How can I list all of the configuration sources or properties in ASP.NET Core?

From .NET Core 3.0+ you can cast your IConfiguration to a IConfigurationRoot and use the GetDebugView extension method. That will generates a human-readable view of the configuration showing where each value came from. E.g.

var root = (IConfigurationRoot)Configuration;
var debugView = root.GetDebugView();

Sample output to debugView:

applicationName=Project.Name (Microsoft.Extensions.Configuration.ChainedConfigurationProvider)
ASPNETCORE_ENVIRONMENT=Development (EnvironmentVariablesConfigurationProvider)
ASPNETCORE_HTTPS_PORT=32774 (EnvironmentVariablesConfigurationProvider)
ASPNETCORE_LOGGING:
  CONSOLE:
    DISABLECOLORS=true (EnvironmentVariablesConfigurationProvider)
ASPNETCORE_URLS=https://+:443;http://+:80 (EnvironmentVariablesConfigurationProvider)
DOTNET_RUNNING_IN_CONTAINER=true (EnvironmentVariablesConfigurationProvider)
DOTNET_USE_POLLING_FILE_WATCHER=1 (EnvironmentVariablesConfigurationProvider)
AllowedHosts=* (JsonConfigurationProvider for 'appsettings.json' (Required))
Kestrel:
  Certificates:
    Development:
      Password=xxxxxxxx-xxxxx-xxxx-xxxx-xxxxxxxxxx (JsonConfigurationProvider for 'secrets.json' (Optional))
EmailOptions:
  EnableSsl=False (JsonConfigurationProvider for 'appsettings.json' (Required))
ENVIRONMENT=Development (Microsoft.Extensions.Configuration.ChainedConfigurationProvider)
HOME=/root (EnvironmentVariablesConfigurationProvider)
HOSTNAME=2cb0f5c24cc0 (EnvironmentVariablesConfigurationProvider)
HTTPS_PORT=32774 (Microsoft.Extensions.Configuration.ChainedConfigurationProvider)
NUGET_FALLBACK_PACKAGES=/root/.nuget/fallbackpackages;/root/.nuget/fallbackpackages2 (EnvironmentVariablesConfigurationProvider)
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin (EnvironmentVariablesConfigurationProvider)
PWD=/app (EnvironmentVariablesConfigurationProvider)
RUNNING_IN_CONTAINER=true (Microsoft.Extensions.Configuration.ChainedConfigurationProvider)
URLS=https://+:443;http://+:80 (Microsoft.Extensions.Configuration.ChainedConfigurationProvider)
USE_POLLING_FILE_WATCHER=1 (Microsoft.Extensions.Configuration.ChainedConfigurationProvider)

You can get a list of all the keys discovered by all configuration sources by doing:

var keys = builder.Build().AsEnumerable().ToList();

I haven't found a way to build each configuration source separately so you could see the sources individually.

In debug mode, you can see the private members and peek into each configuration source: