Provides a fluent API to map discriminator property values to derived types and handle MVC model deserialization.

DerivedDiscriminatorModelBinder.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Mvc;
 
public class DerivedDiscriminatorModelBinder<T> : DefaultModelBinder where T : class
{
    private readonly string discriminatorPropertyName;
    private readonly Dictionary<object, Type> mapping = new Dictionary<object, Type>();

    public DerivedDiscriminatorModelBinder(string discriminatorPropertyName)
    {
        this.discriminatorPropertyName = discriminatorPropertyName;
    }
    public override object BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext)
    {
        if (bindingContext.ModelType == typeof(T))
        {
            var discriminatorValue = bindingContext.ValueProvider.GetValue(CreateSubPropertyName(bindingContext.ModelName, discriminatorPropertyName));
            if (discriminatorValue == null)
                throw new Exception("Discriminator property '" + discriminatorPropertyName + "' does not exist.");
            
            Type derivedType;
            var lookupValue = discriminatorValue.RawValue ?? DBNull.Value;
            if (!mapping.TryGetValue(lookupValue, out derivedType))
                try
                {
                    return base.BindModel(controllerContext, bindingContext);
                }
                catch
                {
                    throw new Exception((lookupValue == DBNull.Value ? "Null discriminator value" : "Discriminator value '" + discriminatorValue.RawValue + "'") + " is not mapped to a derived type and the base type cannot be instantiated.");
                }
            
            var model = Activator.CreateInstance(derivedType);
            bindingContext.ModelMetadata = ModelMetadataProviders.Current.GetMetadataForType(() => model, derivedType);
        }

        return base.BindModel(controllerContext, bindingContext);
    }
        
    public DerivedDiscriminatorModelBinder<T> Map<TDerived>(object discriminatorPropertyValue)
        where TDerived : T, new()
    {
        mapping.Add(discriminatorPropertyValue ?? DBNull.Value, typeof(TDerived));
        return this;
    }
}

public static class ModelBindersExtensions
{
    public static DerivedDiscriminatorModelBinder<T> AddDerivedDiscriminator<T>(this ModelBinderDictionary modelBinderDictionary, string discriminatorPropertyName) where T : class
    {
        var r = new DerivedDiscriminatorModelBinder<T>(discriminatorPropertyName);
        modelBinderDictionary.Add(typeof(T), r);
        return r;
    }
}