How can I use custom data annotations in ASP.NET?

How can I use custom data annotations in ASP.NET?

You can create custom data annotations by inheriting from the ValidationAttribute class and overriding the IsValid method.

Can you give me an example of a custom data annotation?

Sure! Here's a simple example: create a class called 'MustBeAdultAttribute' that checks if a date of birth indicates the person is at least 18 years old.

How do I apply this custom annotation to a model property?

You can apply it by using the attribute above the property in your model class, like this: [MustBeAdult] public DateTime DateOfBirth { get; set; }.

What if I want to add a custom error message?

You can set the ErrorMessage property in your custom attribute class to provide a specific error message when validation fails.