BarController.cs

// Or if you use binding by parameter, don't do anything at all :-)

public ActionResult Create(Bar bar)
{
    if (ModelState.IsValid)
    {
        // Do something;
    }

    // Do something else;
}

FooController.cs

// Use the TryUpdateModel() rather than UpdateModel() - the latter throws an exception when errors are detected.

public ActionResult Edit(FormCollection form)
{
    Foo foo = FindFoo();
    
    TryUpdateModel(foo);

    if (ModelState.IsValid)
    {
        // Do something;
    }

    // Do something else;
}

Global.asax.cs

// Hook up the new model binder in your application.

protected void Application_Start()
{
    ModelBinders.Binders.DefaultBinder = new NHibernateValidatorAwareModelBinder();
}

NHibernateValidatorAwareModelBinder.cs

// Define a new type of IModelBinder, which automatically invokes NHV when binding.

using System.Web.Mvc;

public class NHibernateValidatorAwareModelBinder : DefaultModelBinder
{
    public override object BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext)
    {
        var model = base.BindModel(controllerContext, bindingContext);
        var controller = controllerContext.Controller as Controller;

        if (controller != null)
        {
            var engine = NHibernate.Validator.Cfg.Environment.SharedEngineProvider.GetEngine();
            foreach (var error in engine.Validate(model))
            {
                controller.ModelState.AddModelError(error.PropertyName, error.Message);
            }
        }

        return model;
    }
}