System.Web.Mvc3/Mvc/DefaultModelBinder.cs

namespace System.Web.Mvc {
    using System;
    using System.Collections;
    using System.Collections.Generic;
    using System.ComponentModel;
    using System.Diagnostics.CodeAnalysis;
    using System.Globalization;
    using System.Linq;
    using System.Reflection;
    using System.Runtime.CompilerServices;
    using System.Web.Mvc.Resources;

    public class DefaultModelBinder : IModelBinder {

        private ModelBinderDictionary _binders;
        private static string _resourceClassKey;

        [SuppressMessage("Microsoft.Usage", "CA2227:CollectionPropertiesShouldBeReadOnly", Justification = "Property is settable so that the dictionary can be provided for unit testing purposes.")]
        protected internal ModelBinderDictionary Binders {
            get {
                if (_binders == null) {
                    _binders = ModelBinders.Binders;
                }
                return _binders;
            }
            set {
                _binders = value;
            }
        }

        public static string ResourceClassKey {
            get {
                return _resourceClassKey ?? String.Empty;
            }
            set {
                _resourceClassKey = value;
            }
        }

        private static void AddValueRequiredMessageToModelState(ControllerContext controllerContext, ModelStateDictionary modelState, string modelStateKey, Type elementType, object value) {
            if (value == null && !TypeHelpers.TypeAllowsNullValue(elementType) && modelState.IsValidField(modelStateKey)) {
                modelState.AddModelError(modelStateKey, GetValueRequiredResource(controllerContext));
            }
        }

        internal void BindComplexElementalModel(ControllerContext controllerContext, ModelBindingContext bindingContext, object model) {
            // need to replace the property filter + model object and create an inner binding context
            ModelBindingContext newBindingContext = CreateComplexElementalModelBindingContext(controllerContext, bindingContext, model);

            // validation
            if (OnModelUpdating(controllerContext, newBindingContext)) {
                BindProperties(controllerContext, newBindingContext);
                OnModelUpdated(controllerContext, newBindingContext);
            }
        }

        internal object BindComplexModel(ControllerContext controllerContext, ModelBindingContext bindingContext) {
            object model = bindingContext.Model;
            Type modelType = bindingContext.ModelType;

            // if we're being asked to create an array, create a list instead, then coerce to an array after the list is created
            if (model == null && modelType.IsArray) {
                Type elementType = modelType.GetElementType();
                Type listType = typeof(List<>).MakeGenericType(elementType);
                object collection = CreateModel(controllerContext, bindingContext, listType);

                ModelBindingContext arrayBindingContext = new ModelBindingContext() {
                    ModelMetadata = ModelMetadataProviders.Current.GetMetadataForType(() => collection, listType),
                    ModelName = bindingContext.ModelName,
                    ModelState = bindingContext.ModelState,
                    PropertyFilter = bindingContext.PropertyFilter,
                    ValueProvider = bindingContext.ValueProvider
                };
                IList list = (IList)UpdateCollection(controllerContext, arrayBindingContext, elementType);

                if (list == null) {
                    return null;
                }

                Array array = Array.CreateInstance(elementType, list.Count);
                list.CopyTo(array, 0);
                return array;
            }

            if (model == null) {
                model = CreateModel(controllerContext, bindingContext, modelType);
            }

            // special-case IDictionary<,> and ICollection<>
            Type dictionaryType = TypeHelpers.ExtractGenericInterface(modelType, typeof(IDictionary<,>));
            if (dictionaryType != null) {
                Type[] genericArguments = dictionaryType.GetGenericArguments();
                Type keyType = genericArguments[0];
                Type valueType = genericArguments[1];

                ModelBindingContext dictionaryBindingContext = new ModelBindingContext() {
                    ModelMetadata = ModelMetadataProviders.Current.GetMetadataForType(() => model, modelType),
                    ModelName = bindingContext.ModelName,
                    ModelState = bindingContext.ModelState,
                    PropertyFilter = bindingContext.PropertyFilter,
                    ValueProvider = bindingContext.ValueProvider
                };
                object dictionary = UpdateDictionary(controllerContext, dictionaryBindingContext, keyType, valueType);
                return dictionary;
            }

