TryUpdateModel: Determine model type at runtime

[maxtoroq]

The TryUpdateModel methods require that you explicitly declare the type you are binding at compile time, and if you attempt to bind a subclass instance the subclass properties are ignored. This is because the model metadata is requested using the type parameter:

var modelMetadata = metadataProvider.GetMetadataForType(
    modelAccessor: null,
    modelType: typeof(TModel));

...instead of using the type of the model instance:

var modelMetadata = metadataProvider.GetMetadataForType(
    modelAccessor: null,
    modelType: model.GetType());

Even with this small change you are still forced to provide the TModel type parameter, but at least you can pass a base type for TModel and have a derived instance be properly updated.

[pranavkm]

We now have precedence in doing this via TryValidateModel - https://github.com/aspnet/Mvc/blob/dev/src/Microsoft.AspNet.Mvc.Core/Controller.cs#L1062-L1064

[dougbu]

true though TryUpdateModel<TModel> and TryValidateModel are also inconsistent in MVC 5. so there's a back-compat argument in favour of leaving MVC 6 as-is in this respect. not sure how strong that argument is 😺

if we do make a change here, probably still want to prevent boxing and attempting to update a value type model. the type parameter avoids that problem and moves errors to compile-time.

[maxtoroq]

I didn't suggest removing TModel because the includeExpressions parameter needs it, although you could have one overload with TModel and the others without it.

[dougbu]

@maxtoroq perhaps. then again, the use of TModel in includeExpressions is an argument in favour of using TModel everywhere in this method group. would be confusing otherwise.

[maxtoroq]

@dougbu I agree. Although I haven't tested it, perhaps simply using object for TModel works. If it does then the only change needed is getting the modelType from the model instance.