Filter是延续ASP.NET MVC的产物，同样保留了五种的Filter，分别是Authorization Filter、Resource Filter、Action Filter、Exception Filter及Result Filter。
通过不同的Filter可以有效处理封包进出的加工，本篇将介绍ASP.NET Core的五种Filter运作方式。Filter 介绍
Filter的作用是在Action 执行前或执行后做一些加工处理。
某种程度来看，会跟Middleware很像，但执行的顺序略有不同，用对Filter不仅可以减少代码，还可以提高执行效率。ASP.NET Core 有以下五种Filter 可以使用：
- Authorization FilterAuthorization是五种Filter中优先级最高的，通常用于验证Request合不合法，不合法后面就直接跳过。
- Resource FilterResource是第二优先，会在Authorization之后，Model Binding之前执行。通常会是需要对Model加工处理才用。
- Action Filter最常使用的Filter，封包进出都会经过它，使用上没什么需要特别注意的。跟Resource Filter很类似，但并不会经过Model Binding。
- Exception Filter异常处理的Filter。
- Result Filter当Action完成后，最终会经过的Filter。
Filter 运作方式
ASP.NET Core的每个Request都会先经过已注册的Middleware接着才会执行Filter，除了会依照上述的顺序外，同类型的Filter预设都会以先进后出的方式处里封包。
Response在某些Filter并不会做处理，会直接被pass。Request及Response的运作流程如下图：
- 黄色箭头是正常情况流程
- 灰色箭头是异常处理流程
建立Filter
ASP.NET Core的Filter基本上跟ASP.NET MVC的差不多。
上述的五种Filter范例分别如下：Authorization Filter
AuthorizationFilter.cs
using Microsoft.AspNetCore.Http;using Microsoft.AspNetCore.Mvc.Filters;namespace MyWebsite.Filters{ public class AuthorizationFilter : IAuthorizationFilter { public void OnAuthorization(AuthorizationFilterContext context) { context.HttpContext.Response.WriteAsync($"{GetType().Name} in. \r\n"); } }} 非同步的方式：
// ...public class AuthorizationFilter : IAsyncAuthorizationFilter{ public async Task OnAuthorizationAsync(AuthorizationFilterContext context) { await context.HttpContext.Response.WriteAsync($"{GetType().Name} in. \r\n"); }} Resource Filter
ResourceFilter.cs
using Microsoft.AspNetCore.Http;using Microsoft.AspNetCore.Mvc.Filters;namespace MyWebsite.Filters{ public class ResourceFilter : IResourceFilter { public void OnResourceExecuting(ResourceExecutingContext context) { context.HttpContext.Response.WriteAsync($"{GetType().Name} in. \r\n"); } public void OnResourceExecuted(ResourceExecutedContext context) { context.HttpContext.Response.WriteAsync($"{GetType().Name} out. \r\n"); } }} 非同步的方式：
// ...public class ResourceFilter : IAsyncResourceFilter{ public async Task OnResourceExecutionAsync(ResourceExecutingContext context, ResourceExecutionDelegate next) { await context.HttpContext.Response.WriteAsync($"{GetType().Name} in. \r\n"); await next(); await context.HttpContext.Response.WriteAsync($"{GetType().Name} out. \r\n"); }} Action Filter
ActionFilter.cs
using Microsoft.AspNetCore.Http;using Microsoft.AspNetCore.Mvc.Filters;namespace MyWebsite.Filters{ public class ActionFilter : IActionFilter { public void OnActionExecuting(ActionExecutingContext context) { context.HttpContext.Response.WriteAsync($"{GetType().Name} in. \r\n"); } public void OnActionExecuted(ActionExecutedContext context) { context.HttpContext.Response.WriteAsync($"{GetType().Name} out. \r\n"); } }} 非同步的方式：
// ...public class ActionFilter : IAsyncActionFilter{ public async Task OnActionExecutionAsync(ActionExecutingContext context, ActionExecutionDelegate next) { await context.HttpContext.Response.WriteAsync($"{GetType().Name} in. \r\n"); await next(); await context.HttpContext.Response.WriteAsync($"{GetType().Name} out. \r\n"); }} Result Filter
ResultFilter.cs
using Microsoft.AspNetCore.Http;using Microsoft.AspNetCore.Mvc.Filters;namespace MyWebsite.Filters{ public class ResultFilter : IResultFilter { public void OnResultExecuting(ResultExecutingContext context) { context.HttpContext.Response.WriteAsync($"{GetType().Name} in. \r\n"); } public void OnResultExecuted(ResultExecutedContext context) { context.HttpContext.Response.WriteAsync($"{GetType().Name} out. \r\n"); } }} 非同步的方式：
