Update feature definition returned from ConfigurationFeatureDefinitionProvider to allow determining if properties have been specifically configured

[brental]

I am looking at creating a decorator for IFeatureDefinitionProvider that will apply defaults for a feature if it's feature definition is not contained in configuration or if it is configured but does not have certain properties (such as telemetry) set in its configuration. The ConfigurationFeatureDefinitionProvider currently returns null if the feature is not configured which allows applying defaults in that case. However, if it is in configuration but does not contain values for certain properties those properties are still set. This makes it hard to apply defaults in those cases.

As an example, if the configuration for a feature does not contain telemetry configuration then the Telemetry property is still initialised to a non null. This makes it impossible to tell whether telemetry was specifically configured or just had default values once the decorator has got the feature definition from the ConfigurationFeatureDefinitionProvider. So, would it be possible to update the ConfigurationFeatureDefinitionProvider to not set properties if they arent configured? The feature manager classes seem to handle nulls on nullable feature definition properties so it shouldnt break that functionality, but the change could be done based on a new ConfigurationFeatureDefinitionProviderOption to avoid any breaking changes in behaviour (make it opt in).

The properties that would potentially be useful to set to null if not configured are:

• Allocation (although this looks like it is already null if not configured)
• Variants (currently defaults to empty list)
• Telemetry (currently non null and has default values for each property)

This obviously wouldnt guarantee that behaviour for all IFeatureDefinitionProvider implementations, but we would only be decorating the ConfigurationFeatureDefinitionProvider implementation at the moment. We could also make it clear on the expected behaviour of decorated IFeatureDefinitionProvider implementations when using the decorator.

[CharlesLZY]

Hey, @brental

However, if it is in configuration but does not contain values for certain properties those properties are still set. This makes it hard to apply defaults in those cases.

Could you give us an example about this? I am digesting what you proposed, I feel hard to come up with a scenario that it will be beneficial to distinguish empty list and missing configuration.

[brental]

Hey @CharlesLZY,

Thinking through the scenarios for the Variants list more (since that is the only list out of the properties I mentioned), you are probably right that it would not be useful to differentiate between an empty list and missing configuration. We would probably want to apply any default in either case.

The reason I thought of it being set to null if it is not specifically configured is that is what normally happens with lists, dictionaries, objects, etc properties when binding configuration but there is no configuration for the property (assuming they are nullable). So, it seemed more inline with how configuration binding normally works. However, in this case I think we would probably want to apply any default if the Variants list was either null or an empty list. So, you can probably ignore that part of the original description.

Since the Allocation property looks to already be null if not configured that really just leaves the Telemetry property. It would still be good if that could be considered being set to null if not configured as currently I still cannot see a way to easily to determine if it has been configured or not (and thus whether to apply a default). Would adding an option to make that null if not configured be possible? Again, this is what normally happen if a nullable property were not in the configuration. Doing so would make it similar to the Allocation property and make it possible to apply a default when not configured.