Discuss adding server side validation to value type model properties by default.

[NTaylorMullen]

Today in MVC6 having the following model:

public class Person
{
    public int Id { get; set; }
}

Will result in client side validation with 0 server side validation (unless specifying BindingBehaviorAttribute with BindingBehavior.Required).

This differs from what happened in MVC5; model state would be invalid due to server side validation opting in ValueType properties. However, in WebAPI not providing a value for a ValueType property when model binding was 100% ok and did not result in any ModelState errors.

Given the different behavior in the old MVC/WebAPI systems we should discuss the right approach for this in MVC6.

Related conversation that spurred this: #2720 (comment)

/cc @rynowak @Eilon @dougbu

[rynowak]

Decision here is to bring back the implicit [BindRequired] for value-types. If you need to make something optional, it can be customized:

• using [BindingBehavior(BindingBehavior.Optional)]
• a custom MMP
• using a nullable type (int?)

[rynowak]

We've done more investigation here, and the data above isn't correct about MVC 5's behavior.

In MVC 5 the absence of a field does not result in a validation error for a value type property. You only get the validation error if you submit an empty value.

This is critical for compatibility because Create views that scaffolding generates will omit the Id for the model. With the fix in a6ce9ab - you'll see an error in this very common scenario.

[rynowak]

07fabde