Difference in behavior between UseConfiguration and ConfigureAppConfiguration

[cesarblum]

Ran into this while investigating aspnet/AzureIntegration#81.

Code:

public static void Main(string[] args)
{
    var host = new WebHostBuilder()
        .ConfigureAppConfiguration((context, config) =>
        {
            config.AddCommandLine(args);
        })
        .ConfigureLogging(factory =>
        {
            factory.SetMinimumLevel(LogLevel.Trace);
            factory.AddConsole();
        })
        .UseKestrel()
        .UseStartup<Startup>()
        .Build();

    host.Run();
}

Output:

> dotnet .\bin\Debug\netcoreapp2.0\App.dll --server.urls http://localhost:12345
...
Now listening on: http://localhost:5000
Application started. Press Ctrl+C to shut down.

Note that it bound to the default address.

If I change the code to:

public static void Main(string[] args)
{
    var config = new ConfigurationBuilder()
        .AddJsonFile("appsettings.json")
        .AddCommandLine(args)
        .Build();

    var host = new WebHostBuilder()
        .UseConfiguration(config)
        .ConfigureLogging(factory =>
        {
            factory.SetMinimumLevel(LogLevel.Information);
            factory.AddConsole();
        })
        .UseKestrel()
        .UseStartup<Startup>()
        .Build();

    host.Run();
}

It behaves as expected (binds to http://localhost:12345):

> dotnet .\bin\Debug\netcoreapp2.0\App.dll --server.urls http://localhost:12345
...
Now listening on: http://localhost:12345
Application started. Press Ctrl+C to shut down.

Is this by design?

[JunTaoLuo]

I think the ConfigureAppConfiguration was intended to configure the IConfiguration in the application services whereas UseConfiguration is intended to configure the settings of the WebHostBuilder. Since the url address is a setting on the WebHostBuilder only UseConfiguration will work here. Note that the WebHostBuilder's configuration is added to the application's configuration but the converse is not true; configuring the application's configuration does not affect the WebHostBuilder's configuration. cc @Tratcher to make sure I'm not speaking non-sense.

[Tratcher]

That about covers it. The distinction is somewhat arbitrary at this point.

[JunTaoLuo]

So I guess we are happy with this behaviour and can close the issue? Or is there any possible improvement in the user experience here.

[muratg]

This is by-design.

[filipgoris]

I think this is very confusing.
Consider the following simple apsettings.json:

{
  "server.urls": "http://localhost:12345",
  "Logging": {
    "IncludeScopes": false,
    "LogLevel": {
      "Default": "Trace"
    }
  }
}

With

var config = new ConfigurationBuilder().AddJsonFile("appsettings.json")

and

var host = new WebHostBuilder().UseConfiguration(config)

server.urls is applied, but

var host = new WebHostBuilder().ConfigureLogging((context, logging) =>
{
   logging.AddConfiguration(context.Configuration.GetSection("Logging"))
}

won't apply Log settings.

With

var host = new WebHostBuilder()
   .configureAppConfiguration((context, config) =>
   (
      config.AddJsonFile("appsettings.json", true, true)
   })
   .configureLogging((context, logging) =>
   {
      logging.AddConfiguration(context.Configuration.GetSection("Logging"));
   ))

I can inject log settings, but not server.urls. Meaning I have to load this appsettings.json using both methods.

Or am I seeing this wrong?

[cesarblum]

If you can read the Logging section from you config, there's no reason why you shouldn't be able to read server.urls as well (after having loaded your JSON file in ConfigureAppConfiguration).

[0xorial]

-2 to that design...

[RehanSaeed]

It's still unclear to me when to use UseConfiguration and ConfigureAppConfiguration in relation to configuring Kestrel from appsettings.json:

public static IWebHostBuilder CreateWebHostBuilder(string[] args) =>
    new WebHostBuilder()
        .UseKestrel(
            (builderContext, options) =>
            {
                options.Configure(builderContext.Configuration.GetSection(nameof(ApplicationOptions.Kestrel)));
            })
            ...

Reading the comments, you'd think that you need to use UseConfiguration but CreateDefaultBuilder only calls UseConfiguration with the command line arguments:

builder.UseConfiguration(new ConfigurationBuilder().AddCommandLine(args).Build());

That must mean it's using ConfigureAppConfiguration.

[Tratcher]

That UseKestrel lamda is delay executed after both UseConfiguration and ConfigureAppConfiguration, so it's able to consume config from either source. Note it only reads the new "Kestrel" config section. "server.Urls" is awkward because kestrel doesn't read it directly, the host reads it after UseConfiguration and before ConfigureAppConfiguration and then passes it through a side channel (IServerAddressesFeature) to servers.

Since the new "Kestrel" config section works with appsettings.json and is so much more powerful we opted not to rework "server.Urls" to work with appsettings.json, it's redundant.

[bugproof]

It's not clear and should be documented when to use one over another.