ServerSide Validation through validation object

Vidly/Content/Site.css

@@ -22,3 +22,9 @@ select,
 textarea {
     max-width: 280px;
 }
+.field-validation-error{
+    color:red;
+}
+.input-validation-error{
+    border: 2px solid red;
+}

Vidly/Controllers/CustomersController.cs

@@ -43,6 +43,16 @@ namespace Vidly.Controllers
         [HttpPost]
         public ActionResult Save(Customer customer)
         {
+            if (!ModelState.IsValid)
+            {
+                var viewModel = new CustomerFormViewModel
+                {
+                    Customer = customer,
+                    MembershipTypes = _context.MembershipTypes.ToList()
+                };
+                return View("CustomerForm",viewModel);
+            }
+
             if (customer.Id==0)
                 _context.Customers.Add(customer);
             else

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.");
+        }
+    }
+}

Vidly/Vidly.csproj

@@ -247,6 +247,7 @@
     <Compile Include="Models\IdentityModels.cs" />
     <Compile Include="Models\ManageViewModels.cs" />
     <Compile Include="Models\MembershipType.cs" />
+    <Compile Include="Models\Min18YearsIfAMember.cs" />
     <Compile Include="Models\Movie.cs" />
     <Compile Include="Models\MovieGenre.cs" />
     <Compile Include="Properties\AssemblyInfo.cs" />

Vidly/Views/Customers/CustomerForm.cshtml

@@ -17,17 +17,21 @@ else
 
 @using (@Html.BeginForm("Save", "Customers"))
 {
+    @Html.ValidationSummary(true, "Please fix the following errors.")
     <div class="form-group">
         @Html.LabelFor(m => m.Customer.Name)
         @Html.TextBoxFor(m => m.Customer.Name, new { @class = "form-control" })
-    </div>
+        @Html.ValidationMessageFor(m => m.Customer.Name)
-    <div class="form-group">
-        @Html.LabelFor(m => m.Customer.BirthDate)
-        @Html.TextBoxFor(m => m.Customer.BirthDate, "{0:d MMM yyyy}", new { @class = "form-control" })
     </div>
     <div class="form-group">
         @Html.LabelFor(m => m.Customer.MembershipTypeId)
         @Html.DropDownListFor(m => m.Customer.MembershipTypeId, new SelectList(Model.MembershipTypes, "Id", "Name"), "Select membership Type", new { @class = "form-control" })
+        @Html.ValidationMessageFor(m => m.Customer.MembershipTypeId)
+    </div>
+    <div class="form-group">
+        @Html.LabelFor(m => m.Customer.BirthDate)
+        @Html.TextBoxFor(m => m.Customer.BirthDate, "{0:d MMM yyyy}", new { @class = "form-control" })
+        @Html.ValidationMessageFor(m => m.Customer.BirthDate)
     </div>
     <div class="checkbox">
         <label>