筛选器 filter

筛选器 filter 又称过滤器，是 APS.NET Core 中提供的一种切面编程机制，它允许开发者创建自定义筛选器来处理横切关注点，也就是在APS.NET Core 特定的位置执行自定义的代码，比如在控制器的操作方法之前执行数据检查的代码，或者在ActionResult执行时向响应报文头中写入自定义数据等。

ASP.ENT Core 中筛选器有5种类型：授权筛选器、资源筛选器、操作筛选器、异常筛选器、结果筛选器。

 自定义异常筛选器：

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public class MyExceptionFilter : IAsyncExceptionFilter    {        private readonly ILogger<MyExceptionFilter> _logger;        private readonly IHostEnvironment _env;        public MyExceptionFilter(ILogger<MyExceptionFilter> logger, IHostEnvironment env)        {            _logger = logger;            _env = env;        }          public Task OnExceptionAsync(ExceptionContext context)        {            Exception exception = context.Exception;            _logger.LogError(exception, "UnhandledException occured");            string message;            if (_env.IsProduction() == true)            {                message = "程序中出现未处理的异常";            }            else            {                message = exception.ToString();            }            ObjectResult result = new ObjectResult(new { code = 500, message = message });            result.StatusCode = 500;            context.Result = result;//设置响应报文            context.ExceptionHandled = true;//不再执行默认的异常响应逻辑。            return Task.CompletedTask;        }    }

注册全局异常筛选器：

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builder.Services.Configure<MvcOptions>(options => {     options.Filters.Add<MyExceptionFilter>(); });