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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:

enter image description here

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.


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=, 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:



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"



1 Answer

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Try to enable the 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()



      .UseSetting("detailedErrors", "true")







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

public class Program  


    public static void Main(string[] args)




    public static IWebHost BuildWebHost(string[] args) =>



            .UseSetting("detailedErrors", "true")




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

You must remove such settings before the troubleshooting the troubleshooting is complete, otherwise, it will expose your application to malicious attacks.

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