ServerSide Validation through validation object

Vidly/Models/Customer.cs

@@ -10,11 +10,12 @@ namespace Vidly.Models
     {
         public int Id { get; set; }
 
-        [Required]
+        [Required(ErrorMessage ="Please enter customer's name.")]
         [StringLength(255)]
         public string Name { get; set; }
 
         [Display(Name = "Date of Birth")]
+        [Min18YearsIfAMember]
         public DateTime? BirthDate { get; set; }
 
         public bool IsSubscribedToNewsLetter { get; set; }

Vidly/Models/MembershipType.cs

@@ -14,5 +14,8 @@ namespace Vidly.Models
         public short SignUpFee { get; set; }
         public byte DurationInMonth { get; set; }
         public byte DiscountRate { get; set; }
+
+        public static readonly byte Unknown = 0;
+        public static readonly byte PayAsYouGo = 1;
     }
 }

Vidly/Models/Min18YearsIfAMember.cs

@@ -0,0 +1,26 @@
+using System;
+using System.Collections.Generic;
+using System.ComponentModel.DataAnnotations;
+using System.Linq;
+using System.Web;
+
+namespace Vidly.Models
+{
+    public class Min18YearsIfAMember : ValidationAttribute
+    {
+
+        protected override ValidationResult IsValid(object value, ValidationContext validationContext)
+        {
+            var customer = (Customer)validationContext.ObjectInstance;
+            if (customer.MembershipTypeId == MembershipType.Unknown ||
+                customer.MembershipTypeId == MembershipType.PayAsYouGo)
+                return ValidationResult.Success;
+            if (customer.BirthDate == null)
+                return new ValidationResult("Birthdate is required");
+            var age = DateTime.Today.Year - customer.BirthDate.Value.Year;
+            return (age > 18)
+                ? ValidationResult.Success
+                : new ValidationResult("Customer should be at least 18 years old to go on a membership.");
+        }
+    }
+}