            Type enumerableType = TypeHelpers.ExtractGenericInterface(modelType, typeof(IEnumerable<>));
            if (enumerableType != null) {
                Type elementType = enumerableType.GetGenericArguments()[0];

                Type collectionType = typeof(ICollection<>).MakeGenericType(elementType);
                if (collectionType.IsInstanceOfType(model)) {
                    ModelBindingContext collectionBindingContext = new ModelBindingContext() {
                        ModelMetadata = ModelMetadataProviders.Current.GetMetadataForType(() => model, modelType),
                        ModelName = bindingContext.ModelName,
                        ModelState = bindingContext.ModelState,
                        PropertyFilter = bindingContext.PropertyFilter,
                        ValueProvider = bindingContext.ValueProvider
                    };
                    object collection = UpdateCollection(controllerContext, collectionBindingContext, elementType);
                    return collection;
                }
            }

            // otherwise, just update the properties on the complex type
            BindComplexElementalModel(controllerContext, bindingContext, model);
            return model;
        }

        public virtual object BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext) {
            if (bindingContext == null) {
                throw new ArgumentNullException("bindingContext");
            }

            bool performedFallback = false;

            if (!String.IsNullOrEmpty(bindingContext.ModelName) && !bindingContext.ValueProvider.ContainsPrefix(bindingContext.ModelName)) {
                // We couldn't find any entry that began with the prefix. If this is the top-level element, fall back
                // to the empty prefix.
                if (bindingContext.FallbackToEmptyPrefix) {
                    bindingContext = new ModelBindingContext() {
                        ModelMetadata = bindingContext.ModelMetadata,
                        ModelState = bindingContext.ModelState,
                        PropertyFilter = bindingContext.PropertyFilter,
                        ValueProvider = bindingContext.ValueProvider
                    };
                    performedFallback = true;
                }
                else {
                    return null;
                }
            }

            // Simple model = int, string, etc.; determined by calling TypeConverter.CanConvertFrom(typeof(string))
            // or by seeing if a value in the request exactly matches the name of the model we're binding.
            // Complex type = everything else.
            if (!performedFallback) {
                bool performRequestValidation = ShouldPerformRequestValidation(controllerContext, bindingContext);
                ValueProviderResult vpResult = bindingContext.UnvalidatedValueProvider.GetValue(bindingContext.ModelName, skipValidation: !performRequestValidation);
                if (vpResult != null) {
                    return BindSimpleModel(controllerContext, bindingContext, vpResult);
                }
            }
            if (!bindingContext.ModelMetadata.IsComplexType) {
                return null;
            }

            return BindComplexModel(controllerContext, bindingContext);
        }

        private void BindProperties(ControllerContext controllerContext, ModelBindingContext bindingContext) {
            IEnumerable<PropertyDescriptor> properties = GetFilteredModelProperties(controllerContext, bindingContext);
            foreach (PropertyDescriptor property in properties) {
                BindProperty(controllerContext, bindingContext, property);
            }
        }

        protected virtual void BindProperty(ControllerContext controllerContext, ModelBindingContext bindingContext, PropertyDescriptor propertyDescriptor) {
            // need to skip properties that aren't part of the request, else we might hit a StackOverflowException
            string fullPropertyKey = CreateSubPropertyName(bindingContext.ModelName, propertyDescriptor.Name);
            if (!bindingContext.ValueProvider.ContainsPrefix(fullPropertyKey)) {
                return;
            }

            // call into the property's model binder
            IModelBinder propertyBinder = Binders.GetBinder(propertyDescriptor.PropertyType);
            object originalPropertyValue = propertyDescriptor.GetValue(bindingContext.Model);
            ModelMetadata propertyMetadata = bindingContext.PropertyMetadata[propertyDescriptor.Name];
            propertyMetadata.Model = originalPropertyValue;
            ModelBindingContext innerBindingContext = new ModelBindingContext() {
                ModelMetadata = propertyMetadata,
                ModelName = fullPropertyKey,
                ModelState = bindingContext.ModelState,
                ValueProvider = bindingContext.ValueProvider
            };
            object newPropertyValue = GetPropertyValue(controllerContext, innerBindingContext, propertyDescriptor, propertyBinder);
            propertyMetadata.Model = newPropertyValue;

