How to code a polymorphic model of Binder and a provider in MVC 6

Question

This question was asked earlier in SO and elsewhere in the context of MVC3, and it has bits and beans related to ASP.NET Core RC1 and RC2, but not a single example that actually shows how to do it correctly in MVC 6 .

The following classes exist

 public abstract class BankAccountTransactionModel {

    public long Id { get; set; }
    public DateTime Date { get; set; }
    public decimal Amount { get; set; }
    public readonly string ModelType;
    public BankAccountTransactionModel(string modelType) {
        this.ModelType = modelType;
    }
 }

 public class BankAccountTransactionModel1 : BankAccountTransactionModel{

    public bool IsPending { get; set; }

    public BankAccountTransactionModel1():
            base(nameof(BankAccountTransactionModel1)) {}
 }    

 public class BankAccountTransactionModel2 : BankAccountTransactionModel{

    public bool IsPending { get; set; }

    public BankAccountTransactionModel2():
            base(nameof(BankAccountTransactionModel2)) {}
 }

In my controller, I have something like this

[Route(".../api/[controller]")]  
public class BankAccountTransactionsController : ApiBaseController 
{

 [HttpPost]
 public IActionResult Post(BankAccountTransactionModel model) {

        try {
            if (model == null || !ModelState.IsValid) {
                // failed to bind the model
                return BadRequest(ModelState);
            }

            this.bankAccountTransactionRepository.SaveTransaction(model);

            return this.CreatedAtRoute(ROUTE_NAME_GET_ITEM, new { id = model.Id }, model);
        } catch (Exception e) {

            this.logger.LogError(LoggingEvents.POST_ITEM, e, string.Empty, null);
            return StatusCode(500);
        }
    }

}        

My client can publish either BankAccountTransactionModel1 or BankAccountTransactionModel2, and I would like to use a custom mediator to define a specific concrete model for binding based on the value in the ModelType property, which is defined in the abstract base class BankAccountTransactionModel.

So I did the following

1) , , BankAccountTransactionModel. , BankAccountTransactionModelBinder.

public class BankAccountTransactionModelBinderProvider : IModelBinderProvider {

    public IModelBinder GetBinder(ModelBinderProviderContext context) {

        if (context == null) throw new ArgumentNullException(nameof(context));

        if (context.Metadata.IsComplexType && !context.Metadata.IsCollectionType) {     

            var type1 = context.Metadata.ModelType;
            var type2 = typeof(BankAccountTransactionModel);

            // some other code here?

            // tried this but not sure what to do with it!
            foreach (var property in context.Metadata.Properties) {
                propertyBinders.Add(property, context.CreateBinder(property));
            }

            if (type1 == type2) {
                return new BankAccountTransactionModelBinder(propertyBinders);
            }
        }

        return null;
    }
}

2) BankAccountTransactionModel

 public class BankAccountTransactionModelBinder : IModelBinder {


 private readonly IDictionary<ModelMetadata, IModelBinder> _propertyBinders;

 public BankAccountTransactionModelBinder(IDictionary<ModelMetadata, IModelBinder> propertyBinders){
                this._propertyBinders = propertyBinders;
                  }
 public Task BindModelAsync(ModelBindingContext bindingContext) {

        if (bindingContext == null) throw new ArgumentNullException(nameof(bindingContext));            

        // I would like to be able to read the value of the property
        // ModelType like this or in some way...
        // This does not work and typeValue is...
        var typeValue = bindingContext.ValueProvider.GetValue("ModelType");

        // then once I know whether it is a Model1 or Model2 I would like to
        // instantiate one and get the values from the body of the Http  
        // request into the properties of the instance  

        var model = Activator.CreateInstance(type);

        // read the body of the request in some way and set the 
        // properties of model                                                                                                           

        var key = some key? 
        var result = ModelBindingResult.Success(key, model);

        // Job done  
        return Task.FromResult(result);
    }

 }

3) , Startup.cs

public void ConfigureServices(IServiceCollection services)
    {       
        services.AddMvc(options => {
            options.ModelBinderProviders.Insert(0, new BankAccountTransactionModelBinderProvider());
            options.Filters.Add(typeof (SetUserContextAttribute));
        });

, , . , , BindModelAsync .

, , , - HTTP- , , ModelType JSON. , , BankAccountTransactionModel1 BankAccountTransactionModel , , , JSON .

, , , , , , .

, ModelBinder

var typeValue = bindingContext.ValueProvider.GetValue("ModelType");

. , typeValue -

typeValue
{}
Culture: {}
FirstValue: null
Length: 0
Values: {}
Results View: Expanding the Results View will enumerate the IEnumerable

,

bindingContext.ValueProvider
Count = 2
[0]: {Microsoft.AspNetCore.Mvc.ModelBinding.RouteValueProvider}
[1]: {Microsoft.AspNetCore.Mvc.ModelBinding.QueryStringValueProvider}

, , , , - .

, "" , ?

- , , , ?

.