MVC UpdateModel and Sub Classes vs Base Class

I'm looking to use the UpdateModel method to a Sub Class that retrieved at runtime, would be great if someone could shed the light on whether I'm making a total hash of it and/or whether or not what I'm trying to do is possible.

I'm using a generic action to control the validation of a bunch of partial views; I'm trying to get away from having a specific action per partial view.

Each partial view has a unique Model that derives from a Base Model:

public class ModelA : ModelBase{
     [Required]
     public string SomeStringProperty{get;set;}
...
}
public class ModelB : ModelBase{
     [Required]
     public DateTime? SomeDateProperty{get;set;}
...
}
public class ModelBase{
     public Guid InstanceId{get;set;}
}

I'm using the FormCollection on the Action to get the submitted form elements and their values, this includes the type of model that the View should be using to validate its request. Ignore the security implications of this for this example, I'm aware of them and this is an internal only proof of concept

    [HttpPost]
    public ActionResult ChangeCaseState(int id, FormCollection formCollection)
    {
        Guid instanceId = new Guid(formCollection["instanceId"]);
        string modelType = formCollection["modelType"];
        //Return a specific Model class based on the event/modelType
        var args = GetStateModelClass(modelType, instanceId);

        try
        {
            UpdateModel(args);
            if(Model.IsValid){
             ...
        }
        catch (Exception)
        {
            return View("~/Views/Shared/StateForms/" + modelType + ".ascx", 开发者_如何学运维args);
        }...

And here is the code I'm using to return a Sub Class based on the modelType passed to the controller.

private static ModelBase StateModelClassFactory(string stateModelTypeName, Guid instanceId)
        {
            switch (stateModelTypeName)
            {
                case "modelTypeA":
                    return new ModelA(workflowInstanceId);
                case "modelTypeB":
                    return new ModelB(workflowInstanceId);
    ...
    }

Because the return type of the StateModelClassFactory method is of the Base Class, even though I'm actually returning a Sub Class, the Model Binder used by the UpdateModel method only binds against the values within the Base Class.

Any ideas on how I can solve this problem?

UPDATE:

I created a Customer Model Binder:

public class CustomModelBinder : DefaultModelBinder
    {
      public override object BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext)
            {

And Assigned the new Model Binder to the correct Base Class to see what was going on a little more under the hood:

ModelBinders.Binders.Add(typeof(ModelBase), new CaseController.CustomModelBinder());

When I debug the model binder and inspect the bindingContext, the Model property represets the correct Sub Class, but the ModelType property is that of the Base Class. Should I be looking at changing the ModelType within the BindModel method? If so any pointers on how to do this, the setter on the ModelType seems to have been made redundant. I also noticed that the SomeDateProperty from the Sub Class is actaully in the PropertyMetadata property....Seems so close to behaving as I'd like.

I just ran into this particular issue and found that a better general approach would be just to cast your model to dynamic while passing it into UpdateModel:

[HttpPost]
public ActionResult ChangeCaseState(int id, FormCollection formCollection)
{
    ...try
    {
        UpdateModel((dynamic)args);//!!notice cast to dynamic here
        if(Model.IsValid){
         ...
    }
    catch...

This appears to set all available properties of my type, regardless of whether my variable is delcared with the base type.

There's a work item filed in CodePlex for this issue: http://aspnet.codeplex.com/workitem/8277?ProjectName=aspnet

So I think I've solved my problem. Basically because of the way that I'm retrieving the Model class before calling the UpdateModel, the Model Binder is binding the BaseClass even though the Model was that of the SubClass - this is the code I used to solve my particular problem:

  public class SubClassModelBinder : DefaultModelBinder
{
    public override object BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext)
    {
        var model = bindingContext.Model;
        var metaDataType = ModelMetadataProviders.Current.GetMetadataForType(null, model.GetType());
        bindingContext.ModelMetadata = metaDataType;
        bindingContext.ModelMetadata.Model = model;

        return base.BindModel(controllerContext, bindingContext);
    }
}

And in the Global.asax:

ModelBinders.Binders.Add(typeof(ModelBase), new SubClassModelBinder ());

Thanks to Darin for his inital pointer.

To solve this problem you could write a custom model binder for the base type which based on the value of the string property will return the correct child instance.