Is it correct way to use ModelState.Remove to deal with ModelState?

Question

Im working on a big MVC3 web application and have an annoyance regarding the ModelState.IsValid method.

ModelState is being used in nearly all of my controllers so to validate the data being posted. The views are all based on ViewModels which contain different classes and these classes obviously contain properties which could be marked as [Required].

The problem i am having is the required properties are sometimes not required and im having to use the ModelState.Remove method so that ModelState.IsValid becomes true.

My question is by using ModelState.Remove, is this the correct way of doing things or is there a more efficient approach.

Answer 1

Here's my solution - a RemoveFor() extension method on ModelState, modelled after MVC HTML helpers:

    public static void RemoveFor<TModel>(this ModelStateDictionary modelState,                                           Expression<Func<TModel, object>> expression)     {         string expressionText = ExpressionHelper.GetExpressionText(expression);          foreach (var ms in modelState.ToArray())         {             if (ms.Key.StartsWith(expressionText + ".") || ms.Key == expressionText)             {                 modelState.Remove(ms);             }         }     } 

Here's how it's used :

if (model.CheckoutModel.ShipToBillingAddress == true)  {     // REUSE BILLING ADDRESS FOR SHIPPING ADDRESS     ShoppingCart.ShippingAddress = ShoppingCart.BillingAddress;      // REMOVE MODELSTATE ERRORS FOR SHIPPING ADDRESS     ModelState.RemoveFor<SinglePageStoreModel>(x => model.CheckoutModel.ShippingAddress); } 

So in answer to your question I believe there are definitely use-cases where this is the right way to do it, and a strongly typed helper like this makes it much nicer to look at - and easier to justify if you're concerned about lots of magic strings.

Answer 2

If you're using the same view model with a [Required] property in two different contexts, one where the property is required and one where it isn't, then you'll need to manually alter the ModelState as you're doing.

An alternative is to use a different view model. Perhaps have a base class with all properties except the required property in question. Then derive two view models from it, one with the property where is it required and one with the property where it is not (it's duplication, I know). You may decide to keep them entirely separate altogether and not use inheritance.