Can I apply a prefix to all keys loaded from a configuration provider?

[wazzamatazz]

Say I have an options class like this:

public class MyOptions {
  public string Item1 { get; set; }
  public string Item2 { get; set; }
  public IDictionary<string, Device> Devices { get; set; }
}

public class Device {
  public string DisplayName { get; set; }
  public string ConnectionString { get; set; }
  public IEnumerable<string> SubscriptionItems { get; set; }
}

In my startup code, I might bind these options as follows:

services.Configure<MyOptions>(Configuration.GetSection("MyApp:MyOptions"));

If I wanted to define individual items in the MyOptions.Devices collection in separate files, I could add additional JSON providers to my IConfigurationBuilder and then define additional configuration files e.g.:

// devices.sensor-01.json
{
  "MyApp": {
    "MyOptions": {
      "Devices": {
        "sensor-01": {
          "DisplayName": "Test sensor 1",
          "ConnectionString": "SOME_CONNECTION",
          "SubscriptionItems": [
            "Temperature-01",
            "Temperature-02"
          ]
        }
      }
    }
  }
}

// devices.sensor-02.json
{
  "MyApp": {
    "MyOptions": {
      "Devices": {
        "sensor-02": {
          "DisplayName": "Test sensor 2",
          "ConnectionString": "SOME_CONNECTION",
          "SubscriptionItems": [
            "Temperature-98",
            "Temperature-99"
          ]
        }
      }
    }
  }
}

This works well, but I'd like to know if there is a built-in way to automatically add a prefix to all keys loaded from a given provider? In my example above, I'd like to be able to prefix all keys from JSON files matching devices.*.json with MyApp:MyOptions:Devices:, so that the JSON file itself could be simplified to something like this:

// devices.sensor-01.json (simplified)
{
  "sensor-01": {
    "DisplayName": "Test sensor 1",
    "ConnectionString": "SOME_CONNECTION",
    "SubscriptionItems": [
      "Temperature-01",
      "Temperature-02"
    ]
  }
}

Is this something that is possible out of the box, or would I have to write a custom JSON configuration provider to accomplish this? Looking at the source for the JsonConfigurationFileParser, this could be accomplished by injecting an initial context into the parser containing the prefix, but since the class itself is internal and the EnterContext method is private, there's no obvious way to accomplish it without copying and modifying this code, unless it is possible to tell the configuration builder itself to apply a prefix to keys from certain sources?

[FiniteReality]

I've worked around this before using the IEnumerable overload in the configuration provider; you build one configuration root with the values you want, and then enumerate its key/value pairs, adding a prefix to the key. Something like this:

var rootOne = new ConfigurationBuilder()
    .AddYourSource()
    .Build();

realRoot.AddInMemoryCollection(AddPrefix(rootOne.AsEnumerable(), "prefix"));

static IEnumerable<KeyValuePair<string, string>> AddPrefix(IEnumerable<KeyValuePair<string, string>> values, string prefix)
{
    foreach (var pair in values)
        yield return new(ConfigurationPath.Combine(prefix, pair.Key), pair.Value);
}

Unfortunately, this does not allow for things like reload tokens. I was looking into something like this at one point, but I couldn't figure out a good way to implement that without being confusing.

[wazzamatazz]

Thanks for the suggestion. Being able to reload on change would probably be preferable in the use cases I'm thinking about, unfortunately!