IMiddlewareFactory/IMiddleware 是中间件激活的扩展点。

UseMiddleware 扩展方法检查中间件的已注册类型是否实现 IMiddleware。 如果是，则使用在容器中注册的 IMiddlewareFactory 实例来解析 IMiddleware 实现，而不使用基于约定的中间件激活逻辑。 中间件在应用的服务容器中注册为作用域或瞬态服务。

优点：

• 按客户端请求（作用域服务的注入）激活

• 让中间件强类型化

IMiddleware 按客户端请求（连接）激活，因此作用域服务可以注入到中间件的构造函数中。

IMiddleware

IMiddleware 定义应用的请求管道的中间件。InvokeAsync(HttpContext, RequestDelegate) 方法处理请求，并返回代表中间件执行的 Task。

使用约定激活的中间件：

public class ConventionalMiddleware{    private readonly RequestDelegate _next;    public ConventionalMiddleware(RequestDelegate next)    {        _next = next;    }    public async Task InvokeAsync(HttpContext context, AppDbContext db)    {        var keyValue = context.Request.Query["key"];        if (!string.IsNullOrWhiteSpace(keyValue))        {            db.Add(new Request()                {                    DT = DateTime.UtcNow,                     MiddlewareActivation = "ConventionalMiddleware",                     Value = keyValue                });            await db.SaveChangesAsync();        }        await _next(context);    }}

使用 MiddlewareFactory 激活的中间件：

public class FactoryActivatedMiddleware : IMiddleware{    private readonly AppDbContext _db;    public FactoryActivatedMiddleware(AppDbContext db)    {        _db = db;    }    public async Task InvokeAsync(HttpContext context, RequestDelegate next)    {        var keyValue = context.Request.Query["key"];        if (!string.IsNullOrWhiteSpace(keyValue))        {            _db.Add(new Request()                {                    DT = DateTime.UtcNow,                     MiddlewareActivation = "FactoryActivatedMiddleware",                     Value = keyValue                });            await _db.SaveChangesAsync();        }        await next(context);    }}

程序会为中间件创建扩展：

public static class MiddlewareExtensions{    public static IApplicationBuilder UseConventionalMiddleware(        this IApplicationBuilder builder)    {        return builder.UseMiddleware
    
     ();    }    public static IApplicationBuilder UseFactoryActivatedMiddleware(        this IApplicationBuilder builder)    {        return builder.UseMiddleware
     
      ();    }}
     
    

无法通过 UseMiddleware 将对象传递给工厂激活的中间件：

public static IApplicationBuilder UseFactoryActivatedMiddleware(    this IApplicationBuilder builder, bool option){    // Passing 'option' as an argument throws a NotSupportedException at runtime.    return builder.UseMiddleware
    
     (option);}
    

将工厂激活的中间件添加到 Startup.ConfigureServices 的内置容器中：

public void ConfigureServices(IServiceCollection services){    services.AddDbContext
    
     (options =>        options.UseInMemoryDatabase("InMemoryDb"));    services.AddTransient
     
      ();    services.AddRazorPages();}
     
    

两个中间件均在 Startup.Configure 的请求处理管道中注册：

public void Configure(IApplicationBuilder app, IWebHostEnvironment env){    if (env.IsDevelopment())    {        app.UseDeveloperExceptionPage();    }    else    {        app.UseExceptionHandler("/Error");    }    app.UseConventionalMiddleware();    app.UseFactoryActivatedMiddleware();    app.UseStaticFiles();    app.UseRouting();    app.UseEndpoints(endpoints =>    {        endpoints.MapRazorPages();    });}

IMiddlewareFactory

IMiddlewareFactory 提供中间件的创建方法。 中间件工厂实现在容器中注册为作用域服务。

可在 Microsoft.AspNetCore.Http 包中找到默认的 IMiddlewareFactory 实现（即 MiddlewareFactory）。