文章目录

一、什么是Filter？

二、Exception filter

当系统中出现未经处理的异常的时候，异常筛选器就会执行

1. 新建一个API控制器TestController

2. 在控制器中编写一个会抛异常的方法，代码如下：

   [HttpGet]
    public string Test1()
    {string s = System.IO.File.ReadAllText("D:1.txt");return s;
    }
    

3. 调用方法Test1后，会出现以下界面
   

4. 当系统中出现未处理异常的时候，假设我们需要统一返回给客户端一个如下格式的响应报文：{"code":"500","message":"异常信息"}，可以通过一个实现了IAsyncExceptionFilter接口的类来实现，类的代码如下：

   public class MyExceptionFilter : IAsyncExceptionFilter
    {private readonly IWebHostEnvironment webHost;public MyExceptionFilter(IWebHostEnvironment webHost){this.webHost = webHost;}public Task OnExceptionAsync(ExceptionContext context){string message;if (webHost.IsDevelopment()){message = context.Exception.ToString(); //context.Exception 代表异常信息对象}else{message = "服务器端发生未处理异常！";}ObjectResult objectResult = new ObjectResult(new { code = 500, message = message });context.Result = objectResult;  //context.Result的值会被输入给客户端context.ExceptionHandled = true;    //如果赋值为True，则其他的ExceptionFilter不会再执行return Task.CompletedTask;}
    }
    

5. 全局注册

   builder.Services.Configure(opt =>
    {opt.Filters.Add();
    });
    

6. 调用方法Test1后，会出现以下界面：

   

三、Action filter

1. 实现IAsyncActionFilter接口

2. 多个Action Filter的链式执行，执行顺序如下：
   

3. 在TestController中插入以下代码：

   [HttpGet]
    public string Test1()
    {Console.WriteLine("开始执行 Test1");string s = System.IO.File.ReadAllText("D:1.txt");return s;
    }[HttpGet]
    public DateTime GetDateTime()
    {Console.WriteLine("开始执行 GetDateTime");return DateTime.Now;
    }
    

4. 新建一个实现了IAsyncActionFilter接口的类MyActionFilter ：

   public class MyActionFilter : IAsyncActionFilter
    {public async Task OnActionExecutionAsync(ActionExecutingContext context, ActionExecutionDelegate next){Console.WriteLine("MyActionFilter 开始执行..."); //在Action之前执行ActionExecutedContext result = await next(); //在Action之后执行if(result.Exception == null){Console.WriteLine("MyActionFilter 执行成功了！");}else{Console.WriteLine("MyActionFilter 发生异常了！");}}
    }
    

5. 分别调用两个方法可以看到控制台中显示以下内容：
   

四、实现一个自动启用事务的Action Filter

1. 创建一个类NotTransactionAttribute ，继承自Attribute，且只能标注到方法上

   [AttributeUsage(AttributeTargets.Method)]
    public class NotTransactionAttribute : Attribute
    {
    }
    

2. 创建一个类TransactionScopeFilter，实现了IAsyncActionFilter接口

   public class TransactionScopeFilter : IAsyncActionFilter
    {public async Task OnActionExecutionAsync(ActionExecutingContext context, ActionExecutionDelegate next){//context.ActionDescriptor  当前被执行的Action方法的描述信息ControllerActionDescriptor? controllerActionDescriptor = context.ActionDescriptor as ControllerActionDescriptor;bool isTx = false; //是否进行事务控制if (controllerActionDescriptor != null){//查找当前Action方法的Attribute，是否标记了NotTransactionAttribute//如果标记了NotTransactionAttribute，则返回True，就不进行事务控制bool hasNotTransactionAttribute = controllerActionDescriptor.MethodInfo.GetCustomAttributes(typeof(NotTransactionAttribute), false).Any();isTx = !hasNotTransactionAttribute;}if (isTx){using (TransactionScope ts = new TransactionScope(TransactionScopeAsyncFlowOption.Enabled)){var r = await next();if (r.Exception == null){ts.Complete();}}}else{await next();}}
    }