asp.net核心如何在reuqest管道中运行“默认”RequestDelegate？

Question

I was reading the source code of ApplicationBuilder ( https://source.dot.net/#Microsoft.AspNetCore.Http/Builder/ApplicationBuilder.cs,132 ):

public class ApplicationBuilder : IApplicationBuilder {

   private readonly List<Func<RequestDelegate, RequestDelegate>> _components = new();
   ...
   public IApplicationBuilder Use(Func<RequestDelegate, RequestDelegate> middleware) {   
      _components.Add(middleware);
      return this;
   }

   public RequestDelegate Build() {
      RequestDelegate app = context => {  
         // If we reach the end of the pipeline, but we have an endpoint, then something unexpected has happened
         // This could happen if user code sets an endpoint, but they forgot to add the UseEndpoint middleware.
         var endpoint = context.GetEndpoint();
         var endpointRequestDelegate = endpoint?.RequestDelegate;
         if (endpointRequestDelegate != null) {
            var message = "...";
            throw new InvalidOperationException(message);
         }

         // endpoint is null 
         context.Response.StatusCode = StatusCodes.Status404NotFound;
         return Task.CompletedTask;
      }
   }

   for (var c = _components.Count - 1; c >= 0; c--) { 
      app = _components[c](app);
   }
   return app;
}

As you can see, the RequestDelegate app will always be executed, which means the status code will be set to 404, which doesn't make sense at all.

let's we have the following code:

public void Configure(IApplicationBuilder app, IWebHostEnvironment env) {
   ...
   app.UseRouting();

   app.UseEndpoints(endpoints => {
      endpoints.MapGet("routing", async context => {
         await context.Response.WriteAsync("Request Was Routed");
      });
   });

   app.Use(async (context, next) => { // let's call it terminal middleware
      await context.Response.WriteAsync("Terminal Middleware Reached");
   });
}

a default startup url without routing segement will reach to the terminal middleware the request has no assocaited endpoint and according the source code, the response's status code will be set to 404, which doesn't make sense at all. So my guess is, when UseEndpoints() is the last middleware as:

public void Configure(IApplicationBuilder app, IWebHostEnvironment env) {
   ...
   app.UseRouting();

   app.UseEndpoints(endpoints => {
      endpoints.MapGet("routing", async context => {
         await context.Response.WriteAsync("Request Was Routed");
      });
   });
}

The status code will be set to 404

But when there are middlwares after UseEndpoints() like:

public void Configure(IApplicationBuilder app, IWebHostEnvironment env) {
   ...
   app.UseRouting();

   app.UseEndpoints(endpoints => {
      endpoints.MapGet("routing", async context => {
         await context.Response.WriteAsync("Request Was Routed");
      });
   });

   app.Use(async (context, next) => { 
      await context.Response.WriteAsync("Terminal Middleware Reached");
   });
}

asp.net core will check HttpContext 's reponse, if the response contain sth, then the the last app RequestDelegate in the source code won't run, is my understanding correct?

Answer 1

Shortly yes, but there are some things where probably you should consider to care of, more in the comments below.

public RequestDelegate Build() 
{
    RequestDelegate app = context => 
    {  
        var endpoint = context.GetEndpoint();
        var endpointRequestDelegate = endpoint?.RequestDelegate;

        // I was not able to produce this exception, or it is handled by the framework 
        // probably returns again 404 and do not executes all pending middlewares for sure.
        if (endpointRequestDelegate != null) 
        {
           var message = "...";
           throw new InvalidOperationException(message);
        }

        // however, hire is set 404 by default, case no endpoint found
        context.Response.StatusCode = StatusCodes.Status404NotFound;
        return Task.CompletedTask;
    }

    // Then each middleware in the pipeline will be executed solong there is nothing writen 
    // to the response body, at this point execution is broken and 
    // the response is returned to the client. 
    for (var c = _components.Count - 1; c >= 0; c--) 
    { 
        app = _components[c](app);
    }

    return app;
}

// To execute the code as you expect make sure to add all middlewares 
// in that order that Asp.Net expects

    static string msg = string.Empty;
    public void Configure(IApplicationBuilder app)
    {
        app.UseRouting();

        app.Use(async (context, next) =>
        {
            msg += "Middleware 1; ";

            await next();
        });

        app.Use(async (context, next) =>
        {
            msg += "Middleware 2; ";

            await next();
        });

        // use app.UseEndpoints... to register your endpoints, for exceptions and other staff
        // make the needed middlewares above and take care of their order
        app.UseEndpoints(x =>
        {
            x.Map("routing", async request =>
            {
                msg += "Routing Middleware Reached";
                await request.Response.Body.WriteAsync(Encoding.UTF8.GetBytes(msg));
            });

            //x.Map("/", async request =>
            //{
            //    msg += "Terminal Middleware Reached";
            //    await request.Response.Body.WriteAsync(Encoding.UTF8.GetBytes(msg));
            //});
        });

        // do not use like below, consider to put the middlewares before app.UseEndpoints...
        app.Use(async (context, next) =>
        {
            msg += "Middleware 3; ";

            await next();
        });

        app.Use(async (context, next) =>
        {
            msg += "Terminal Middleware Reached";
            await context.Response.Body.WriteAsync(Encoding.UTF8.GetBytes(msg));
        });

        app.Use(async (context, next) =>
        {
            msg += "Middleware 4; ";

            await next();
        });
    }