最近问了几个面试同一个问题：如果有多个自定义Middleware，如何控制它们的执行顺序（比如先判断用户合法再写访问日志）。居然大部分人答不上来？！

对此，你有什么看法？

ASP.NET Core Middleware的调用顺序

这里我直接贴上官方文档^[1]中的图片:

ASP.NET Core 请求管道包含一系列请求委托，依次调用。而调用顺序实际上就是我们在Startup.cs中注册（使用UseMiddlewareExtensions.UseMiddleware方法）它们的顺序。

示例代码如下：

public void ConfigureServices(IServiceCollection services)
{//注册生命周期services.AddTransient<TransientMiddleware>();services.AddScoped<ScopedMiddleware>();
}public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{//注册调用顺序app.UseMiddleware<TransientMiddleware>();app.UseMiddleware<ScopedMiddleware>();
}

但这种方式，对于调整自定义Middleware的需求，需要经常修改Startup.cs，而且会使代码比较凌乱，可读性较差。

可以使用下面的方式，简化注册代码：

public void ConfigureServices(IServiceCollection services)
{services.AddMiddlewares();
}
public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{app.UseMiddlewares();
}

实现方式

1.定义Attribute

MiddlewareRegisterAttribute将放在每个Middleware实现类上，表明它是需要被注册的Middleware。

[AttributeUsage(AttributeTargets.Class)]
public class MiddlewareRegisterAttribute : Attribute
{//注册顺序public int Sort { get; set; } = int.MaxValue;//生命周期public ServiceLifetime Lifetime { get; set; } = ServiceLifetime.Scoped;
}

2.定义注册信息类

MiddlewareRegisterInfo用于存放Middleware的注册信息，供注册方法调用。

public class MiddlewareRegisterInfo
{public MiddlewareRegisterInfo(Type type,MiddlewareRegisterAttribute attribute){Type = type;Sort = attribute.Sort;Lifetime = attribute.Lifetime;}public Type Type { get; private set; }public int Sort { get; private set; }public ServiceLifetime Lifetime { get; private set; }
}

2.实现注册扩展方法

读取Assembly中的Type, 如果存在MiddlewareRegisterAttribute就把它放入List<MiddlewareRegisterInfo>列表中，最后根据Sort属性顺序依次注册，代码如下：

public static class MiddlewareRegisterExtensions
{private static readonly IEnumerable<MiddlewareRegisterInfo> _middlewareRegisterInfos = GetMiddlewareRegisterInfos();public static IServiceCollection AddMiddlewares(this IServiceCollection services){foreach (var middlewareRegisterInfo in _middlewareRegisterInfos){switch (middlewareRegisterInfo.Lifetime){case ServiceLifetime.Singleton:services.AddSingleton(middlewareRegisterInfo.Type);break;case ServiceLifetime.Transient:services.AddTransient(middlewareRegisterInfo.Type);break;default:services.AddScoped(middlewareRegisterInfo.Type);break;}}return services;}public static IApplicationBuilder UseMiddlewares(this IApplicationBuilder applicationBuilder){foreach (var middlewareRegisterInfo in _middlewareRegisterInfos){applicationBuilder.UseMiddleware(middlewareRegisterInfo.Type);}return applicationBuilder;}private static List<MiddlewareRegisterInfo> GetMiddlewareRegisterInfos(){var middlewareRegisterInfos = new List<MiddlewareRegisterInfo>();//所有包含Middleware的Assemblyvar assemblies = new Assembly[] { typeof(Startup).Assembly };foreach (var assembly in assemblies){foreach (var type in assembly.GetTypes().Where(x => !x.IsAbstract)){var attribute = type.GetCustomAttribute<MiddlewareRegisterAttribute>();if (attribute != null){middlewareRegisterInfos.Add(new MiddlewareRegisterInfo(type, attribute));}}}return middlewareRegisterInfos.OrderBy(p=>p.Sort).ToList();}
}

测试一下

创建3个Middleware，功能仅仅是输出日志：

[MiddlewareRegister(Sort = 100)]
public class OneMiddleware : IMiddleware
{private readonly ILogger<OneMiddleware> logger;public OneMiddleware(ILogger<OneMiddleware> logger){this.logger = logger;}public async Task InvokeAsync(HttpContext context, RequestDelegate next){logger.LogInformation("One");await next(context);}
}

Sort用100、200、300，方便后面修改排序。

运行效果如下图：

现在，将ThreeMiddleware的Sort改为150，调整注册顺序，再次运行，效果如下图：

结论

当然，这个解决方案也存在一些缺点，比如修改排序的位置移到每个Middleware，比较分散。但整体来说，代码更易读！

如果你有其他实现方式，欢迎到公众号后台留言指教。

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参考资料

[1]

官方文档: https://docs.microsoft.com/zh-cn/aspnet/core/fundamentals/middleware