ServerSide Validation through validation object

Vidly/Models/Min18YearsIfAMember.cs

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+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.");
+        }
+    }
+}