ASP.NET Core 1.1 runs fine locally but when publishing to Azure says "An error occurred while starting the application."

Question

I've been developing an ASP.NET Core web app, based largely on the MVC template provided in Visual Studio 2017 RC2. It runs just fine in local debug mode, but when I try to publish it to an Azure hosted web app, I get this error:

An error occurred while starting the application.

.NET Core X86 v4.1.1.0 | Microsoft.AspNetCore.Hosting version 1.1.0-rtm-22752 | Microsoft Windows 6.2.9200

I've tried setting stdoutLogEnabled="true" in the web.config file, but it seems to have no effect, the error is the same.

Update:

With some help I managed to retrieve the log, and it says:

Application startup exception: System.TypeLoadException: Could not load type 'System.IO.File' from assembly 'mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e'.        at Microsoft.Extensions.DependencyModel.FileWrapper.OpenRead(String path)    at Microsoft.Extensions.DependencyModel.DependencyContextLoader.LoadEntryAssemblyContext(IDependencyContextReader reader)    at Microsoft.Extensions.DependencyModel.DependencyContextLoader.Load(Assembly assembly)        at Microsoft.Extensions.DependencyModel.DependencyContext.Load(Assembly assembly)        at Microsoft.AspNetCore.Mvc.Internal.DefaultAssemblyPartDiscoveryProvider.DiscoverAssemblyParts(String entryPointAssemblyName)        at Microsoft.Extensions.DependencyInjection.MvcCoreServiceCollectionExtensions.GetApplicationPartManager(IServiceCollection services)        at Microsoft.Extensions.DependencyInjection.MvcCoreServiceCollectionExtensions.AddMvcCore(IServiceCollection services)        at Microsoft.Extensions.DependencyInjection.MvcServiceCollectionExtensions.AddMvc(IServiceCollection services)        at Bla.Api.Startup.ConfigureServices(IServiceCollection services) in C:\Users\user\Source\Workspaces\Bla\Bla.Api\src\Bla.Api\Startup.cs:line 73 --- End of stack trace from previous location where exception was thrown ---    at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()    at Microsoft.AspNetCore.Hosting.ConventionBasedStartup.ConfigureServices(IServiceCollection services)    at Microsoft.AspNetCore.Hosting.Internal.WebHost.EnsureApplicationServices()    at Microsoft.AspNetCore.Hosting.Internal.WebHost.BuildApplication()  Hosting environment: Production     Content root path: D:\home\site\wwwroot     Now listening on: http://localhost:1264     Application started. Press Ctrl+C to shut down. 

The line of code it refers to at line 73 is:

services.AddMvc(); 

Update:

My global.json file looks like this (where Bla.Api is the name of the project, and the file sits in the solution root folder).

{   "projects": [ "Bla.Api" ],   "sdk": {     "version": "1.1.0"   } } 

Answer 1

Since many different problems can cause this error page, I can strongly recommend the following in order to determine the root cause quickly and easily, without wrestling Azure (or any server/platform for that matter) to get logs.

You can enable extremely helpful developer friendly error messages at startup by setting the .UseSetting("detailedErrors", "true") and .CaptureStartupErrors(true) actions in your Program.cs file.

For ASP.NET Core 1.x

public static void Main(string[] args) {   var host = new WebHostBuilder()       .UseKestrel()       .UseContentRoot(Directory.GetCurrentDirectory())       .UseSetting("detailedErrors", "true")       .UseIISIntegration()       .UseStartup<Startup>()       .CaptureStartupErrors(true)       .Build();    host.Run(); } 

(2018/07) Update for ASP.NET Core 2.1

public class Program   {     public static void Main(string[] args)     {         BuildWebHost(args).Run();     }      public static IWebHost BuildWebHost(string[] args) =>         WebHost.CreateDefaultBuilder(args)             .CaptureStartupErrors(true)             .UseSetting("detailedErrors", "true")             .UseStartup<Startup>()             .Build(); } 

• These settings should be removed as soon as your troubleshooting is complete so as not to expose your application to malicious attacks.

Answer 2

Connect via an sftp client and delete everything in the site/wwwroot folder manually. Republish

I have had nothing but problems since I migrated an application I have hosted on Azure to .net core from MVC 4.

At one point a few weeks ago I was unable to get a project to run after a successful publish. I even tried twice to delete the entire App Service profile and recreate it with the same name. However when I appended a '2' to the App Service name (to create a never before used app service) publishing the exact same project with 0 changes worked perfectly. What exactly does a delete do if I can publish successfully to a new app service but not a deleted and recreated one? Remove Existing Files At Destination was checked in each publish, that didn't do anything either.

I had the same error today as pictured in the OP in my #2 site. It occurred after attempting to update a number of asp nuget packages and re-deploy. Really not wanting to have to move on to iteration myApp3 of my app service, I decided to use the FTP information provided in the azure overview page. I navigated to Site/wwwroot and deleted everything inside from the FTP client. I then published the application, and it worked. I can only conclude that the 'Delete' checkbox doesn't work properly.