public class GenericRequired: RequiredAttribute
{
    public GenericRequired()
    {
        this.ErrorMessage = "{0} Blah blah"; 
    }
}

It seems that RequiredAttribute doesn t implement IClientValidatable, so if you override the RequiredAttribute it breaks client side validation.

So this is what I did and it works. Hope this helps someone.

public class CustomRequiredAttribute : RequiredAttribute, IClientValidatable
{
    public CustomRequiredAttribute()
    {
        this.ErrorMessage = "whatever your error message is";
    }

    public IEnumerable<ModelClientValidationRule> GetClientValidationRules(ModelMetadata metadata, ControllerContext context)
    {
        yield return new ModelClientValidationRule
        {
            ErrorMessage = this.ErrorMessage,
            ValidationType = "required"
        };
    }
}

Here is a way to do it without subclassing RequiredAttribute. Just make some attribute adapter classes. Here I m using ErrorMessageResourceType/ErrorMessageResourceName (with a resource) but you could easily set ErrorMessage, or even check for the existence of overrides before setting these.

Global.asax.cs:

public class MvcApplication : HttpApplication {
    protected void Application_Start() {
        // other stuff here ...
        DataAnnotationsModelValidatorProvider.RegisterAdapter(
            typeof(RequiredAttribute), typeof(CustomRequiredAttributeAdapter));
        DataAnnotationsModelValidatorProvider.RegisterAdapter(
            typeof(StringLengthAttribute), typeof(CustomStringLengthAttributeAdapter));
    }
}

private class CustomRequiredAttributeAdapter : RequiredAttributeAdapter {
    public CustomRequiredAttributeAdapter(ModelMetadata metadata, ControllerContext context, RequiredAttribute attribute)
        : base(metadata, context, attribute)
    {
        attribute.ErrorMessageResourceType = typeof(Resources);
        attribute.ErrorMessageResourceName = "ValRequired";
    }
}

private class CustomStringLengthAttributeAdapter : StringLengthAttributeAdapter {
    public CustomStringLengthAttributeAdapter(ModelMetadata metadata, ControllerContext context, StringLengthAttribute attribute)
        : base(metadata, context, attribute)
    {
        attribute.ErrorMessageResourceType = typeof(Resources);
        attribute.ErrorMessageResourceName = "ValStringLength";
    }
}

This example would have you create a resource file named Resources.resx with ValRequired as the new required default message and ValStringLength as the string length exceeded default message. Note that for both, {0} receives the name of the field, which you can set with [Display(Name = "Field Name")].

Just change

[Required] 

to

[Required(ErrorMessage = "Your Message, Bla bla bla aaa!")]

Edit: I posted this because I was looking for this solution with [Required] attribute and had problem to find it. Creating new Q/A don t look that good, since this question already exists.

I was trying to solve the same thing and I found very simple solution in MVC 4.

First, you would probably have to store error messages in some place. I used custom resources, well described at http://afana.me/post/aspnet-mvc-internationalization.aspx.

Important is, that I can get any resource (even error message) by simply calling ResourcesProject.Resources.SomeCustomError or ResourcesProject.Resources.MainPageTitle etc. Everytime I try to access Resources class, it takes culture info from current thread and returns right language.

I have error message for field validation in ResourcesProject.Resources.RequiredAttribute. To set this message to appear in View, simply update [Required] attribute with these two parameters.

ErrorMessageResourceType - Type which will be called (in our example ResourcesProject.Resources)

ErrorMessageResourceName - Property, which will be called on the type above (In our case RequiredAttribute)

Here is a very simplified login model, which shows only username validation message. When the field is empty, it will take the string from ResourcesProject.Resources.RequiredAttribute and display this as an error.

    public class LoginModel
    {        
        [Required(ErrorMessageResourceType = typeof(Resources.Resources), ErrorMessageResourceName="RequiredAttribute")]
        [Display(Name = "User name")]
        public string UserName { get; set; }
}

You can write your own attribute:

public class MyRequiredAttribute : ValidationAttribute
{
    MyRequiredAttribute() : base(() => "{0} blah blah blah blaaaaaah")
    {

    }

    public override bool IsValid(object value)
    {
        if (value == null)
        {
            return false;
        }
        string str = value as string;
        if (str != null)
        {
            return (str.Trim().Length != 0);
        }
        return true;
    }
}

This is copy of RequiredAttribute from Reflector with changed error message.

using Resources;
using System;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
using System.Linq;
using System.Reflection;
using System.Resources;
using System.Web;
using System.Web.Mvc;

public class CustomRequiredAttribute : RequiredAttribute,  IClientValidatable
{
    public CustomRequiredAttribute()
    {
        ErrorMessageResourceType = typeof(ValidationResource);
        ErrorMessageResourceName = "RequiredErrorMessage";
    }


    public IEnumerable<ModelClientValidationRule> GetClientValidationRules(ModelMetadata metadata, ControllerContext context)
    {


        yield return new ModelClientValidationRule
        {
            ErrorMessage = GetRequiredMessage(metadata.DisplayName),
            ValidationType = "required"
        };
    }


    private string GetRequiredMessage(string displayName) {

        return displayName + " " + Resources.ValidationResource.RequiredErrorMessageClient;        

    }


}

App_GlobalResources/ValidationResource.resx

Now use data annotation

[CustomRequired]

This worked for me. Carefully read the comments inside the code. (Based on Chad s answer).

 // Keep the name the same as the original, it helps trigger the original javascript 
 // function for client side validation.

        public class RequiredAttribute : System.ComponentModel.DataAnnotations.RequiredAttribute, IClientValidatable
            {
                public RequiredAttribute()
                {
                    this.ErrorMessage = "Message" // doesnt get called again. only once.
                }

                public IEnumerable<ModelClientValidationRule> GetClientValidationRules(ModelMetadata metadata, ControllerContext context)
                {
                    yield return new ModelClientValidationRule
                    {
                        ErrorMessage = "Message", // this gets called on every request, so it s contents can be dynamic.
                        ValidationType = "required"
                    };
                }
            }