Remove DefaultValue support from model binding and do it only in the invoker for action parameters

[rynowak]

MutableObjectModelBinder performs reflection to look up properties and their default values.

https://github.com/aspnet/Mvc/blob/dev/src/Microsoft.AspNet.Mvc.ModelBinding/Binders/MutableObjectModelBinder.cs#L298

We should move this into ModelMetadata so it can be cached, and so that the model binding system is more data driven.

[rynowak]

We should also consider just removing this code.

See: https://msdn.microsoft.com/en-us/library/system.componentmodel.defaultvalueattribute%28v=vs.110%29.aspx

A DefaultValueAttribute will not cause a member to be automatically initialized with the attribute's value.
You must set the initial value in your code.

[rynowak]

Legacy info:

MVC5 and WebAPI2 respected [DefaultValue(...)] applied to a parameter of an action method.

MVC futures and WebAPI2 respected [DefaultValue(...)] for binding model properties.

[Eilon]

Remove DefaultValue support from model binding and do it only in the invoker for action parameters.

[rynowak]

Remove DefaultValue support from model binding and do it only in the invoker for action parameters.

Does this include bound-controller-properties? This affects the shape of the implementation.

Is the goal to limit the spread of a feature that we need for compat? Or to be consistent?

[Eilon]

Bound controller properties are an interesting case. They're more akin to action parameters, so that makes it more interesting.

Having said that, I'm fine not supporting DefaultValue on controller properties for now, and we could add that in the future if there's demand.

[rynowak]

The interesting thing about doing it on both properties/parameters is that the implementation concerns would probably dictate that it be first-class at that point -- ParameterModel, ParameterDescriptor, customizing via convention, etc.

Seems to me like that's a bit overkill, will just do it at a low-level for now and just for parameters 👍

[Eilon]

Yeah that sounds like the best plan for now. Thanks!