Data Annotation

using System;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
using System.Linq;

namespace MyAwesomeMVCApp.Models{
  public class Customer
  {
      [Required]
      [RegularExpression("^[A-Z]{3,3}[0-9]{4,4}$")]
      public string CustomerCode { get; set; }

      [Required]
      [StringLength(25)]
      public string CustomerName { get; set; }

  }

}

The [Required], [StringLength] and [RegularExpression] method decorators in the above code sample are the data annotations.

When do these validations happen

As soon as the model object is instantiated from the request or response data, the data annotation validations are run and if any errors are found, the errors are sent to ModelState class.