ConfigurationBinder: Allow specifying default values

[divega]

After PR #282 is pushed we will have the ability to bind the values of configuration sections to scalars and also entire configuration sections to structural objects, e.g.:

var timeout = config.Get<int>("timeout");

I have been looking at how this API could be used and I think a lot of friction would go away if we accepting inline default values, e.g.:

var timeout = config.Get<int>("timeout", defaultValue: 30);
// or even simpler, inferring the type from the default value:
var timeout = config.Get("timeout", defaultValue: 30);

Design & implementation notes:

1. This default values can work with structural objects as well: an instance with default property values is passed and any properties with matching configuration values are overwritten. Do we still need the public Bind() method after this?
2. The private BindInstance() method already returns the passed instance if no value could be read or no properties could be set. Seems that we can leverage that behavior.

[HaoK]

Personally I think its much more clear with the current explicit Get/Bind semantics

Where Bind() you pass in your instance
Get returns you a new instance

[HaoK]

I see where you were going with using the defaultValue for bind instance but I think that's a bit weird. I would think the only time you want to use the defaultValue is if config was null, not to use it as the base instance and override. That seems really weird to me

[HaoK]

My initial take on this was to just add a bunch of overloads to Get which allow specifying the default value, and leaving BindInstance alone completely.

[divega]

The way I understand your concern is that for mutable instances the parameter means something different from 'defaultValue'. It seems to me that:

1. What the overload of the method will do for mutable instances (overwrite property values with values from configuration) is still very useful.
2. It would be harder to block the scenario than to just let it work.

This is what I was trying to tell you a few days ago when I said I wasn't sure about the name of the parameter.

[HaoK]

The way my PR is written today we don't use the defaultValue for bind... Because its just a default :)

[divega]

So you mean it replaces the object completely?

[HaoK]

The proper name for what you describe is initialValue. No, today we only use defaultValue if config is null.

[HaoK]

that's a very simple and understandable behavior. If its not found in configuration, use the default value. It has no mapping on Bind, since you already have an instance for bind. That IS your default value :)

[HaoK]

To put this another way, if I already have an object that I'm trying to bind, and I want to replace it with a different value? I can just assign the variable no? Going through Bind to do the replacement just feels weird

[HaoK]

I currently only used default value in one place, when config is null via the core Get method:

https://github.com/aspnet/Configuration/pull/309/files#diff-ad58cb8470446e5e357497cee03e8e19R33