How can I use data annotations for validation in ASP.NET?

How can I use data annotations for validation in ASP.NET?

You can use data annotations in ASP.NET to enforce validation rules on your model properties. For example, you can use attributes like [Required], [StringLength], and [Range] to specify validation criteria.

Can you give me an example of using the [Required] attribute?

Sure! If you have a model class like this:

public class User {
    [Required(ErrorMessage = "Username is required.")]
    public string Username { get; set; }
}

The [Required] attribute ensures that the Username property must have a value.

What happens if the validation fails?

If validation fails, the model state will be invalid, and you can check this in your controller. You can return validation errors to the user, typically through the view or an API response.

Are there any other validation attributes I should know about?

Yes, there are several useful attributes like [EmailAddress], [Phone], and [RegularExpression] that help enforce specific formats and rules.