            // validation
            ModelState modelState = bindingContext.ModelState[fullPropertyKey];
            if (modelState == null || modelState.Errors.Count == 0) {
                if (OnPropertyValidating(controllerContext, bindingContext, propertyDescriptor, newPropertyValue)) {
                    SetProperty(controllerContext, bindingContext, propertyDescriptor, newPropertyValue);
                    OnPropertyValidated(controllerContext, bindingContext, propertyDescriptor, newPropertyValue);
                }
            }
            else {
                SetProperty(controllerContext, bindingContext, propertyDescriptor, newPropertyValue);

                // Convert FormatExceptions (type conversion failures) into InvalidValue messages
                foreach (ModelError error in modelState.Errors.Where(err => String.IsNullOrEmpty(err.ErrorMessage) && err.Exception != null).ToList()) {
                    for (Exception exception = error.Exception; exception != null; exception = exception.InnerException) {
                        if (exception is FormatException) {
                            string displayName = propertyMetadata.GetDisplayName();
                            string errorMessageTemplate = GetValueInvalidResource(controllerContext);
                            string errorMessage = String.Format(CultureInfo.CurrentCulture, errorMessageTemplate, modelState.Value.AttemptedValue, displayName);
                            modelState.Errors.Remove(error);
                            modelState.Errors.Add(errorMessage);
                            break;
                        }
                    }
                }
            }
        }

        internal object BindSimpleModel(ControllerContext controllerContext, ModelBindingContext bindingContext, ValueProviderResult valueProviderResult) {
            bindingContext.ModelState.SetModelValue(bindingContext.ModelName, valueProviderResult);

            // if the value provider returns an instance of the requested data type, we can just short-circuit
            // the evaluation and return that instance
            if (bindingContext.ModelType.IsInstanceOfType(valueProviderResult.RawValue)) {
                return valueProviderResult.RawValue;
            }

            // since a string is an IEnumerable<char>, we want it to skip the two checks immediately following
            if (bindingContext.ModelType != typeof(string)) {

                // conversion results in 3 cases, as below
                if (bindingContext.ModelType.IsArray) {
                    // case 1: user asked for an array
                    // ValueProviderResult.ConvertTo() understands array types, so pass in the array type directly
                    object modelArray = ConvertProviderResult(bindingContext.ModelState, bindingContext.ModelName, valueProviderResult, bindingContext.ModelType);
                    return modelArray;
                }

                Type enumerableType = TypeHelpers.ExtractGenericInterface(bindingContext.ModelType, typeof(IEnumerable<>));
                if (enumerableType != null) {
                    // case 2: user asked for a collection rather than an array
                    // need to call ConvertTo() on the array type, then copy the array to the collection
                    object modelCollection = CreateModel(controllerContext, bindingContext, bindingContext.ModelType);
                    Type elementType = enumerableType.GetGenericArguments()[0];
                    Type arrayType = elementType.MakeArrayType();
                    object modelArray = ConvertProviderResult(bindingContext.ModelState, bindingContext.ModelName, valueProviderResult, arrayType);

                    Type collectionType = typeof(ICollection<>).MakeGenericType(elementType);
                    if (collectionType.IsInstanceOfType(modelCollection)) {
                        CollectionHelpers.ReplaceCollection(elementType, modelCollection, modelArray);
                    }
                    return modelCollection;
                }
            }

            // case 3: user asked for an individual element
            object model = ConvertProviderResult(bindingContext.ModelState, bindingContext.ModelName, valueProviderResult, bindingContext.ModelType);
            return model;
        }

        private static bool CanUpdateReadonlyTypedReference(Type type) {
            // value types aren't strictly immutable, but because they have copy-by-value semantics
            // we can't update a value type that is marked readonly
            if (type.IsValueType) {
                return false;
            }

