Flow hosting configuration through to application

[DamianEdwards]

There's currently no way to get access to the configuration constructed and used by the hosting layer, from the application. This is useful in cases where the application wants to get access to values that have already been parsed into an IConfiguration by the hosting layer, e.g. command line arguments.

I propose we add a new property to IHostingEnvironment: IConifguration Configuration { get; }

This would be set by WebHostBuilder when it's initializing the IHostingEnvironment instance (e.g. somewhere around here).

That way, an application can simply accept IHostingEnvironment in its constructor to get access to the hosting's configuration.

Example usage:

public class Startup
{
    public static void Main(string[] args) => WebApplication.Run<Startup>(args);

    public Startup(IHostingEnvironment hosting)
    {

        var builder = new ConfigurationBuilder()
            .AddJsonFile("appsettings.json")
            .AddEnvironmentVariables()
            .AddConfiguration(hosting.Configuration);

        Configuration = builder.Build();

        Configuration.Bind(Options);
    }

    public IConfigurationRoot Configuration { get; set; }

    public StartupOptions Options { get; } = new StartupOptions();

    public class StartupOptions
    {
        public bool EnableThing { get; set; }
    }
        ...

Required change to configuration to support this aspnet/Configuration#339

[davidfowl]

/cc @lodejard @Tratcher Any comments?

[Tratcher]

Someone asked for this a few months ago specifically for the command line parameters. It was eventually closed for some reason. @Eilon had some interest in that discussion.

[DamianEdwards]

To be clear, you could do this right now (if you don't launch via commands) like the following, but I think something more formal like the above is nicer.

public class Startup
{
    private static string[] _args;
    public static void Main(string[] args) => WebApplication.Run<Startup>(_args = args);

    public Startup(IHostingEnvironment env)
    {

        var builder = new ConfigurationBuilder()
            .AddJsonFile("appsettings.json")
            .AddEnvironmentVariables()
            .AddCommandLine(_args);

        Configuration = builder.Build();

        Configuration.Bind(Options);
    }

    public IConfigurationRoot Configuration { get; set; }

    public StartupOptions Options { get; } = new StartupOptions();

    public class StartupOptions
    {
        public bool EnableThing { get; set; }
    }

[Tratcher]

Note even the commands work if you change your command to "web": "MyWebApp"

[Tratcher]

But you have to include --server=Microsoft.AspNet.Server.Kestrel

[davidfowl]

This is a cleaner way to flow that data. It works irregardless of how you host your application. Unlike the previous request to only flow command line arguments, this flows all of the configuration.

[Eilon]

@Tratcher yeah I can't find the issue you're referring to. I did find this semi-related issue: #399