ASP.NET MVC5学习笔记之Action参数模型绑定值提供体系

　　这一节我们关注模型绑定的值提供体系，先来介绍几个重要的接口

一. IValueProvider，接口定义如下:

public interface IValueProvider
    {
        
        bool ContainsPrefix(string prefix);
       
        ValueProviderResult GetValue(string key);
    }

从上面可以看出，IValueProvider定义了两个方法， 一个是检测是否包含指定的前缀，一个是通过指定的Key获取查询结果.这里前缀的概念主要是针对复杂类型的绑定，复杂类型包含属性，而属性的类型又是一个复杂类型，这样一层层下来，当我们在绑定类型的属性时，我们必须有一种机制确定该属性的值是从属于某个对象的，这就有了前缀的概念。系统定义了以下几种类型的绑定语法:

1.简单类型

　　prefix == 变量的名称

2. 复杂类型

　　prefix 变量名称

　　prefix.Name

　　prefix.Address.Name

3. 数组

　　a. 同名数据项

　　　　多个同名数据项， ValueProviderResult直接转换成数组

　　b. 基于索引的数组绑定

　　　　[0].Name

　　　　[0].PhoneNo

　　　　[0].Email

　　　　[1].Name

　　　　[1].PhoneNo

　　　　[1].Email

4，集合IEnumerable<T> 与数组类似

5. 字典

　　[0].Key

　　[0].Value.Name

　　[0].Value.EmailAddress

　　[1].Key

　　[2].Value.Name

　　[3].Value.EmailAddress

 二. ValueProviderResult类型

 [Serializable]
    public class ValueProviderResult
    {
        protected ValueProviderResult();
       
        public ValueProviderResult(object rawValue, string attemptedValue, CultureInfo culture);

        public string AttemptedValue { get; protected set; }
       
        public CultureInfo Culture { get; protected set; }
       
        public object RawValue { get; protected set; }

        public object ConvertTo(Type type);
       
        public virtual object ConvertTo(Type type, CultureInfo culture);
    }

   AttemptedValue表示从值的字符串表示，RawValue 表示值的原始值. 同时看到定义类型转换接口。 这里转换的代码值得研究一下:

public virtual object ConvertTo(Type type, CultureInfo culture)
        {
            if (type == null)
            {
                throw new ArgumentNullException("type");
            }

            CultureInfo cultureToUse = culture ?? Culture;
            return UnwrapPossibleArrayType(cultureToUse, RawValue, type);
        }

        private static object UnwrapPossibleArrayType(CultureInfo culture, object value, Type destinationType)
        {
            if (value == null || destinationType.IsInstanceOfType(value))
            {
                return value;
            }

            // array conversion results in four cases, as below
            Array valueAsArray = value as Array;
            if (destinationType.IsArray)
            {
                Type destinationElementType = destinationType.GetElementType();
                if (valueAsArray != null)
                {
                    // case 1: both destination + source type are arrays, so convert each element
                    IList converted = Array.CreateInstance(destinationElementType, valueAsArray.Length);
                    for (int i = 0; i < valueAsArray.Length; i++)
                    {
                        converted[i] = ConvertSimpleType(culture, valueAsArray.GetValue(i), destinationElementType);
                    }
                    return converted;
                }
                else
                {
                    // case 2: destination type is array but source is single element, so wrap element in array + convert
                    object element = ConvertSimpleType(culture, value, destinationElementType);
                    IList converted = Array.CreateInstance(destinationElementType, 1);
                    converted[0] = element;
                    return converted;
                }
            }
            else if (valueAsArray != null)
            {
                // case 3: destination type is single element but source is array, so extract first element + convert
                if (valueAsArray.Length > 0)
                {
                    value = valueAsArray.GetValue(0);
                    return ConvertSimpleType(culture, value, destinationType);
                }
                else
                {
                    // case 3(a): source is empty array, so can't perform conversion
                    return null;
                }
            }
            // case 4: both destination + source type are single elements, so convert
            return ConvertSimpleType(culture, value, destinationType);
        }

 1. 如果值是目标类型的实例，直接返回

 2. 尝试转换为数组，这里列了4种情况。

 3. 单一类型转换

 再来看一下ConvertSimpleType的代码:

private static object ConvertSimpleType(CultureInfo culture, object value, Type destinationType)
        {
            if (value == null || destinationType.IsInstanceOfType(value))
            {
                return value;
            }

            // if this is a user-input value but the user didn't type anything, return no value
            string valueAsString = value as string;
            if (valueAsString != null && String.IsNullOrWhiteSpace(valueAsString))
            {
                return null;
            }

            // In case of a Nullable object, we extract the underlying type and try to convert it.
            Type underlyingType = Nullable.GetUnderlyingType(destinationType);

            if (underlyingType != null)
            {
                destinationType = underlyingType;
            }