            // arrays are mutable, but because we can't change their length we shouldn't try
            // to update an array that is referenced readonly
            if (type.IsArray) {
                return false;
            }

            // special-case known common immutable types
            if (type == typeof(string)) {
                return false;
            }

            return true;
        }

        [SuppressMessage("Microsoft.Globalization", "CA1304:SpecifyCultureInfo", MessageId = "System.Web.Mvc.ValueProviderResult.ConvertTo(System.Type)", Justification = "The target object should make the correct culture determination, not this method.")]
        [SuppressMessage("Microsoft.Design", "CA1031:DoNotCatchGeneralExceptionTypes", Justification = "We're recording this exception so that we can act on it later.")]
        private static object ConvertProviderResult(ModelStateDictionary modelState, string modelStateKey, ValueProviderResult valueProviderResult, Type destinationType) {
            try {
                object convertedValue = valueProviderResult.ConvertTo(destinationType);
                return convertedValue;
            }
            catch (Exception ex) {
                modelState.AddModelError(modelStateKey, ex);
                return null;
            }
        }

        internal ModelBindingContext CreateComplexElementalModelBindingContext(ControllerContext controllerContext, ModelBindingContext bindingContext, object model) {
            BindAttribute bindAttr = (BindAttribute)GetTypeDescriptor(controllerContext, bindingContext).GetAttributes()[typeof(BindAttribute)];
            Predicate<string> newPropertyFilter = (bindAttr != null)
                ? propertyName => bindAttr.IsPropertyAllowed(propertyName) && bindingContext.PropertyFilter(propertyName)
                : bindingContext.PropertyFilter;

            ModelBindingContext newBindingContext = new ModelBindingContext() {
                ModelMetadata = ModelMetadataProviders.Current.GetMetadataForType(() => model, bindingContext.ModelType),
                ModelName = bindingContext.ModelName,
                ModelState = bindingContext.ModelState,
                PropertyFilter = newPropertyFilter,
                ValueProvider = bindingContext.ValueProvider
            };

            return newBindingContext;
        }

        protected virtual object CreateModel(ControllerContext controllerContext, ModelBindingContext bindingContext, Type modelType) {
            Type typeToCreate = modelType;

            // we can understand some collection interfaces, e.g. IList<>, IDictionary<,>
            if (modelType.IsGenericType) {
                Type genericTypeDefinition = modelType.GetGenericTypeDefinition();
                if (genericTypeDefinition == typeof(IDictionary<,>)) {
                    typeToCreate = typeof(Dictionary<,>).MakeGenericType(modelType.GetGenericArguments());
                }
                else if (genericTypeDefinition == typeof(IEnumerable<>) || genericTypeDefinition == typeof(ICollection<>) || genericTypeDefinition == typeof(IList<>)) {
                    typeToCreate = typeof(List<>).MakeGenericType(modelType.GetGenericArguments());
                }
            }

            // fallback to the type's default constructor
            return Activator.CreateInstance(typeToCreate);
        }

        protected static string CreateSubIndexName(string prefix, int index) {
            return String.Format(CultureInfo.InvariantCulture, "{0}[{1}]", prefix, index);
        }

        protected static string CreateSubIndexName(string prefix, string index) {
            return String.Format(CultureInfo.InvariantCulture, "{0}[{1}]", prefix, index);
        }

        protected internal static string CreateSubPropertyName(string prefix, string propertyName) {
            if (String.IsNullOrEmpty(prefix)) {
                return propertyName;
            }
            else if (String.IsNullOrEmpty(propertyName)) {
                return prefix;
            }
            else {
                return prefix + "." + propertyName;
            }
        }

        protected IEnumerable<PropertyDescriptor> GetFilteredModelProperties(ControllerContext controllerContext, ModelBindingContext bindingContext) {
            PropertyDescriptorCollection properties = GetModelProperties(controllerContext, bindingContext);
            Predicate<string> propertyFilter = bindingContext.PropertyFilter;

            return from PropertyDescriptor property in properties
                   where ShouldUpdateProperty(property, propertyFilter)
                   select property;
        }

