. net 5 Web custom middleware implementation returns the default picture

background ： During the request for a large number of map tiles , There may be a loss of tiles , At this time, you need to display the default tile image on the map , Previously, it was implemented in the front-end related map package , It is now implemented in the background. When the request for tiles fails , Returns the default tile image , And support the default picture configuration .

solve ：

1. establish .net 5 Web project

2. introduce Microsoft.Extensions.Configuration, For reading appsettings.json

3. Create a new middleware class DefaultImageMiddleware

   public class DefaultImageMiddleware
    {
        private readonly RequestDelegate _next;

        public static string DefaultImagePath { get; set; }

        /// <summary>
        ///  When the pipeline execution reaches our middleware, it receives... From the previous middleware RequestDelegate entrust 
        /// </summary>
        /// <param name="next"></param>
        /// <param name="defaultImagePath"> Default tile picture </param>
        public DefaultImageMiddleware(RequestDelegate next, string defaultImagePath)
        {
            this._next = next;
            DefaultImagePath = defaultImagePath;
        }
        /// <summary>
        ///   to HTTP Request the pipeline to join the business 
        /// </summary>
        /// <param name="context"></param>
        /// <returns></returns>
        public async Task Invoke(HttpContext context)
        {
            await _next(context);// take context Pass on to the next middleware 
            if (context.Response.StatusCode == 404)
            {// Request resources 404
                var contentType = context.Request.Headers["accept"].ToString().ToLower();
                await SetDefaultImage(context);
                if (contentType.StartsWith("image"))
                {
                    await SetDefaultImage(context);
                }
            }
        }
        /// <summary>
        ///  Return to the default picture 
        /// </summary>
        /// <param name="context"></param>
        /// <returns></returns>
        private async Task SetDefaultImage(HttpContext context)
        {
            try
            {
                string path = Path.Combine(Directory.GetCurrentDirectory(), DefaultImagePath);

                FileStream fs = File.OpenRead(path);
                byte[] bytes = new byte[fs.Length];
                await fs.ReadAsync(bytes, 0, bytes.Length);

                await context.Response.Body.WriteAsync(bytes, 0, bytes.Length);
            }
            catch (Exception ex)
            {
                await context.Response.WriteAsync(ex.Message);
            }
        }
    }

Be careful ： Custom middleware classes must have constructors and Invoke Method , The former is to accept the next middleware Commission ; The latter is part of your middleware business implementation ;

4. stay Startup.cs in Configure Register custom middleware

    public class Startup
    {
        /// requires using Microsoft.Extensions.Configuration;
        private readonly IConfiguration Configuration;

        public Startup(IConfiguration configuration)
        {
            Configuration = configuration;
        }

        // For more information on how to configure your application, visit https://go.microsoft.com/fwlink/?LinkID=398940
        public void ConfigureServices(IServiceCollection services)
        {
        }

        // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
        public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
        {
            if (env.IsDevelopment())
            {
                app.UseDeveloperExceptionPage();    
            }

            app.UseRouting();
            app.UseStaticFiles();
            string url = Configuration["defaultImagePath"];
            app.UseMiddleware<DefaultImageMiddleware>(url);// Register the middleware that returns the default picture 
            app.UseEndpoints(endpoints =>
            {
                endpoints.MapGet("/", async context =>
                {
                    await context.Response.WriteAsync("Hello World!");
                });
            });
        }
    }

5. stay appsettings.json Add the relevant configuration

{
  "Logging": {
    "LogLevel": {
      "Default": "Information",
      "Microsoft": "Warning",
      "Microsoft.Hosting.Lifetime": "Information"
    }
  },
  "AllowedHosts": "*",
  "defaultImagePath": "wwwroot\\default.jpg"// Default tile image path 
}

This is asking for pictures 404 when , You can return to the specified default picture