            // String doesn't provide convertibles to interesting types, and thus it will typically throw rather than succeed.
            if (valueAsString == null)
            {
                // If the source type implements IConvertible, try that first
                IConvertible convertible = value as IConvertible;
                if (convertible != null)
                {
                    try
                    {
                        return convertible.ToType(destinationType, culture);
                    }
                    catch
                    {
                    }
                }
            }

            // Last resort, look for a type converter
            TypeConverter converter = TypeDescriptor.GetConverter(destinationType);
            bool canConvertFrom = converter.CanConvertFrom(value.GetType());
            if (!canConvertFrom)
            {
                converter = TypeDescriptor.GetConverter(value.GetType());
            }
            if (!(canConvertFrom || converter.CanConvertTo(destinationType)))
            {
                // EnumConverter cannot convert integer, so we verify manually
                if (destinationType.IsEnum && value is int)
                {
                    return Enum.ToObject(destinationType, (int)value);
                }

                string message = String.Format(CultureInfo.CurrentCulture, MvcResources.ValueProviderResult_NoConverterExists,
                                               value.GetType().FullName, destinationType.FullName);
                throw new InvalidOperationException(message);
            }

            try
            {
                object convertedValue = (canConvertFrom)
                                            ? converter.ConvertFrom(null /* context */, culture, value)
                                            : converter.ConvertTo(null /* context */, culture, value, destinationType);
                return convertedValue;
            }
            catch (Exception ex)
            {
                string message = String.Format(CultureInfo.CurrentCulture, MvcResources.ValueProviderResult_ConversionThrew,
                                               value.GetType().FullName, destinationType.FullName);
                throw new InvalidOperationException(message, ex);
            }
        }

　　这里也考虑了几种情况转换
1. 值是目标类型的实例直接返回
2. 值是空串返回null
3. 可空类型取其下的真正类型
4. 尝试利用IConvertible转换
5. 利用目标类型和值类型的TypeConverter
6. 检查目标类型是Enum和值类型是否int

 三. ValueProviderFactory 

 public abstract class ValueProviderFactory
    {
        public abstract IValueProvider GetValueProvider(ControllerContext controllerContext);
    }

表示的值提供对象的创健工厂.

四. ValueProvider创建工厂和具体ValueProvider介绍

 ValueProvider的调用入口是Controller.ValueProvider属性，它是调用ValueProviderFactories.Factories.GetValueProvider()返回值,看看ValueProviderFactories的定义

public static class ValueProviderFactories
    {
        private static readonly ValueProviderFactoryCollection _factories = new ValueProviderFactoryCollection()
        {
            new ChildActionValueProviderFactory(),
            new FormValueProviderFactory(),
            new JsonValueProviderFactory(),
            new RouteDataValueProviderFactory(),
            new QueryStringValueProviderFactory(),
            new HttpFileCollectionValueProviderFactory(),
        };

        public static ValueProviderFactoryCollection Factories
        {
            get { return _factories; }
        }
    }

可以了解到系统内置几种ValueProviderFactory, 下面依次来了解.

a. ChildActionValueProviderFactory 创建ChildActionValueProvider， 提供在HtmlHelper.Action方法附加的路由信息

public sealed class ChildActionValueProviderFactory : ValueProviderFactory
    {
        public override IValueProvider GetValueProvider(ControllerContext controllerContext)
        {
            if (controllerContext == null)
            {
                throw new ArgumentNullException("controllerContext");
            }

            return new ChildActionValueProvider(controllerContext);
        }
    }

b. FormValueProviderFactory 创建FormValueProvider , 提供请求表单的值

public sealed class FormValueProviderFactory : ValueProviderFactory
    {
        private readonly UnvalidatedRequestValuesAccessor _unvalidatedValuesAccessor;

        public FormValueProviderFactory()
            : this(null)
        {
        }

        // For unit testing
        internal FormValueProviderFactory(UnvalidatedRequestValuesAccessor unvalidatedValuesAccessor)
        {
            _unvalidatedValuesAccessor = unvalidatedValuesAccessor ?? (cc => new UnvalidatedRequestValuesWrapper(cc.HttpContext.Request.Unvalidated));
        }

        public override IValueProvider GetValueProvider(ControllerContext controllerContext)
        {
            if (controllerContext == null)
            {
                throw new ArgumentNullException("controllerContext");
            }

            return new FormValueProvider(controllerContext, _unvalidatedValuesAccessor(controllerContext));
        }
    }

c. JsonValueProviderFactory 处理json请求类型(application/json)， 创建DictionaryValueProvider，

 public sealed class JsonValueProviderFactory : ValueProviderFactory
    {
        private static void AddToBackingStore(EntryLimitedDictionary backingStore, string prefix, object value)
        {
            IDictionary<string, object> d = value as IDictionary<string, object>;
            if (d != null)
            {
                foreach (KeyValuePair<string, object> entry in d)
                {
                    AddToBackingStore(backingStore, MakePropertyKey(prefix, entry.Key), entry.Value);
                }
                return;
            }

