.net core, n-layered app, should services layer ha

问题:

Straightforward question is: are Microsoft.Extensions.Options.IOptions meant to be used only within the context of umbrella app (web app in this case) or in class libraries also?

Example:

In a n-layered, asp.net core app we have services layer that is dependant on some settings coming from appsettings.json file.

What we first started with is something along these lines in Startup.cs:

  services.Configure<Services.Options.XOptions>(options =>
  {
    options.OptionProperty1 = Configuration["OptionXSection:OptionXProperty"];
  });

And then in service constructor:

ServiceConstructor(IOptions<XOptions> xOptions){}

But that assumes that in our Service layer we have dependecy on Microsoft.Extensions.Options.

We're not sure if this is recomended way or is there some better practice?

It just feels a bit awkward our services class library should be aware of DI container implementation.

回答1:

You can register POCO settings for injection too, but you lose some functionalities related to when the appsettings.json gets edited.

services.AddTransient<XOptions>(
    provider => provider.GetRequiredService<IOptionsSnapshot<XOptions>>().Value);

Now when you inject XOptions in constructor, you will get the class. But when your edit your appsettings.json, the value won't be updated until the next time it's resolved which for scoped services would be on next request and singleton services never.

On other side injecting IOptionsSnapshot<T> .Value will always get you the current settings, even when appsettings.json is reloaded (assuming you registered it with .AddJsonFile("appsettings.json", reloadOnSave: true)).

The obvious reason to keep the functionality w/o pulling Microsoft.Extensions.Options package into your service/domain layer will be create your own interface and implementation.

// in your shared service/domain assembly
public interface ISettingsSnapshot<T> where T : class
{
    T Value { get; }
}

and implement it on the application side (outside of your services/domain assemblies), i.e. MyProject.Web (where ASP.NET Core and the composition root is)

public class OptionsSnapshotWrapper<T> : ISettingsSnapshot<T>
{
    private readonly IOptionsSnapshot<T> snapshot;

    public OptionsSnapshotWrapper(IOptionsSnapshot<T> snapshot) 
    {
        this.snapshot = snapshot ?? throw new ArgumentNullException(nameof(snapshot));
    }

    public T Value => snapshot.Value;
}

and register it as

services.AddSingleton(typeof(ISettingsSnapshot<>), typeof(OptionsSnapshotWrapper<T>));

Now you have removed your dependency on IOptions<T> and IOptionsSnapshot<T> from your services but retain all up advantages of it like updating options when appsettings.json is edited. When you change DI, just replace OptionsSnapshotWrapper<T> with your new implementation.