Hands-On RESTful Web Services with ASP.NET Core 3

Custom validation attributes

ASP.NET Core provides a way for us to create custom validations for our requests by extending ValidationAttribute, which means we can create custom validators for our types. Let's create a more appropriate validation for our Currency attribute:

using System.Collections.Generic;using System.ComponentModel.DataAnnotations;using System.Linq;namespace SampleAPI.Requests{    public class CurrencyAttribute : ValidationAttribute    {        private readonly IList<string> _acceptedCurrencyCodes =         new List<string>{            "EUR",            "USD",            "GBP"        };        protected override ValidationResult IsValid(object value,         ValidationContext validationContext)        {            return _acceptedCurrencyCodes.Any(c => c == value.ToString()) ?                ValidationResult.Success                 : new