// ...public class ResultFilter : IAsyncResultFilter{ public async Task OnResultExecutionAsync(ResultExecutingContext context, ResultExecutionDelegate next) { await context.HttpContext.Response.WriteAsync($"{GetType().Name} in. \r\n"); await next(); await context.HttpContext.Response.WriteAsync($"{GetType().Name} out. \r\n"); }} Exception Filter
ExceptionFilter.cs
using Microsoft.AspNetCore.Http;using Microsoft.AspNetCore.Mvc.Filters;namespace MyWebsite.Filters{ public class ExceptionFilter : IExceptionFilter { public void OnException(ExceptionContext context) { context.ExceptionHandled = true; // 表明异常已处理，客户端可得到正常返回 context.HttpContext.Response.WriteAsync($"{GetType().Name} in. \r\n"); } }} 非同步的方式：
// ...public class ExceptionFilter : IAsyncExceptionFilter{ public Task OnExceptionAsync(ExceptionContext context) { context.ExceptionHandled = true;// 表明异常已处理，客户端可得到正常返回 context.HttpContext.Response.WriteAsync($"{GetType().Name} in. \r\n"); return Task.CompletedTask; }} 注册Filter
Filter有两种注册方式，一种是全局注册，另一种是用[Attribute]局部注册的方式，只套用在特定的Controller或Action。
全局注册
在Startup.ConfigureServices的MVC服务中注册Filter，这样就可以套用到所有的Request。如下：
// ...public class Startup{ public void ConfigureServices(IServiceCollection services) { services.AddMvc(config => { config.Filters.Add(new ResultFilter()); config.Filters.Add(new ExceptionFilter()); config.Filters.Add(new ResourceFilter()); }); }} 局部注册
ASP.NET Core在局部注册Filter的方式跟ASP.NET MVC有一点不一样，要通过[TypeFilter(type)]。
[TypeFilter(type)]就可以局部注册Filter。如下： // ...namespace MyWebsite.Controllers{ [TypeFilter(typeof(AuthorizationFilter))] public class HomeController : Controller { [TypeFilter(typeof(ActionFilter))] public void Index() { Response.WriteAsync("Hello World! \r\n"); } [TypeFilter(typeof(ActionFilter))] public void Error() { throw new System.Exception("Error"); } }} [TypeFilter(type)]用起来有点冗长，想要像过去ASP.NET MVC用[Attribute]注册Filter的话，只要将Filter继承Attribute即可。如下：
public class AuthorizationFilter : Attribute, IAuthorizationFilter{ // ...}public class ActionFilter : Attribute, IActionFilter{ // ...} [Attribute] 注册就可以改成如下方式：
// ...namespace MyWebsite.Controllers{ [AuthorizationFilter] public class HomeController : Controller { [ActionFilter] public void Index() { Response.WriteAsync("Hello World! \r\n"); } [ActionFilter] public void Error() { throw new System.Exception("Error"); } }} 执行结果
http://localhost:5000/Home/Index 输出结果如下：
AuthorizationFilter in.ResourceFilter in.ActionFilter in.Hello World!ActionFilter out.ResultFilter in.ResultFilter out.ResourceFilter out.
http://localhost:5000/Home/Error 输出结果如下：
AuthorizationFilter in.ResourceFilter in.ActionFilter in.ActionFilter out.ExceptionFilter in.ResourceFilter out.
执行顺序
预设注册同类型的Filter 是以先进后出的方式处里封包，注册层级也会影响执行顺序。
但也可以通过实现 IOrderedFilter 更改执行顺序。例如：
public class ActionFilter : Attribute, IActionFilter, IOrderedFilter{ public string Name { get; set; } public int Order { get; set; } = 0; public void OnActionExecuting(ActionExecutingContext context) { context.HttpContext.Response.WriteAsync($"{GetType().Name}({Name}) in. \r\n"); } public void OnActionExecuted(ActionExecutedContext context) { context.HttpContext.Response.WriteAsync($"{GetType().Name}({Name}) out. \r\n"); }} 在注册Filter 时带上Order，数值越小优先权越高。
// ...public class Startup{ public void ConfigureServices(IServiceCollection services) { services.AddMvc(config => { config.Filters.Add(new ActionFilter() { Name = "Global", Order = 3 }); }); }} // ...namespace MyWebsite.Controllers{ [ActionFilter(Name = "Controller", Order = 2)] public class HomeController : Controller { [ActionFilter(Name = "Action", Order = 1)] public void Index() { Response.WriteAsync("Hello World! \r\n"); } }} 变更执行顺序后的输出内容：
ActionFilter(Action) in. ActionFilter(Controller) in. ActionFilter(Global) in. Hello World! ActionFilter(Global) out. ActionFilter(Controller) out. ActionFilter(Action) out.
参考
老司机发车啦：