        [SuppressMessage("Microsoft.Globalization", "CA1304:SpecifyCultureInfo", MessageId = "System.Web.Mvc.ValueProviderResult.ConvertTo(System.Type)", Justification = "ValueProviderResult already handles culture conversion appropriately.")]
        private static void GetIndexes(ModelBindingContext bindingContext, out bool stopOnIndexNotFound, out IEnumerable<string> indexes) {
            string indexKey = CreateSubPropertyName(bindingContext.ModelName, "index");
            ValueProviderResult vpResult = bindingContext.ValueProvider.GetValue(indexKey);

            if (vpResult != null) {
                string[] indexesArray = vpResult.ConvertTo(typeof(string[])) as string[];
                if (indexesArray != null) {
                    stopOnIndexNotFound = false;
                    indexes = indexesArray;
                    return;
                }
            }

            // just use a simple zero-based system
            stopOnIndexNotFound = true;
            indexes = GetZeroBasedIndexes();
        }

        protected virtual PropertyDescriptorCollection GetModelProperties(ControllerContext controllerContext, ModelBindingContext bindingContext) {
            return GetTypeDescriptor(controllerContext, bindingContext).GetProperties();
        }

        protected virtual object GetPropertyValue(ControllerContext controllerContext, ModelBindingContext bindingContext, PropertyDescriptor propertyDescriptor, IModelBinder propertyBinder) {
            object value = propertyBinder.BindModel(controllerContext, bindingContext);

            if (bindingContext.ModelMetadata.ConvertEmptyStringToNull && Object.Equals(value, String.Empty)) {
                return null;
            }

            return value;
        }

        protected virtual ICustomTypeDescriptor GetTypeDescriptor(ControllerContext controllerContext, ModelBindingContext bindingContext) {
            return TypeDescriptorHelper.Get(bindingContext.ModelType);
        }

        // If the user specified a ResourceClassKey try to load the resource they specified.
        // If the class key is invalid, an exception will be thrown.
        // If the class key is valid but the resource is not found, it returns null, in which
        // case it will fall back to the MVC default error message.
        private static string GetUserResourceString(ControllerContext controllerContext, string resourceName) {
            string result = null;

            if (!String.IsNullOrEmpty(ResourceClassKey) && (controllerContext != null) && (controllerContext.HttpContext != null)) {
                result = controllerContext.HttpContext.GetGlobalResourceObject(ResourceClassKey, resourceName, CultureInfo.CurrentUICulture) as string;
            }

            return result;
        }

        private static string GetValueInvalidResource(ControllerContext controllerContext) {
            return GetUserResourceString(controllerContext, "PropertyValueInvalid") ?? MvcResources.DefaultModelBinder_ValueInvalid;
        }

        private static string GetValueRequiredResource(ControllerContext controllerContext) {
            return GetUserResourceString(controllerContext, "PropertyValueRequired") ?? MvcResources.DefaultModelBinder_ValueRequired;
        }

        private static IEnumerable<string> GetZeroBasedIndexes() {
            for (int i = 0; ; i++) {
                yield return i.ToString(CultureInfo.InvariantCulture);
            }
        }

        protected static bool IsModelValid(ModelBindingContext bindingContext) {
            if (bindingContext == null) {
                throw new ArgumentNullException("bindingContext");
            }
            if (String.IsNullOrEmpty(bindingContext.ModelName)) {
                return bindingContext.ModelState.IsValid;
            }
            return bindingContext.ModelState.IsValidField(bindingContext.ModelName);
        }

        protected virtual void OnModelUpdated(ControllerContext controllerContext, ModelBindingContext bindingContext) {
            Dictionary<string, bool> startedValid = new Dictionary<string, bool>(StringComparer.OrdinalIgnoreCase);

            foreach (ModelValidationResult validationResult in ModelValidator.GetModelValidator(bindingContext.ModelMetadata, controllerContext).Validate(null)) {
                string subPropertyName = CreateSubPropertyName(bindingContext.ModelName, validationResult.MemberName);

                if (!startedValid.ContainsKey(subPropertyName)) {
                    startedValid[subPropertyName] = bindingContext.ModelState.IsValidField(subPropertyName);
                }

                if (startedValid[subPropertyName]) {
                    bindingContext.ModelState.AddModelError(subPropertyName, validationResult.Message);
                }
            }
        }

        protected virtual bool OnModelUpdating(ControllerContext controllerContext, ModelBindingContext bindingContext) {
            // default implementation does nothing
            return true;
        }

        protected virtual void OnPropertyValidated(ControllerContext controllerContext, ModelBindingContext bindingContext, PropertyDescriptor propertyDescriptor, object value) {
            // default implementation does nothing
        }

        protected virtual bool OnPropertyValidating(ControllerContext controllerContext, ModelBindingContext bindingContext, PropertyDescriptor propertyDescriptor, object value) {
            // default implementation does nothing
            return true;
        }

        [SuppressMessage("Microsoft.Design", "CA1031:DoNotCatchGeneralExceptionTypes", Justification = "We're recording this exception so that we can act on it later.")]
        protected virtual void SetProperty(ControllerContext controllerContext, ModelBindingContext bindingContext, PropertyDescriptor propertyDescriptor, object value) {

            ModelMetadata propertyMetadata = bindingContext.PropertyMetadata[propertyDescriptor.Name];
            propertyMetadata.Model = value;
            string modelStateKey = CreateSubPropertyName(bindingContext.ModelName, propertyMetadata.PropertyName);

            // If the value is null, and the validation system can find a Required validator for
            // us, we'd prefer to run it before we attempt to set the value; otherwise, property
            // setters which throw on null (f.e., Entity Framework properties which are backed by
            // non-nullable strings in the DB) will get their error message in ahead of us.
            //
            // We are effectively using the special validator -- Required -- as a helper to the
            // binding system, which is why this code is here instead of in the Validating/Validated
            // methods, which are really the old-school validation hooks.
            if (value == null && bindingContext.ModelState.IsValidField(modelStateKey)) {
                ModelValidator requiredValidator = ModelValidatorProviders.Providers.GetValidators(propertyMetadata, controllerContext).Where(v => v.IsRequired).FirstOrDefault();
                if (requiredValidator != null) {
                    foreach (ModelValidationResult validationResult in requiredValidator.Validate(bindingContext.Model)) {
                        bindingContext.ModelState.AddModelError(modelStateKey, validationResult.Message);
                    }
                }
            }

            bool isNullValueOnNonNullableType =
                value == null &&
                !TypeHelpers.TypeAllowsNullValue(propertyDescriptor.PropertyType);

            // Try to set a value into the property unless we know it will fail (read-only
            // properties and null values with non-nullable types)
            if (!propertyDescriptor.IsReadOnly && !isNullValueOnNonNullableType) {
                try {
                    propertyDescriptor.SetValue(bindingContext.Model, value);
                }
                catch (Exception ex) {
                    // Only add if we're not already invalid
                    if (bindingContext.ModelState.IsValidField(modelStateKey)) {
                        bindingContext.ModelState.AddModelError(modelStateKey, ex);
                    }
                }
            }

            // Last chance for an error on null values with non-nullable types, we'll use
            // the default "A value is required." message.
            if (isNullValueOnNonNullableType && bindingContext.ModelState.IsValidField(modelStateKey)) {
                bindingContext.ModelState.AddModelError(modelStateKey, GetValueRequiredResource(controllerContext));
            }
        }

        private static bool ShouldPerformRequestValidation(ControllerContext controllerContext, ModelBindingContext bindingContext) {
            if (controllerContext == null || controllerContext.Controller == null || bindingContext == null || bindingContext.ModelMetadata == null) {
                // To make unit testing easier, if the caller hasn't specified enough contextual information we just default
                // to always pulling the data from a collection that goes through request validation.
                return true;
            }

