Provide access to IHost.Services inside 'UseMiddleware' method.

[StevenRasmussen]

Perhaps there's already a way to accomplish this but I was unable to figure it out. What I would like is:

commandLineBuilder.UseMiddleware(context =>
{
    //  'services' here would be the 'IHost.Services'
    var someService = services.GetService<ISomeService>();
}

I see that in the UseHost extension method, the IHost is placed in the BindingContext.ServiceProvider here:

invocation.BindingContext.AddService(typeof(IHost), _ => host);

However, the BindingContext.ServiceProvider is "internal" and so there's no way to get the IHost. Otherwise something like this would be possible:

commandLineBuilder.UseMiddleware(context =>
{
    var host = context.BindingContext.ServiceProvider.GetService<IHost>();
    var someService = host.Services.GetService<ISomeService>();
}

Directly exposing the BindingContext.ServiceProvider is probably not the answer... perhaps providing another extension method including the IHost.Services would be best (if it's possible):

public static CommandLineBuilder UseMiddleware(
    this CommandLineBuilder builder, 
    Action<InvocationContext> onInvoke, 
    IServiceProvider services, // IHost.Services
    MiddlewareOrder order = MiddlewareOrder.Default);

Or perhaps just the IHost itself:

public static CommandLineBuilder UseMiddleware(
    this CommandLineBuilder builder, 
    Action<InvocationContext> onInvoke, 
    IHost host,
    MiddlewareOrder order = MiddlewareOrder.Default);

I'd be happy to try and contribute a PR if some guidance is provided on direction here.

[fredrikhr]

@StevenRasmussen here's what you can do today if inside a middleware that runs later than the UseHost middleware:

1. Construct a ModelBinder<IHost>
2. Call CreateInstace on the model binder and pass the binding context of the Invocation context.
3. You get an instace of IHost that you can use normally, i.e. access the Services property.

I agree that this way is not straightforward. I am considering adding an extension method GetHost on the Invocation context and putting that extension into the hosting package.

That would reduce the code to

IHost host = invocationContext.GetHost();

We really need it to be an extension though, in order to maintain the decoupled dependencies of the generic host.

[StevenRasmussen]

Just to be clear, this is what you're suggesting?

public static IHost GetHost(this InvocationContext invocationContext)
{
    var modelBinder = new ModelBinder<IHost>();
    return (IHost)modelBinder.CreateInstance(invocationContext.BindingContext);
}

Does this create a different IHost instance than what the rest of the application would use as part of the UseHost extension method?

[fredrikhr]

@StevenRasmussen correct, this is what I am suggesting. The IHost instance will be resolved from the internal service provider, which returns a singleton that is created inside UseHost. That means, your code snippet above will return the exact same instance that a command handler would get (if the handler had an IHost parameter). The code snippet above will not create a new separate instance.

[StevenRasmussen]

Thanks for confirming! This would be great to have in the project here but I can certainly just create this method myself as well ;) Thanks for pointing me in the right direction!

[reduckted]

I am considering adding an extension method GetHost on the Invocation context and putting that extension into the hosting package.

Awesome! I made the exact same suggestion in #1025 (comment) before I discovered this issue.