Why is AddCommandLine(args) called twice in CreateDefaultBuilder(string[] args)？

[Surbowl]

I find that this method is called twice in WebHost.cs -> CreateDefaultBuilder(string[] args)

aspnetcore/src/DefaultBuilder/src/WebHost.cs

Lines 164 to 167 in 450e047

	if (args != null)
	{
	builder.UseConfiguration(new ConfigurationBuilder().AddCommandLine(args).Build());
	}

and

aspnetcore/src/DefaultBuilder/src/WebHost.cs

Lines 187 to 190 in 450e047

	if (args != null)
	{
	config.AddCommandLine(args);
	}

But I see the following code in WebHostBuilder.cs -> BuildCommonServices(out AggregateException? hostingStartupErrors)

aspnetcore/src/Hosting/Hosting/src/WebHostBuilder.cs

Lines 271 to 273 in 450e047

	var builder = new ConfigurationBuilder()
	.SetBasePath(_hostingEnvironment.ContentRootPath)
	.AddConfiguration(_config, shouldDisposeConfiguration: true);

The new builder seems to contain the value of _config, and _config already has CommandLine config in it, so I don't quite understand why it's called twice.
I'm sorry my English isn't very good😳

[Tratcher]

There are two stages to config. In the first one a few sources are loaded so the host itself can be configured. This is used to read things like ASPNETCORE_ENVIRONMENT, ContentRoot, etc..

aspnetcore/src/DefaultBuilder/src/WebHost.cs

Lines 164 to 167 in b7b3001

	if (args != null)
	{
	builder.UseConfiguration(new ConfigurationBuilder().AddCommandLine(args).Build());
	}

Later the host builds a config for the application. It starts with a copy of the host config so the app can re-use those values, and layers everything else on top of that. This is where things like AppSettings.Development.json are added, using the environment and the content root from the host config.

aspnetcore/src/Hosting/Hosting/src/WebHostBuilder.cs

Lines 271 to 275 in b7b3001

	var builder = new ConfigurationBuilder()
	.SetBasePath(_hostingEnvironment.ContentRootPath)
	.AddConfiguration(_config, shouldDisposeConfiguration: true);
	_configureAppConfigurationBuilder?.Invoke(_context, builder);

[Surbowl]

Thank you for your answer! but I still don't quite understand what this code does

aspnetcore/src/DefaultBuilder/src/WebHost.cs

Lines 187 to 190 in 450e047

	if (args != null)
	{
	config.AddCommandLine(args);
	}

the application config is a copy of the host config, and host config already contains args, so why add args again in configureDelegate😳

[Tratcher]

Ah, that one is for precedence. Sources added later override the earlier ones, so command line args were added again so they could override things like appsettings.json.

[Surbowl]

Thank you Tratcher. I get it ❤