            // We should perform request validation only if both the controller and the model ask for it. This is the
            // default behavior for both. If either the controller (via [ValidateInput(false)]) or the model (via [AllowHtml])
            // opts out, we don't validate.
            return (controllerContext.Controller.ValidateRequest && bindingContext.ModelMetadata.RequestValidationEnabled);
        }

        private static bool ShouldUpdateProperty(PropertyDescriptor property, Predicate<string> propertyFilter) {
            if (property.IsReadOnly && !CanUpdateReadonlyTypedReference(property.PropertyType)) {
                return false;
            }

            // if this property is rejected by the filter, move on
            if (!propertyFilter(property.Name)) {
                return false;
            }

            // otherwise, allow
            return true;
        }

        internal object UpdateCollection(ControllerContext controllerContext, ModelBindingContext bindingContext, Type elementType) {
            bool stopOnIndexNotFound;
            IEnumerable<string> indexes;
            GetIndexes(bindingContext, out stopOnIndexNotFound, out indexes);
            IModelBinder elementBinder = Binders.GetBinder(elementType);

            // build up a list of items from the request
            List<object> modelList = new List<object>();
            foreach (string currentIndex in indexes) {
                string subIndexKey = CreateSubIndexName(bindingContext.ModelName, currentIndex);
                if (!bindingContext.ValueProvider.ContainsPrefix(subIndexKey)) {
                    if (stopOnIndexNotFound) {
                        // we ran out of elements to pull
                        break;
                    }
                    else {
                        continue;
                    }
                }

                ModelBindingContext innerContext = new ModelBindingContext() {
                    ModelMetadata = ModelMetadataProviders.Current.GetMetadataForType(null, elementType),
                    ModelName = subIndexKey,
                    ModelState = bindingContext.ModelState,
                    PropertyFilter = bindingContext.PropertyFilter,
                    ValueProvider = bindingContext.ValueProvider
                };
                object thisElement = elementBinder.BindModel(controllerContext, innerContext);

                // we need to merge model errors up
                AddValueRequiredMessageToModelState(controllerContext, bindingContext.ModelState, subIndexKey, elementType, thisElement);
                modelList.Add(thisElement);
            }

            // if there weren't any elements at all in the request, just return
            if (modelList.Count == 0) {
                return null;
            }

            // replace the original collection
            object collection = bindingContext.Model;
            CollectionHelpers.ReplaceCollection(elementType, collection, modelList);
            return collection;
        }

        internal object UpdateDictionary(ControllerContext controllerContext, ModelBindingContext bindingContext, Type keyType, Type valueType) {
            bool stopOnIndexNotFound;
            IEnumerable<string> indexes;
            GetIndexes(bindingContext, out stopOnIndexNotFound, out indexes);

            IModelBinder keyBinder = Binders.GetBinder(keyType);
            IModelBinder valueBinder = Binders.GetBinder(valueType);

            // build up a list of items from the request
            List<KeyValuePair<object, object>> modelList = new List<KeyValuePair<object, object>>();
            foreach (string currentIndex in indexes) {
                string subIndexKey = CreateSubIndexName(bindingContext.ModelName, currentIndex);
                string keyFieldKey = CreateSubPropertyName(subIndexKey, "key");
                string valueFieldKey = CreateSubPropertyName(subIndexKey, "value");

                if (!(bindingContext.ValueProvider.ContainsPrefix(keyFieldKey) && bindingContext.ValueProvider.ContainsPrefix(valueFieldKey))) {
                    if (stopOnIndexNotFound) {
                        // we ran out of elements to pull
                        break;
                    }
                    else {
                        continue;
                    }
                }

                // bind the key
                ModelBindingContext keyBindingContext = new ModelBindingContext() {
                    ModelMetadata = ModelMetadataProviders.Current.GetMetadataForType(null, keyType),
                    ModelName = keyFieldKey,
                    ModelState = bindingContext.ModelState,
                    ValueProvider = bindingContext.ValueProvider
                };
                object thisKey = keyBinder.BindModel(controllerContext, keyBindingContext);

                // we need to merge model errors up
                AddValueRequiredMessageToModelState(controllerContext, bindingContext.ModelState, keyFieldKey, keyType, thisKey);
                if (!keyType.IsInstanceOfType(thisKey)) {
                    // we can't add an invalid key, so just move on
                    continue;
                }

                // bind the value
                ModelBindingContext valueBindingContext = new ModelBindingContext() {
                    ModelMetadata = ModelMetadataProviders.Current.GetMetadataForType(null, valueType),
                    ModelName = valueFieldKey,
                    ModelState = bindingContext.ModelState,
                    PropertyFilter = bindingContext.PropertyFilter,
                    ValueProvider = bindingContext.ValueProvider
                };
                object thisValue = valueBinder.BindModel(controllerContext, valueBindingContext);

                // we need to merge model errors up
                AddValueRequiredMessageToModelState(controllerContext, bindingContext.ModelState, valueFieldKey, valueType, thisValue);
                KeyValuePair<object, object> kvp = new KeyValuePair<object, object>(thisKey, thisValue);
                modelList.Add(kvp);
            }

            // if there weren't any elements at all in the request, just return
            if (modelList.Count == 0) {
                return null;
            }

            // replace the original collection
            object dictionary = bindingContext.Model;
            CollectionHelpers.ReplaceDictionary(keyType, valueType, dictionary, modelList);
            return dictionary;
        }

        // This helper type is used because we're working with strongly-typed collections, but we don't know the Ts
        // ahead of time. By using the generic methods below, we can consolidate the collection-specific code in a
        // single helper type rather than having reflection-based calls spread throughout the DefaultModelBinder type.
        // There is a single point of entry to each of the methods below, so they're fairly simple to maintain.

        private static class CollectionHelpers {

            private static readonly MethodInfo _replaceCollectionMethod = typeof(CollectionHelpers).GetMethod("ReplaceCollectionImpl", BindingFlags.Static | BindingFlags.NonPublic);
            private static readonly MethodInfo _replaceDictionaryMethod = typeof(CollectionHelpers).GetMethod("ReplaceDictionaryImpl", BindingFlags.Static | BindingFlags.NonPublic);

            [MethodImpl(MethodImplOptions.NoInlining | MethodImplOptions.NoOptimization)]
            public static void ReplaceCollection(Type collectionType, object collection, object newContents) {
                MethodInfo targetMethod = _replaceCollectionMethod.MakeGenericMethod(collectionType);
                targetMethod.Invoke(null, new object[] { collection, newContents });
            }

            private static void ReplaceCollectionImpl<T>(ICollection<T> collection, IEnumerable newContents) {
                collection.Clear();
                if (newContents != null) {
                    foreach (object item in newContents) {
                        // if the item was not a T, some conversion failed. the error message will be propagated,
                        // but in the meanwhile we need to make a placeholder element in the array.
                        T castItem = (item is T) ? (T)item : default(T);
                        collection.Add(castItem);
                    }
                }
            }

            [MethodImpl(MethodImplOptions.NoInlining | MethodImplOptions.NoOptimization)]
            public static void ReplaceDictionary(Type keyType, Type valueType, object dictionary, object newContents) {
                MethodInfo targetMethod = _replaceDictionaryMethod.MakeGenericMethod(keyType, valueType);
                targetMethod.Invoke(null, new object[] { dictionary, newContents });
            }

            private static void ReplaceDictionaryImpl<TKey, TValue>(IDictionary<TKey, TValue> dictionary, IEnumerable<KeyValuePair<object, object>> newContents) {
                dictionary.Clear();
                foreach (KeyValuePair<object, object> item in newContents) {
                    // if the item was not a T, some conversion failed. the error message will be propagated,
                    // but in the meanwhile we need to make a placeholder element in the dictionary.
                    TKey castKey = (TKey)item.Key; // this cast shouldn't fail
                    TValue castValue = (item.Value is TValue) ? (TValue)item.Value : default(TValue);
                    dictionary[castKey] = castValue;
                }
            }
        }
    }
}