            IList l = value as IList;
            if (l != null)
            {
                for (int i = 0; i < l.Count; i++)
                {
                    AddToBackingStore(backingStore, MakeArrayKey(prefix, i), l[i]);
                }
                return;
            }

            // primitive
            backingStore.Add(prefix, value);
        }

        private static object GetDeserializedObject(ControllerContext controllerContext)
        {
            if (!controllerContext.HttpContext.Request.ContentType.StartsWith("application/json", StringComparison.OrdinalIgnoreCase))
            {
                // not JSON request
                return null;
            }

            StreamReader reader = new StreamReader(controllerContext.HttpContext.Request.InputStream);
            string bodyText = reader.ReadToEnd();
            if (String.IsNullOrEmpty(bodyText))
            {
                // no JSON data
                return null;
            }

            JavaScriptSerializer serializer = new JavaScriptSerializer();
            object jsonData = serializer.DeserializeObject(bodyText);
            return jsonData;
        }

        public override IValueProvider GetValueProvider(ControllerContext controllerContext)
        {
            if (controllerContext == null)
            {
                throw new ArgumentNullException("controllerContext");
            }

            object jsonData = GetDeserializedObject(controllerContext);
            if (jsonData == null)
            {
                return null;
            }

            Dictionary<string, object> backingStore = new Dictionary<string, object>(StringComparer.OrdinalIgnoreCase);
            EntryLimitedDictionary backingStoreWrapper = new EntryLimitedDictionary(backingStore);
            AddToBackingStore(backingStoreWrapper, String.Empty, jsonData);
            return new DictionaryValueProvider<object>(backingStore, CultureInfo.CurrentCulture);
        }

        private static string MakeArrayKey(string prefix, int index)
        {
            return prefix + "[" + index.ToString(CultureInfo.InvariantCulture) + "]";
        }

        private static string MakePropertyKey(string prefix, string propertyName)
        {
            return (String.IsNullOrEmpty(prefix)) ? propertyName : prefix + "." + propertyName;
        }

        private class EntryLimitedDictionary
        {
            private static int _maximumDepth = GetMaximumDepth();
            private readonly IDictionary<string, object> _innerDictionary;
            private int _itemCount = 0;

            public EntryLimitedDictionary(IDictionary<string, object> innerDictionary)
            {
                _innerDictionary = innerDictionary;
            }

            public void Add(string key, object value)
            {
                if (++_itemCount > _maximumDepth)
                {
                    throw new InvalidOperationException(MvcResources.JsonValueProviderFactory_RequestTooLarge);
                }

                _innerDictionary.Add(key, value);
            }

            private static int GetMaximumDepth()
            {
                NameValueCollection appSettings = ConfigurationManager.AppSettings;
                if (appSettings != null)
                {
                    string[] valueArray = appSettings.GetValues("aspnet:MaxJsonDeserializerMembers");
                    if (valueArray != null && valueArray.Length > 0)
                    {
                        int result;
                        if (Int32.TryParse(valueArray[0], out result))
                        {
                            return result;
                        }
                    }
                }

                return 1000; // Fallback default
            }
        }
    }

d. RouteDataValueProviderFactory 创建RouteDataValueProvider, 提供路由信息相关值

 public sealed class RouteDataValueProviderFactory : ValueProviderFactory
    {
        public override IValueProvider GetValueProvider(ControllerContext controllerContext)
        {
            if (controllerContext == null)
            {
                throw new ArgumentNullException("controllerContext");
            }

            return new RouteDataValueProvider(controllerContext);
        }
    }

e. QueryStringValueProviderFactory 创建QueryStringValueProvider, 提供查询字符串值

public sealed class QueryStringValueProviderFactory : ValueProviderFactory
    {
        private readonly UnvalidatedRequestValuesAccessor _unvalidatedValuesAccessor;

        public QueryStringValueProviderFactory()
            : this(null)
        {
        }

        // For unit testing
        internal QueryStringValueProviderFactory(UnvalidatedRequestValuesAccessor unvalidatedValuesAccessor)
        {
            _unvalidatedValuesAccessor = unvalidatedValuesAccessor ?? (cc => new UnvalidatedRequestValuesWrapper(cc.HttpContext.Request.Unvalidated));
        }

        public override IValueProvider GetValueProvider(ControllerContext controllerContext)
        {
            if (controllerContext == null)
            {
                throw new ArgumentNullException("controllerContext");
            }

            return new QueryStringValueProvider(controllerContext, _unvalidatedValuesAccessor(controllerContext));
        }
    }

f. HttpFileCollectionValueProviderFactory创建HttpFileCollectionValueProvider上传文件值提供

public sealed class HttpFileCollectionValueProviderFactory : ValueProviderFactory
    {
        public override IValueProvider GetValueProvider(ControllerContext controllerContext)
        {
            if (controllerContext == null)
            {
                throw new ArgumentNullException("controllerContext");
            }

            return new HttpFileCollectionValueProvider(controllerContext);
        }
    }

 整体ValueProvider的继承体系统如下图: