Setting ModelState values in custom model binder - asp.net-mvc-2

Question

I am using custom model binder in ASP.NET MVC 2 that looks like this:
public override object BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext)
{
if (controllerContext == null)
{
throw new ArgumentNullException("controllerContext");
}
if (bindingContext == null)
{
throw new ArgumentNullException("bindingContext");
}
BaseContentObject obj = (BaseContentObject)base.BindModel(controllerContext, bindingContext);
if(string.IsNullOrWhiteSpace(obj.Slug))
{
// creating new object
obj.Created = obj.Modified = DateTime.Now;
obj.ModifiedBy = obj.CreatedBy = controllerContext.HttpContext.User.Identity.Name;
// slug is not provided thru UI, derivate it from Title; property setter removes chars that are not allowed
obj.Slug = obj.Title;
ModelStateDictionary modelStateDictionary = bindingContext.ModelState;
modelStateDictionary.SetModelValue("Slug", new ValueProviderResult(obj.Slug, obj.Slug, null));
...
When I get back from this binder into controller action, my business object that is provided as a parameter to the action is correctly altered (the lines obj.Created = .... work).
However, the ModelState is not updated. I know this because I have Required on my business object's Slug property and although I altered ModelStateDictionary in my custom model binder, providing a Slug to it (as you can see above), the ModelState.IsValid is still false.
If I put ModelState["Slug"] in my Watch window in Debug session, it says it has Errors (1), so apparently it is empty and as such fails.
How can I correctly alter the ModelState inside the custom model binder code?

Answer 1

Apparently there is no way to revalidate the ModelState once you change a value of some key. The IsValid remains false because setting a new value to some key does not trigger revalidation.
The solution is to first remove the key that triggered IsValid to be false and recreate it and assign the value to it. When you do that the ModelState automatically revalidates and if everything is fine, IsValid returns true.
Like this:
bindingContext.ModelState.Remove("Slug");
bindingContext.ModelState.Add("Slug", new ModelState());
bindingContext.ModelState.SetModelValue("Slug", new ValueProviderResult(obj.Slug, obj.Slug, null));