详解ASP.NET Core 中基于工厂的中间件激活的实现方法

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<ConventionalMiddleware>();
 }

 public static IApplicationBuilder UseFactoryActivatedMiddleware(
  this IApplicationBuilder builder)
 {
  return builder.UseMiddleware<FactoryActivatedMiddleware>();
 }
}

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<FactoryActivatedMiddleware>(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<AppDbContext>(options =>
  options.UseInMemoryDatabase("InMemoryDb"));

 services.AddTransient<FactoryActivatedMiddleware>();

 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）。

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