Filter与FilterProvider之间的关系
根据用途和执行时机的不同，MVC主要分为以下5种类型的过虑器：AuthenticationFilter、AuthorizationFilter、ActionFilter、ExceptionFilter、ResultFilter。下面我们来看一个IFilter接口，如下所示：
public class Filter{ ???public const int DefaultOrder = -1; ???public Filter(object instance, FilterScope scope, int? order) ???{ ???????//省略 ???????if (order == null) ???????{ ???????????IMvcFilter mvcFilter = instance as IMvcFilter; ???????????if (mvcFilter != null) ???????????{ ???????????????order = mvcFilter.Order; ???????????} ???????} ???????Instance = instance; ???????Order = order ?? DefaultOrder; ???????Scope = scope; ???} ???public object Instance { get; protected set; } ???public int Order { get; protected set; } ???public FilterScope Scope { get; protected set; }}public enum FilterScope{ ???First = 0, ???Global = 10, ???Controller = 20, ???Action = 30, ???Last = 100,}一个Filter对象就是对一个过滤器的封装，它将过滤器对象封装在Instance属性中。表示排序的Order属性，默认为-1，值越小越优先。还有一个表示范围的Scope属性，它是一个枚举类型。其中Global、Controller、Action分别表示应用到整个应用范围内、某个Controller上、某个Action上，另两个First、Last表明是应用的第一个还是最后一个。如果前面的Order属性的值一样大，那么相同的Order的Filter则会按照Scope属性值越小越优先。
所有的Filter均是通过FilterProvider来提供的，该接口的定义如下所示：
public interface IFilterProvider{ ???IEnumerable<Filter> GetFilters(ControllerContext controllerContext, ActionDescriptor actionDescriptor);}该接口只定义了一个GetFilters方法，用于获取某个Action方法上所有的Filter。它具有两个参数ControllerContext表示当前的Controller的上下文，ActionDescriptor描述目标Action方法。
用于提供Filter的FilterProvider是通过静态类型FilterProviders来注册的。如下所示：
public static class FilterProviders{ ???static FilterProviders() ???{ ???????Providers = new FilterProviderCollection(); ???????Providers.Add(GlobalFilters.Filters); ???????Providers.Add(new FilterAttributeFilterProvider()); ???????Providers.Add(new ControllerInstanceFilterProvider()); ???} ???public static FilterProviderCollection Providers { get; private set; }}从上我们看到MVC提供了三种原生的FilterProvider，它们分别是：GlobalFilters.Filters用于提供全局、FilterAttributeFilterProvider用于提供Controller类和Action方法上、ControllerInstanceFilterProvider用于提供Controller这个别特的过滤器。下面来看FilterProvider是如何提供Filter的。
1、FilterAttributeFilterProvider
我们通常是将过滤器定义成特性的方式标注到某个Controller或Action上。这样的过滤器特性一般以FilterAttribute作为基类。它实现了IMvcFilter接口，IMvcFilter只有两个只读属性成员Order和AllowMultiple。在上面的Filter类中我们看到如果实例可以转换为IMvcFilter接口，那么则将IMvcFilter的Order值赋给Filter的Order属性。我们在某个Action方法或Controller上标记某个特性时，可以在上面指定Order值，如[Authorize(Order =3)]。如下所示：
[AttributeUsage(AttributeTargets.Class | AttributeTargets.Method, Inherited = true, AllowMultiple = false)]public abstract class FilterAttribute : Attribute, IMvcFilter{ ???private static readonly ConcurrentDictionary<Type, bool> _multiuseAttributeCache = new ConcurrentDictionary<Type, bool>(); ???private int _order = Filter.DefaultOrder; ???public bool AllowMultiple ???{ ???????get { return AllowsMultiple(GetType()); } ???} ???public int Order ???{ ???????get { return _order; } ???????set ???????{ ???????????if (value < Filter.DefaultOrder) ???????????{ ???????????????throw new ArgumentOutOfRangeException("value", MvcResources.FilterAttribute_OrderOutOfRange); ???????????} ???????????_order = value; ???????} ???} ???private static bool AllowsMultiple(Type attributeType) ???{ ???????return _multiuseAttributeCache.GetOrAdd( ???????????attributeType, ???????????type => type.GetCustomAttributes(typeof(AttributeUsageAttribute), true) ???????????????????????.Cast<AttributeUsageAttribute>() ???????????????????????.First() ???????????????????????.AllowMultiple); ???}}public interface IMvcFilter{ ???bool AllowMultiple { get; } ???int Order { get; }}从应用在FilterAttribute的AttributeUsageAttribute特性上我们可以看到此特性可以应用到类和方法上，AllowMultiple默认为false，不允许多个。
