Asp.net MVC DefaultModelBinder分析

今天看见一同事写了一段代码很是奇怪，大致结构如下：

 public ActionResult Demo(string name, dynamic obj)
        {
            if (obj != null)
            {
                return Content("obj is not null");
            }
            return Content("obj is  null");
        }

在调用action时obj参数不传，如@{Html.RenderAction("Demo",new {name="majiang"});}。

可是在实际运行中obj永远不等于null。这是为什么了？ 其实一切答案都在DefaultModelBinder

首先看看BindModel方法

        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);
        }

很显然object是复杂类型 应该关注方法BindComplexModel

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;
        }

在它里面有关键的一句
if (model == null) {
                model = CreateModel(controllerContext, bindingContext, modelType);
            }

        protected override 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);
        }

很明显 在绑定的时候发现复杂类型实例为null，这时DefaultModelBinder会创建一个默认实例。这就是为什么action参数为object的时候，再绑定后都不为null。然而string类型的数据他们走的是BindSimpleModel。

为了便于大家调试代码，证实以上结论，大家可以创建一个类继承与DefaultModelBinder，如CustBinder

然后再Application_Start() 中添加一句   ModelBinders.Binders.DefaultBinder = new CustBinder();

CustBinder代码：

  public class CustBinder : DefaultModelBinder
    {
        public override object BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext)
        {
            //return base.BindModel(controllerContext, 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);
                ValueProviderResult vpResult = (bindingContext.ValueProvider as IUnvalidatedValueProvider).GetValue(bindingContext.ModelName, skipValidation: !performRequestValidation);
                if (vpResult != null)
                {
                    return BindSimpleModel(controllerContext, bindingContext, vpResult);
                }
            }
            if (!bindingContext.ModelMetadata.IsComplexType)
            {
                return null;
            }

            return BindComplexModel(controllerContext, bindingContext);
        }

        protected override 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);
        }

        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;
        }

        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;
        }

    
        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 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;
        }

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

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

        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;
            }
        }

        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;
                }
            }
        }

    }

    internal delegate bool TryGetValueDelegate(object dictionary, string key, out object value);
    internal static class TypeHelpers
    {

        private static readonly Dictionary<Type, TryGetValueDelegate> _tryGetValueDelegateCache = new Dictionary<Type, TryGetValueDelegate>();
        private static readonly ReaderWriterLockSlim _tryGetValueDelegateCacheLock = new ReaderWriterLockSlim();

        private static readonly MethodInfo _strongTryGetValueImplInfo = typeof(TypeHelpers).GetMethod("StrongTryGetValueImpl", BindingFlags.NonPublic | BindingFlags.Static);

        public static readonly Assembly MsCorLibAssembly = typeof(string).Assembly;
        public static readonly Assembly MvcAssembly = typeof(Controller).Assembly;
        public static readonly Assembly SystemWebAssembly = typeof(HttpContext).Assembly;

        // method is used primarily for lighting up new .NET Framework features even if MVC targets the previous version
        // thisParameter is the 'this' parameter if target method is instance method, should be null for static method
        public static TDelegate CreateDelegate<TDelegate>(Assembly assembly, string typeName, string methodName, object thisParameter) where TDelegate : class
        {
            // ensure target type exists
            Type targetType = assembly.GetType(typeName, false /* throwOnError */);
            if (targetType == null)
            {
                return null;
            }

            return CreateDelegate<TDelegate>(targetType, methodName, thisParameter);
        }

        public static TDelegate CreateDelegate<TDelegate>(Type targetType, string methodName, object thisParameter) where TDelegate : class
        {
            // ensure target method exists
            ParameterInfo[] delegateParameters = typeof(TDelegate).GetMethod("Invoke").GetParameters();
            Type[] argumentTypes = Array.ConvertAll(delegateParameters, pInfo => pInfo.ParameterType);
            MethodInfo targetMethod = targetType.GetMethod(methodName, argumentTypes);
            if (targetMethod == null)
            {
                return null;
            }

            TDelegate d = Delegate.CreateDelegate(typeof(TDelegate), thisParameter, targetMethod, false /* throwOnBindFailure */) as TDelegate;
            return d;
        }

        public static TryGetValueDelegate CreateTryGetValueDelegate(Type targetType)
        {
            TryGetValueDelegate result;

            _tryGetValueDelegateCacheLock.EnterReadLock();
            try
            {
                if (_tryGetValueDelegateCache.TryGetValue(targetType, out result))
                {
                    return result;
                }
            }
            finally
            {
                _tryGetValueDelegateCacheLock.ExitReadLock();
            }

            Type dictionaryType = ExtractGenericInterface(targetType, typeof(IDictionary<,>));

            // just wrap a call to the underlying IDictionary<TKey, TValue>.TryGetValue() where string can be cast to TKey
            if (dictionaryType != null)
            {
                Type[] typeArguments = dictionaryType.GetGenericArguments();
                Type keyType = typeArguments[0];
                Type returnType = typeArguments[1];

                if (keyType.IsAssignableFrom(typeof(string)))
                {
                    MethodInfo strongImplInfo = _strongTryGetValueImplInfo.MakeGenericMethod(keyType, returnType);
                    result = (TryGetValueDelegate)Delegate.CreateDelegate(typeof(TryGetValueDelegate), strongImplInfo);
                }
            }

            // wrap a call to the underlying IDictionary.Item()
            if (result == null && typeof(IDictionary).IsAssignableFrom(targetType))
            {
                result = TryGetValueFromNonGenericDictionary;
            }

            _tryGetValueDelegateCacheLock.EnterWriteLock();
            try
            {
                _tryGetValueDelegateCache[targetType] = result;
            }
            finally
            {
                _tryGetValueDelegateCacheLock.ExitWriteLock();
            }

            return result;
        }

        public static Type ExtractGenericInterface(Type queryType, Type interfaceType)
        {
            Func<Type, bool> matchesInterface = t => t.IsGenericType && t.GetGenericTypeDefinition() == interfaceType;
            return (matchesInterface(queryType)) ? queryType : queryType.GetInterfaces().FirstOrDefault(matchesInterface);
        }

        public static object GetDefaultValue(Type type)
        {
            return (TypeAllowsNullValue(type)) ? null : Activator.CreateInstance(type);
        }

        public static bool IsCompatibleObject<T>(object value)
        {
            return (value is T || (value == null && TypeAllowsNullValue(typeof(T))));
        }

        public static bool IsNullableValueType(Type type)
        {
            return Nullable.GetUnderlyingType(type) != null;
        }

        private static bool StrongTryGetValueImpl<TKey, TValue>(object dictionary, string key, out object value)
        {
            IDictionary<TKey, TValue> strongDict = (IDictionary<TKey, TValue>)dictionary;

            TValue strongValue;
            bool retVal = strongDict.TryGetValue((TKey)(object)key, out strongValue);
            value = strongValue;
            return retVal;
        }

        private static bool TryGetValueFromNonGenericDictionary(object dictionary, string key, out object value)
        {
            IDictionary weakDict = (IDictionary)dictionary;

            bool containsKey = weakDict.Contains(key);
            value = (containsKey) ? weakDict[key] : null;
            return containsKey;
        }

        public static bool TypeAllowsNullValue(Type type)
        {
            return (!type.IsValueType || IsNullableValueType(type));
        }
    }