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 = ; 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, );
                     converted[] = 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 > )
                 {
                     value = valueAsArray.GetValue();
                     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 = ; 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 = ;

             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 > )
                     {
                         int result;
                         if (Int32.TryParse(valueArray[], out result))
                         {
                             return result;
                         }
                     }
                 }

                 return ; // 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: