The DataAnnotations namespace provides a set of built-in validation attributes that are applied declaratively to a class or property. DataAnnotations also contains formatting attributes like `DataType` that help with formatting and don't provide any validation.
The validation attributes specify behavior that you want to enforce on the model properties they're applied to:
* The `Required` and `MinimumLength` attributes indicate that a property must have a value; but nothing prevents a user from entering white space to satisfy this validation.
* The `RegularExpression` attribute is used to limit what characters can be input. In the preceding code, "Genre":
* The `Range` attribute constrains a value to within a specified range.
* The `StringLength` attribute lets you set the maximum length of a string property, and optionally its minimum length.
* Value types (such as `decimal`, `int`, `float`, `DateTime`) are inherently required and don't need the `[Required]` attribute.
Having validation rules automatically enforced by ASP.NET Core helps make your app more robust. It also ensures that you can't forget to validate something and inadvertently let bad data into the database.
