Validation in ASP.NET MVC. FluentValidationModelProvider

ModelState always corresponds to model binding system. For any parameter that you action has got, and any validation errors when binding it, ModelState is populated. ModelState is of course on server side, and there's no way to check for it on client side. You should and can check it only in controller actually.

The pattern you posted as an example, is approved pattern for handling post requests in asp.net mvc. First check for ModelState.IsValid gives you information whether client posted values cotain validation errors or not. If there're errors, you return same view populated for client to check values and correct them. If supplied values are valid, ModelState.IsValid returns true and you try to save it to repository. But that repository also may do its internal validation additionally and throw FluentValidation.ValidationException. That's where you need the catch - to catch that validation exception and add it to ModelState, so that mvc system could show validation errors to client

Modify that catch a little bit

[AcceptVerbs(HttpVerbs.Post)] 
public ActionResult Create(Dinner dinner) {
    if(ModelState.IsValid) {
        try {
            dinner.HostedBy = "SomeUser"; 

            dinnerRepository.Add(dinner);
            dinnerRepository.Save();

            return RedirectToAction("Details", new {id = dinner.DinnerID }); 
        } 
        catch(ValidationException ex) 
        {
            ValidationResult result = new ValidationResult(ex.Errors);
            result.AddToModelState(ModelState, string.Empty);
        } 
    } 
    return View(dinner); 
}