This adds a type already used by ASP.NET Core's new WebApplcation and WebApplicationBuilder that allows reading from configuration (e.g. appsettings.json and DOTNET_/ASPNETCORE_ environment variables) while still being able to add new configuration sources without explicitly rebuilding config. Every time a source is added via the IConfigurationBuilder interface, the IConfiguration updates automatically and immediately.

New API

namespace Microsoft.Extensions.Configuration
{
+    public sealed class ConfigurationManager : IConfigurationRoot, IConfigurationBuilder, IDisposable
+    {
+        public ConfigurationManager();
+        public string? this[string key] { get; set; }
+        public IConfigurationSection GetSection(string key);
+        public void Dispose();
+    }

The members that are required to implement IConfigurationBuilder will be implemented explicitly, so members like IList<IConfigurationSource> IConfigurationBuilder.Sources don't pollute intellisense. Extension methods are generally used to add configuration sources.

Usage Example

WebApplicationBuilder essentially already exposes this type as a public property. The problem it is trying to solve is being able to read config from appsettings.json and DOTNET_/ASPNETCORE_ while configuring the host's IServiceCollection while at the same time being able to add new configuration sources or even change the content root without having to introduce additional build stages.

    using var config = new ConfigurationManager();

    config.AddEnvironmentVariables(prefix: "MyCustomPrefix_");

    if (config["FileConfig"] == "enabled")
    {
        config.AddJsonFile("MyConfig.json", optional: true, reloadOnChange: true);
    } 

    string myValueFromJson = config["JsonConfigValue"];
    // ...

We have docs where we demonstrate manually building an IConfigurationBuilder, reading from the built IConfiguration, and then throwing that away to add a new config source. This is an alternative to that.

Fixes #51770