Updated Wiki: Getting started

Create a new solution, containing 2 projects. One is MVC project and another one is class library

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Localized resources supposed to be in the LocalizedResources project. MvcDemoApp has dependencies from LocalizedResources project.

Add new DisplayResources.resx file to the LocalizedResources project.

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Add new T4 template to LocalizedResources  project and rename  it to match your ResX file:

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Add the following content to the T4 template:

Save the T4 Template file. A new class, containing the constants will be generated for you:

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Every time you add or modify ResX file you need to rerun T4 template in order to generated constant keys for your meesages.

Now the constants are ready to be used with DataAnnotations attributes.

Add a new Person class to your MVC Project:

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Use generated constants for providing a resource key for DisplayAttribute. The Display Attribute requires additional parameterResourceType to run, but we will replace it generally for all our Model classes.

For this purpose add a new DataAnnotationsLocalizer class to LocalizedDataAnnotations project:

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replace its content with following C# code:

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.ComponentModel.DataAnnotations;
using System.Linq;
using System.Reflection;
using System.Text;
using System.Threading.Tasks;

namespace LocalizedResources
{
    public sealed class DataAnnotationsLocalizer
    {
        public static void SetDefaultResourceType(Assembly assembly, Type resources, params Type[] localizableAttributeTypes)
        {
            foreach (var item in GetTypesInNamespace(assembly, null))
            {
                SetDefaultResourceType(item, resources, localizableAttributeTypes);

            }
        }

        public static void SetDefaultResourceType(Assembly assembly, string nameSpace, Type resources, params Type[] localizableAttributeTypes)
        {
            foreach (var item in GetTypesInNamespace(assembly, nameSpace))
            {
                SetDefaultResourceType(item, resources, localizableAttributeTypes);
            }
        }

        public static void SetDefaultResourceType(Type item, Type resources, params Type[] localizableAttributeTypes)
        {
            foreach (PropertyDescriptor prop in TypeDescriptor.GetProperties(item))
            {
                foreach (var localizableAttrType in localizableAttributeTypes)
                {
                    Attribute dAttr = ((Attribute)prop.Attributes[localizableAttrType]);
                    if (dAttr != null)
                    {
                        if (dAttr is DisplayAttribute && (dAttr as DisplayAttribute).Name != null)
                            ((DisplayAttribute)dAttr).ResourceType = resources;
                        if (dAttr is ValidationAttribute && (dAttr as ValidationAttribute).ErrorMessageResourceName != null)
                            ((ValidationAttribute)dAttr).ErrorMessageResourceType = resources;

                    }
                }

            }
        }

        public static void ReplaceDefaultLocalizedMessage<T>(Assembly assembly, Type resources, string messageKey) where T : ValidationAttribute
        {
            foreach (var item in GetTypesInNamespace(assembly, null))
            {
                foreach (PropertyDescriptor prop in TypeDescriptor.GetProperties(item))
                {
                    ValidationAttribute dAttr = ((ValidationAttribute)prop.Attributes[typeof(T)]);
                    if (dAttr != null)
                    {
                        dAttr.ErrorMessageResourceType = resources;
                        dAttr.ErrorMessageResourceName = messageKey;

                    }

                }

            }
        }

        public static void ReplaceDefaultLocalizedMessage<T>(Assembly assembly, string nameSpace, Type resources, string messageKey) where T : ValidationAttribute
        {
            foreach (var item in GetTypesInNamespace(assembly, nameSpace))
            {
                ReplaceDefaultLocalizedMessage<T>(item ,resources, messageKey);

            }
        }

        public static void ReplaceDefaultLocalizedMessage<T>(Type item, Type resources, string messageKey) where T : ValidationAttribute
        {
            foreach (PropertyDescriptor prop in TypeDescriptor.GetProperties(item))
            {
                ValidationAttribute dAttr = ((ValidationAttribute)prop.Attributes[typeof(T)]);
                if (dAttr != null)
                {
                    dAttr.ErrorMessageResourceType = resources;
                    dAttr.ErrorMessageResourceName = messageKey;
                }

            }
        }

        private static Type[] GetTypesInNamespace(Assembly assembly, string nameSpace)
        {
            if (nameSpace != null)
                return assembly.GetTypes().Where(t => String.Equals(t.Namespace, nameSpace, StringComparison.Ordinal)).ToArray();
            else
                return assembly.GetTypes();
        }
    }
}

This class is a helper, that will allow us to set a resource source for our DataAnnotationAttributes.

It can be done, for example, in the Global.asax:

protected void Application_Start()
        {
                        DataAnnotationsLocalizer.SetDefaultResourceType(typeof(Person).Assembly, typeof(DisplayNames), typeof(DisplayAttribute));

The SetDefaultResourceType call above, sets a ResourceType  propery of DisplayAttribute to point to the proper Resource type. 

