c# - 在asp.net核心响应之前和之后附加自定义HTML输出？

Question

我有一个简单的 .net 核心应用程序，它发出一个 API 输出。
我的 Configure方法很简单:

public void Configure(IApplicationBuilder app, IWebHostEnvironment env  )
        {

            if (env.IsDevelopment())
            {
                app.UseDeveloperExceptionPage();
            }
            app.UseRouting();
            app.UseAuthorization();
            app.UseEndpoints(endpoints => { endpoints.MapControllers(); });
    }

这是 API 的当前输出:

仅出于测试目的，我想在响应前后添加 HTML 标记:
类似于(在 DOM 中手动编辑):

所以我添加了这个:

public void Configure(IApplicationBuilder app, IWebHostEnvironment env  )
        {
            

          app.Use(async (context, next) =>
         {
             await context.Response.WriteAsync("<b> Hi</b>");
             await next ();
             await context.Response.WriteAsync("<b> Bye </b>");
         });




            if (env.IsDevelopment())
            {
                app.UseDeveloperExceptionPage();
            }
            app.UseRouting();
            app.UseAuthorization();
            app.UseEndpoints(endpoints => { endpoints.MapControllers(); });
    }

但是当我运行它时，我得到:

fail:Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware1An unhandled exception has occurred while executing the request. System.InvalidOperationException: Headers are read-only, response hasalready started.With this HTML :

我一直在寻找解决方案，但没有找到，如何去做。
问题:
为什么会发生？我以为我可以通过调用 next() 来控制管道并做任何我想做的事情在管道上。
如何在前后添加自定义 HTML 标签？
编辑:
如果我将代码移到 Configure 的末尾方法，我看到 the regular output ，没有得到异常，但没有 HTML 标签。
编辑 #2:
我也试过 OnStarting event ，但仍然没有成功(我得到一个空页面):

 app.Use(async (context, next) =>
        {
          
            context.Response.OnStarting(async state =>
            {
                if (state is HttpContext httpContext)
                {
                    var request = httpContext.Request;
                    var response = httpContext.Response;
                    await response .WriteAsync("<b> Bye </b>"); // <----
               }
            }, context);
            await next();
             
        });

Answer 1

使用以下中间件，我可以在操作结果前后添加 html 标签:

public class BeforeAfterMiddleware
{
    private readonly RequestDelegate _next;
    public BeforeAfterMiddleware(RequestDelegate next)
    {
        _next = next;
    }

    public async Task Invoke(HttpContext context)
    {
        //Redirect response into a memorystream
        using var actionResponse = new MemoryStream();
        var httpResponse = context.Response.Body;
        context.Response.Body = actionResponse;

        //Call the handler action
        await _next.Invoke(context);

        //Read the handler response from the memory stream
        actionResponse.Seek(0, SeekOrigin.Begin);
        var reader = new StreamReader(actionResponse);
        using var bufferReader = new StreamReader(actionResponse);
        string body = await bufferReader.ReadToEndAsync();

        //Remove the handler's response from the memory stream
        context.Response.Clear();

        //Write data to the memorystream
        await context.Response.WriteAsync("<h1>HI</h1>");
        await context.Response.WriteAsync(body);
        await context.Response.WriteAsync("<h1>BYE</h1>");

        //Copy memorystream to the response stream
        context.Response.Body.Seek(0, SeekOrigin.Begin);
        await context.Response.Body.CopyToAsync(httpResponse);
        context.Request.Body = httpResponse;
    }
}

它只是将响应重定向到 MemoryStream

然后在

之前和之后使用一些文本更改它

最后将 memoryStream 重定向回响应流

用法: app.UseMiddleware<BeforeAfterMiddleware>();