
Workshop
Going container for .NET Core
in the cloud
Alex Thissen
Cloud architect at Xpirit, The Netherlands
@alexthissen