How to debug startup in Web Core API?

Question

I have an MVC website that uses a Web Core API. After making a minor change and a deployment I unexpectedly got an error; Response status code does not indicate success: 500 (Internal Server Error). So I enabled the log file for the Web Core API (see https://docs.microsoft.com/en-us/aspnet/core/host-and-deploy/iis/troubleshoot?view=aspnetcore-2.0#aspnet-core-module-stdout-log) and I get this error message;

Application startup exception: System.Collections.Generic.KeyNotFoundException: The given key was not present in the dictionary. at System.Collections.Generic.Dictionary2.get_Item(TKey key) at Microsoft.AspNetCore.Mvc.Internal.DefaultAssemblyPartDiscoveryProvider.CandidateResolver.ComputeClassification(String dependency) at Microsoft.AspNetCore.Mvc.Internal.DefaultAssemblyPartDiscoveryProvider.CandidateResolver.ComputeClassification(String dependency) at Microsoft.AspNetCore.Mvc.Internal.DefaultAssemblyPartDiscoveryProvider.CandidateResolver.ComputeClassification(String dependency) at Microsoft.AspNetCore.Mvc.Internal.DefaultAssemblyPartDiscoveryProvider.CandidateResolver.d__4.MoveNext() at System.Linq.Enumerable.d__172.MoveNext() at System.Linq.Enumerable.WhereSelectEnumerableIterator`2.MoveNext() at Microsoft.Extensions.DependencyInjection.MvcCoreServiceCollectionExtensions.GetApplicationPartManager(IServiceCollection services) at Microsoft.Extensions.DependencyInjection.MvcCoreServiceCollectionExtensions.AddMvcCore(IServiceCollection services) at Properties.API.Startup.ConfigureServices(IServiceCollection services) --- 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: Staging Content root path: C:\inetpub\wwwroot\SIR Now listening on: http://localhost:33007 Application started. Press Ctrl+C to shut down.

UPDATE: The line where it is falling over is;

services.AddMvcCore().AddJsonFormatters();

in ConfigureServices.

How do I debug this to find out what is causing this?

public class Startup {
 public Startup(IHostingEnvironment env) {
  var builder = new ConfigurationBuilder()
   .SetBasePath(env.ContentRootPath)
   .AddJsonFile("appSettings.json", optional: false, reloadOnChange: true)
   .AddJsonFile($ "appSettings.{env.EnvironmentName}.json", optional: true, reloadOnChange: true)
   .AddEnvironmentVariables();

  Configuration = builder.Build();
 }

.
.
.

    public void ConfigureServices(IServiceCollection services)
    {
        services.AddCors();
        services.AddMvcCore().AddJsonFormatters();
        services.Configure<IISOptions>(options => new IISOptions
        {
            AutomaticAuthentication = true,
            ForwardClientCertificate = false,
            ForwardWindowsAuthentication = false
        });

        var connectionStringMSurveyV2 = Configuration.GetConnectionString("MSurveyV2Db");
        services.AddScoped<MSurveyV2Db>(_ => new MSurveyV2Db(connectionStringMSurveyV2));
        var connectionStringSir = Configuration.GetConnectionString("SirDb");
        services.AddScoped<SirDb>(_ => new SirDb(connectionStringSir));
        services.AddScoped<IPropertiesRepo, PropertiesRepo>();
        services.AddScoped<ISirUoW, SirUoW>();
        services.AddScoped<IPropertyUoW, PropertyUoW>();
        services.AddScoped<Services.IMailService, Services.MailService>();
    }

. . .

Exception happens inside ConfigureServices, which code is not shown. — Evk
I am wondering how you worked that out? Even so I have posted the ConfigureServices anyway. — arame3333
It says so right in stack trace you posted: " at Properties.API.Startup.ConfigureServices(...)" — Evk
And exception is thrown by AddMvcCore. Do you know the difference between AddMvcCore and AddMvc, and why you are using AddMvcCore specifically? — Evk
So you don't really know the difference. Because, Core in AddMvcCore means basically "only add core components", not "add asp.net Core". While AddMvc also calls AddMvcCore internally, but also add more essential components on top of that. — Evk

Saša Ćetković Saša Ćetković · Accepted Answer · 2018-05-31T07:58:16

You didn't say which version of ASP.NET Core you are using. If it's 2.0+, you can configure NLog in Program.cs to load before Startup:

public static class Program
{
    public static void Main(string[] args)
    {
        var logger = NLogBuilder.ConfigureNLog("nlog.config").GetCurrentClassLogger();

        try
        {
            var config = new ConfigurationBuilder()
                    .SetBasePath(Directory.GetCurrentDirectory())
                    .AddJsonFile("hosting.json", optional: true)
                    .Build();

            BuildWebHost(args, config).Run();
        }
        catch (System.Exception ex)
        {
            logger.Error(ex, "An error occurred during program startup");
            throw;
        }
    }

    private static IWebHost BuildWebHost(string[] args, IConfigurationRoot config)
    {
        return WebHost.CreateDefaultBuilder(args)
            .UseConfiguration(config)
            .UseStartup<Startup>()
            .UseNLog()
            .Build();
    }
}

How to debug startup in Web Core API?

1 Answers