asp.net-core-mvc Getting started with asp.net-core-mvc Dependency injection basics

Example

Almost any controller needs some external dependencies to work. Here is a way to configure a dependency object (or its factory) and pass it to a controller. Doing so will help to sustain a separation of concerns, keep code clear and testable.

Say, we have an interface and its implementation that needs some values from config in its constructor:

public interface ISomeDependency
{
    async Task<IEnumerable<string>> GetItemsAsync(string key);
}

public class SomeDependency : ISomeDependency
{
    public SomeDependency(string connectionString)
    {
        ...
    }
    ...
}

It's used in some controller class:

public class SomeController : Controller
{
    private reanonly ISomeDependency dependency;

    public SomeController(ISomeDependency dependency)
    {
        ...
        this.dependency = dependency;
    }

    ...

    public async Task<IEnumerable<string>> Get(string key) =>
        await dependency.GetItemsAsync(key);
}

One can inject this dependency in the controller constructor calling services.AddTransient inside Startup.ConfigureServices method:

public class Startup
{
    public Startup(IHostingEnvironment env)
    {
        var builder = new ConfigurationBuilder().
            .SetBasePath(env.ContentRootPath)
            .AddJsonFile("appsettings.json", optional: true, reloadOnChange: true)
        ...
        Configuration = builder.Build();
    }

    public IConfigurationRoot Configuration { get; }

    public void ConfigureServices(IServiceCollection services)
    {
        ...
        services.AddTransient(serviceProvider =>
            new MyDependency(Configuration["Data:ConnectionString"]));
    }

    ...
}

Here Data:ConnectionString is a path to a setting in appsettings.json file:

{
  ...
  },
  "Data": {
    "ConnectionString": "some connection string"
  }
}

Lifetime management

To manage a lifetime of the injected object, along with AddTransient another two options exist: AddSingleton and AddScoped. The last one means that lifetime of the object is scoped to a HTTP request.