change umbraco content in middleware (example, may need some work!)

maintain.cs

using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Http;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Logging;

using System.IO;
using System.Threading.Tasks;

using Umbraco.Cms.Core.Composing;
using Umbraco.Cms.Core.DependencyInjection;
using Umbraco.Cms.Web.Common.ApplicationBuilder;

namespace MiddleLib
{
    /// <summary>
    ///  example of how to replace content in middleware in umbraco
    ///  (for example if you want to put under maintance page up ?
    /// </summary>

    public class MyComposer : IComposer
    {
        public void Compose(IUmbracoBuilder builder)
        {
            builder.Services.Configure<UmbracoPipelineOptions>(options =>
            {
                options.AddFilter(new UmbracoPipelineFilter(
                    "MaintainTemplate",
                    applicationBuilder =>
                    {
                        applicationBuilder.UseWhen(
                            ctx => !IsBackendRequest(ctx.Request.Path),
                            ab => ab.UseMiddleware<MaintainTemplateMiddleWare>());
                    },
                    applicationBuilder => { },
                    applicationBuilder => { }
                ));
            });
        }

        private static string[] backofficePaths = new string[] 
        {
            "/umbraco", 
            "/App_Plugins",
            "/api", // keep alive ??
            "/media" // let media through ? (because of the back office???)
        };

        private bool IsBackendRequest(PathString path)
        {
            foreach (var backofficepath in backofficePaths) {
                if (path.StartsWithSegments(backofficepath))
                    return true;
            }

            return false;
        }
    }

    public class MaintainTemplateMiddleWare
    {
        private readonly RequestDelegate _next;
        private readonly IConfiguration _config;
        private ILogger<MaintainTemplateMiddleWare> _logger;
        public MaintainTemplateMiddleWare(RequestDelegate next,
            IConfiguration config,
            ILogger<MaintainTemplateMiddleWare> logger)
        {
            _next = next;
            _logger = logger;
            _config = config;

            _logger.LogInformation("Starting middleware");
        }

        public async Task InvokeAsync(HttpContext context)
        {

            _logger.LogInformation("Middleware tiggered {url}", context.Request.Path);
            if (_config.GetValue<bool>("Maintain:Enabled", false))
            {
                using (var stream = GetTemplate())
                {
                    await stream.CopyToAsync(context.Response.Body);
                }
            }
            else
            {
                await _next(context);
            }
        }

        private Stream GetTemplate()
        {
            var text = "<h1>Maintenance Template</h1>";
            var stream = new MemoryStream();
            var writer = new StreamWriter(stream);
            
            writer.Write(text);
            writer.Flush();
            stream.Position = 0;
            return stream;

        }
    }
}

Note - one does not simple set the body content .... you have to copy into it. see https://stackoverflow.com/a/58688017