How does client side validation know that ID is required without DataAnnotation attribute

I have a partal view for my create a开发者_运维问答nd edit views.

In that partial is a: @Html.HiddenFor(model => model.ID)

In my POCO that property is not decorated with the [Required] attribute but when I go to the create page and click save the client side validation kicks in and says ID is required.

Why does it think that?

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Integers are required by default unless you define it as int? (nullable int)

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Value types are implicitly validated by the MVC framework.

To turn this off set DataAnnotationsModelValidatorProvider.AddImplicitRequiredAttributeForValueTypes to false in global.asax