获取Controller类和Action方法上的特性是通过FilterAttributeFilterProvider来提供的。如下所示：
public class FilterAttributeFilterProvider : IFilterProvider{ ???private readonly bool _cacheAttributeInstances; ???public FilterAttributeFilterProvider() ???????: this(true) ???{ ???} ???public FilterAttributeFilterProvider(bool cacheAttributeInstances) ???{ ???????_cacheAttributeInstances = cacheAttributeInstances; ???} ???protected virtual IEnumerable<FilterAttribute> GetActionAttributes(ControllerContext controllerContext, ActionDescriptor actionDescriptor) ???{ ???????return actionDescriptor.GetFilterAttributes(_cacheAttributeInstances); ???} ???protected virtual IEnumerable<FilterAttribute> GetControllerAttributes(ControllerContext controllerContext, ActionDescriptor actionDescriptor) ???{ ???????return actionDescriptor.ControllerDescriptor.GetFilterAttributes(_cacheAttributeInstances); ???} ???public virtual IEnumerable<Filter> GetFilters(ControllerContext controllerContext, ActionDescriptor actionDescriptor) ???{ ???????// Results are low in number in the common case so use yield return to avoid creating intermediate collections or nested enumerables ???????if (controllerContext.Controller != null) ???????{ ???????????foreach (FilterAttribute attr in GetControllerAttributes(controllerContext, actionDescriptor)) ???????????{ ???????????????yield return new Filter(attr, FilterScope.Controller, order: null); ???????????} ???????????foreach (FilterAttribute attr in GetActionAttributes(controllerContext, actionDescriptor)) ???????????{ ???????????????yield return new Filter(attr, FilterScope.Action, order: null); ???????????} ???????} ????????????????}}从上面的静态类FilterProviders中，我们可以看到MVC默认注册的是FilterAttributeFilterProvider的无参构造函数，然后它传一个true调用有参构造函数，该_cacheAttributeInstances字段表示是否缓存获取到的Filter。因为是通过反射获取Controller和Action上的Filter，所以MVC默认缓存获取到的Filter，提高性能。
该类实现的GetFilters方法，它会先后调用GetControllerAttributes方法和GetActionAttributes方法，分别获取Cotroller和Action上Filter。分别用于描述 Controller和Action的ControllerDescriptor和ActionDescriptor类实现了ICustomAttributeProvider接口。我们可以调用相应的方法获取应用在对应的Controller类型或Action方法上包括FilterAttribute在内的所有特性。如下所示：
public abstract class ControllerDescriptor : ICustomAttributeProvider, IUniquelyIdentifiable{ ???public virtual object[] GetCustomAttributes(Type attributeType, bool inherit) ???{ ???????if (attributeType == null) ???????{ ???????????throw new ArgumentNullException("attributeType"); ???????} ???????return (object[])Array.CreateInstance(attributeType, 0); ???} ???public virtual IEnumerable<FilterAttribute> GetFilterAttributes(bool useCache) ???{ ???????return GetCustomAttributes(typeof(FilterAttribute), inherit: true).Cast<FilterAttribute>(); ???