ASP.NET MVC 3 Remote Validation

Unfortunately that's how this attribute is implemented. Here's an excerpt from the constructor of this attribute:

public RemoteAttribute(string action, string controller, string areaName) : this()
{
    if (string.IsNullOrWhiteSpace(action))
    {
        throw new ArgumentException(MvcResources.Common_NullOrEmpty, "action");
    }
    if (string.IsNullOrWhiteSpace(controller))
    {
        throw new ArgumentException(MvcResources.Common_NullOrEmpty, "controller");
    }
    this.RouteData["controller"] = controller;
    this.RouteData["action"] = action;
    if (!string.IsNullOrWhiteSpace(areaName))
    {
        this.RouteData["area"] = areaName;
    }
}

Notice the IsNullOrWhiteSpace test against the areaName at the end that's killing everything?

You could fix it by writing a custom remote attribute:

public class MyRemoteAttribute : RemoteAttribute
{
    public MyRemoteAttribute(string action, string controller, string area)
        : base(action, controller, area)
    {
        this.RouteData["area"] = area;
    }
}

and then:

[MyRemote("IsValidUserName", "Validation", "", ErrorMessage = "Invalid username")]
public string Username { get; set; }

Now the proper data-val-remote-url="/Validation/IsValidUserName" will be generated.