How is content root path handled when clearing configuration sources?

[AnthonyMastrean]

My team produces Windows services and depends on this content root path behavior that changed in .NET 7.

Starting with .NET 7, when a hosted application is launched with the current directory set to the System special folder, it defaults the ContentRootPath property to AppContext.BaseDirectory.

https://learn.microsoft.com/en-us/dotnet/core/compatibility/extensions/7.0/contentrootpath-hosted-app

However, due to security configurations, our customers expect any configuration files in the content root path to be read-only. We must provide writable configuration files in an application-specific subfolder of Environment.SpecialFolder.CommonApplicationData (AKA C:\ProgramData).

The recommendation for adding a JSON configuration source before the appsettings files has been to "clear sources" or to start with an empty host builder and build up sources from scratch.

• [API Proposal]: Possibility to specify a custom naming for appsettings.json files #67142 (comment)
• builder.SetBasePath does not work as expected #85068 (comment)
• Unable to configure host environment from a JSON settings file when using Host.CreateApplicationBuilder #97930 (comment)
• [API Proposal]: Host.CreateEmptyApplicationBuilder #81280 (comment)

I don't know about you, but I'm uncomfortable with these recommendations when I can't reproduce the same default configuration sources in the same order. From what I can tell, the following configuration sources are applied:

• An empty in-memory collection (I can't figure out why it's empty yet or what it would contain)
• An in-memory collection containing the "corrected" content root path (via an internal method, I dislike copying special framework methods into my application code)
• Environment variables prefixed with DOTNET_
• Command line args
• The appsettings.json file
• The appsettings.{Environment}.json file
• User secrets
• Environment variables (no prefix)
• Command line args (again?)

https://source.dot.net/#Microsoft.Extensions.Hosting/HostApplicationBuilder.cs,84

Here are the options I'm testing:

1. Clear sources. Host settings and ContentRootPath may already be applied thanks to the private/internal apply defaults and initialize calls... trust that you don't have to readd those other sources that you don't have access to (SetDefaultContentRoot)?

var builder = Host.CreateApplicationBuilder(args);
builder.Configuration.Sources.Clear();

// add earlier default sources
builder.Configuration.AddEnvironmentVariables(prefix: "DOTNET_");
if (args is { Length: > 0 })
{
    builder.Configuration.AddCommandLine(args);
}

// add custom source in the right place
builder.Configuration.AddJsonFile(Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.CommonApplicationData), "ExampleApp", "example.conf"), optional: true, reloadOnChange: true);

// add later default sources
builder.Configuration.AddJsonFile("appsettings.json", optional: true, reloadOnChange: true);
builder.Configuration.AddJsonFile($"appsettings.{builder.Environment.EnvironmentName}.json", optional: true, reloadOnChange: true);
if (builder.Environment.IsDevelopment())
{
    builder.Configuration.AddSecrets<Program>(optional: true, reloadOnChange: true);
}
builder.Configuration.AddEnvironmentVariables();
if (args is { Length: > 0 })
{
    builder.Configuration.AddCommandLine(args);
}

2. Stack sources (no clearing). We add our custom source and re-add everything that it displaced so that they're in the right effective order after all.

var builder = Host.CreateApplicationBuilder(args);

// add custom source to the "end" of the sources
builder.Configuration.AddJsonFile(Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.CommonApplicationData), "ExampleApp", "example.conf"), optional: true, reloadOnChange: true);

// add every default source that was displaced
builder.Configuration.AddJsonFile("appsettings.json", optional: true, reloadOnChange: true);
builder.Configuration.AddJsonFile($"appsettings.{builder.Environment.EnvironmentName}.json", optional: true, reloadOnChange: true);
if (builder.Environment.IsDevelopment())
{
    builder.Configuration.AddSecrets<Program>(optional: true, reloadOnChange: true);
}
builder.Configuration.AddEnvironmentVariables();
if (args is { Length: > 0 })
{
    builder.Configuration.AddCommandLine(args);
}

3. Empty host builder... we don't have enough experience with this one. Content root path does not seem to be fixed and we lose so many other features (default logging and services) that we desire.

var builder = Host.CreateEmptyApplicationBuilder(new() { Args = args });

// uh... 🤷‍♂️

Should we just clear sources and move on? Are those earlier sources that I can't recreate just sorted out somehow (closed over by IHostEnvironment and service registration)?

What am I missing?