}}public abstract class ActionDescriptor : ICustomAttributeProvider, IUniquelyIdentifiable{ ???public virtual object[] GetCustomAttributes(Type attributeType, bool inherit) ???{ ???????if (attributeType == null) ???????{ ???????????throw new ArgumentNullException("attributeType"); ???????} ???????return (object[])Array.CreateInstance(attributeType, 0); ???} ???public virtual IEnumerable<FilterAttribute> GetFilterAttributes(bool useCache) ???{ ???????return GetCustomAttributes(typeof(FilterAttribute), inherit: true).Cast<FilterAttribute>(); ???}}2、ControllerInstanceFilterProvider
Controller本身就是一个过滤器，如下所示：
public abstract class Controller : ControllerBase, IActionFilter, IAuthenticationFilter, IAuthorizationFilter, IDisposable, IExceptionFilter, IResultFilter, IAsyncController, IAsyncManagerContainer{ ???//省略}对于Controller这个特殊的过滤器，其对应的FilterProvider类型，如下所示：
public class ControllerInstanceFilterProvider : IFilterProvider{ ???public IEnumerable<Filter> GetFilters(ControllerContext controllerContext, ActionDescriptor actionDescriptor) ???{ ???????if (controllerContext.Controller != null) ???????{ ???????????// Use FilterScope.First and Order of Int32.MinValue to ensure controller instance methods always run first ???????????yield return new Filter(controllerContext.Controller, FilterScope.First, Int32.MinValue); ???????} ???}}它的GetFilter方法跟据ControllerContext获取对应的Controller对象，并以此创建对应的Filter。
3、GlobalFilters.Filters
全局过滤器，它不是显示的应用于某个Controller、某个Action上，而是默认应用到整个程序的所有Controller类型的Action方法上。如下所示：
public static class GlobalFilters{ ???static GlobalFilters() ???{ ???????Filters = new GlobalFilterCollection(); ???} ???public static GlobalFilterCollection Filters { get; private set; }}GlobalFilters就只有一个属性GlobalFilterCollection，GlobalFilterCollection它是一个元素类型为Filter的集合，它显示地实现了IFilterProvider接口的GetFilters方法，返回的就是它自己，如下所示：
public sealed class GlobalFilterCollection : IEnumerable<Filter>, IFilterProvider{ ???private List<Filter> _filters = new List<Filter>(); ???????public void Add(object filter) ???{ ???????AddInternal(filter, order: null); ???} ???public void Add(object filter, int order) ???{ ???????AddInternal(filter, order); ???} ???????private void AddInternal(object filter, int? order) ???{ ???????ValidateFilterInstance(filter); ???????_filters.Add(new Filter(filter, FilterScope.Global, order)); ???} ???????IEnumerable<Filter> IFilterProvider.GetFilters(ControllerContext controllerContext, ActionDescriptor actionDescriptor) ???{ ???????return this; ???} //省略}到目前为止，MVC默认提供的3种原生的FilterProvider以及各自采用的Filter提供机制介绍完毕。MVC的5种过滤器最终被封装成相应的Filter对象，但是它们的执行时机和方式是不同的，所以在执行之前需要根据被封装的过滤器类型对所有的Filter进行分组。就是当ControllerActionInvoker被调用时，它会利用静态类型FilterProviders得到所有注册的IFilterProvider类型，然后利用当前的ControllerContext和ActionDescriptor对象得到Filter，然后根据其Instance属性表示的过滤器类型，将它分组，最终得到一个具有如下所示FilterInfo类型的对象。
public class FilterInfo{ ???public IList<IActionFilter> ActionFilters { get; } ???public IList<IAuthenticationFilter> AuthenticationFilters { get; } ???public IList<IAuthorizationFilter> AuthorizationFilters { get; } ???public IList<IExceptionFilter> ExceptionFilters { get; } ???public IList<IResultFilter> ResultFilters { get; }}简说MVC Filter
原文地址：http://www.cnblogs.com/koeltp/p/7427106.html