Now our application is ready to run with localized DisplayAttribute. There is an Example of Controller and View classes:

Controller:

public class DemoController : Controller
    {
        //
        // GET: /Demo/

        public ActionResult Index()
        {
            return View(new Person());
        }

        [HttpPost]
        public ActionResult Save(Person p)
        {
            return View("Index", p);
        }

        public ActionResult SetLanguage(string language)
        {
            Session["SelectedCultureId"] = language;
            return RedirectToAction("Index");
        }

        protected override void Initialize(System.Web.Routing.RequestContext requestContext)
        {
            string culture = (string)requestContext.HttpContext.Session["SelectedCultureId"];
            if (culture != null)
            {
                System.Threading.Thread.CurrentThread.CurrentUICulture = new System.Globalization.CultureInfo(culture);
                System.Threading.Thread.CurrentThread.CurrentCulture = new System.Globalization.CultureInfo(culture);
            }
            base.Initialize(requestContext);
        }

    }

View:

@model MvcDemoApp.Models.Person
@{
    ViewBag.Title = "Demo";
    Layout = "~/Views/Shared/_Layout.cshtml";
}

<h2>Localization via T4 Template Demo</h2>

@Html.ActionLink("Englisch", "SetLanguage", "Demo", new { language = "en-us" }, null)
@Html.ActionLink("Deutsch", "SetLanguage", "Demo", new { language = "de-de" }, null)

@using(Html.BeginForm("Save", "Demo"))
{
    @Html.ValidationSummary(true);
<fieldset>
    <legend>Person</legend>
    @Html.LabelFor(m => Model.FirstName)
    @Html.EditorFor(m => Model.FirstName)
    @Html.ValidationMessageFor(m => Model.FirstName)
    <br />
    @Html.LabelFor(m => Model.LastName)
    @Html.EditorFor(m => Model.LastName)
    @Html.ValidationMessageFor(m => Model.LastName)
    <br />
    @Html.LabelFor(m => Model.Phone)
    @Html.EditorFor(m => Model.Phone)
    @Html.ValidationMessageFor(m => Model.Phone)
    <br />
    @Html.LabelFor(m => Model.Address)
    @Html.EditorFor(m => Model.Address)
    @Html.ValidationMessageFor(m => Model.Address)
    <br />
    <input type="submit" name="btnSubmit" id="SubmitButton" value="Submit" />
</fieldset>
}

If you run this application, and switch the language to German, you will see the following result:

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That works fine, although if we post a form, we will see that the Validation messages are not localized:

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The problem is that the default .NET installation does not contain  a translation for other languages. In order to get ValidationAttributes to be localized you need to install .NET language pack

http://www.microsoft.com/de-de/download/details.aspx?id=3324

What is if you are not happy with default Microsoft localization for ValidationAttributes?

Actually, it is not a problem. Create your local copy of messages for each ValidationAttribute:

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And then use DataAnottationsLocalizer class to set your custom Validation messages:

protected void Application_Start()
{
DataAnnotationsLocalizer.SetDefaultResourceType(typeof(Person).Assembly, typeof(DisplayNames), typeof(DisplayAttribute));

 

 DataAnnotationsLocalizer.ReplaceDefaultLocalizedMessage<RequiredAttribute>(

 

 typeof(Person).Assembly,typeof(Person).Namespace,typeof(ErrorMessages),ErrorMessages.Keys.RequiredMessage);

 

 DataAnnotationsLocalizer.ReplaceDefaultLocalizedMessage<MaxLengthAttribute>(

 

 typeof(Person).Assembly,typeof(Person).Namespace,typeof(ErrorMessages),ErrorMessages.Keys.MaxLengthMessage);

 

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In this way you can overwrite each ValidationAttribute that default message you want to be changed.

 That works fine, but what is if you need a custom localized validation message for a certain field? It works also. Specify  a ErrorMessageResourceName on the required ValidationAttribute

public class Person:IValidatableObject 
    {
        [Display(Name = DisplayNames.Keys.FirstName)]
        [Required(ErrorMessageResourceName=ErrorMessages.Keys.CustomModelError)]
        public string FirstName { get; set; }

And call aDataAnnotationsLocalizer for required attribute type in your ApplicationStart:

DataAnnotationsLocalizer.ReplaceDefaultLocalizedMessage<RequiredAttribute>(
                typeof(Person).Assembly, typeof(Person).Namespace, typeof(ErrorMessages), ErrorMessages.Keys.RequiredMessage);

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Provided solution works fine in both MVC and non MVC scenarios. For example if you want to validate an Object outside of MVC, you will get the same localized messages:

class Program
    {
        static void Main(string[] args)
        {
            DataAnnotationsLocalizer.SetDefaultResourceType(typeof(Person).Assembly, typeof(DisplayNames), typeof(DisplayAttribute));
            DataAnnotationsLocalizer.SetDefaultResourceType(typeof(Person).Assembly,
                typeof(ErrorMessages),
                typeof(RequiredAttribute),
                typeof(MaxLengthAttribute));

            DataAnnotationsLocalizer.ReplaceDefaultLocalizedMessage<RequiredAttribute>(
                typeof(Person).Assembly, typeof(Person).Namespace, typeof(ErrorMessages), ErrorMessages.Keys.RequiredMessage);
            DataAnnotationsLocalizer.ReplaceDefaultLocalizedMessage<MaxLengthAttribute>(
                typeof(Person).Assembly, typeof(Person).Namespace, typeof(ErrorMessages), ErrorMessages.Keys.MaxLengthMessage);

            try
            {
                System.Threading.Thread.CurrentThread.CurrentUICulture = new System.Globalization.CultureInfo("de-de");
                System.Threading.Thread.CurrentThread.CurrentCulture = new System.Globalization.CultureInfo("de-de");

                Person p = new Person { Address = "asdkjalsdkjalskdjasldkjalskdjaskldjaskldjalasdasdasdasdasdasdskdjalskdjaskldjalsdkjalsdk" };

                List<ValidationResult> res = new List<ValidationResult>();
                Validator.TryValidateObject(p, new ValidationContext(p, null, null), res, true);

                foreach (var item in res)
                {
                    Console.WriteLine(item.ErrorMessage);
                }
            }
            catch (ValidationException)
            {

            }

            Console.ReadLine();

        }
    }

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Download a full working example from Source Code session.