Generic startup issue

[Andrey26]

Issue:
When attempting to execute dotnet swagger tofile on dll with generic Startup class if fails to locate it.
I do not think that It is Swashbuckle library issue because .NET code fails to locate dll.

Reproduction scenario:

1. From VS create a new ASP.NET Core Web Application. You can select ASP.NET Core Web API to have request and working API.
   
2. In Startup.cs, rewrite Startup to Startup<T>. Therefore in Program.cs, rewrite

webBuilder.UseStartup();

to (for example)

webBuilder.UseStartup<Startup<int>>();

3. From cmd execute dotnet new tool-manifest in root folder of the project
4. From cmd execute dotnet tool install --local --version ${​​​​swaggerVersion}​​​​ Swashbuckle.AspNetCore.Cli
   where ${​​​​swaggerVersion}​​​​ - is a version of Swashbuckle.AspNetCore.Swagger.dll located together with .dll
5. From cmd execute dotnet tool restore
6. From cmd execute dotnet swagger tofile --output <json documentation file name to generate> <full path to .dll> <version of Web Service API>

Result:

Unhandled exception. System.InvalidOperationException: A type named 'StartupProduction' or 'Startup' could not be found in assembly 'WebApplication1'.
   at Microsoft.AspNetCore.Hosting.StartupLoader.FindStartupType(String startupAssemblyName, String environmentName)
   at Microsoft.AspNetCore.Hosting.WebHostBuilder.BuildCommonServices(AggregateException& hostingStartupErrors)
--- End of stack trace from previous location ---
   at Microsoft.AspNetCore.Hosting.WebHostBuilder.<>c__DisplayClass14_2.<BuildCommonServices>b__2(IServiceProvider _)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitFactory(FactoryCallSite factoryCallSite, RuntimeResolverContext context)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor`2.VisitCallSiteMain(ServiceCallSite callSite, TArgument argument)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitCache(ServiceCallSite callSite, RuntimeResolverContext context, ServiceProviderEngineScope serviceProviderEngine, RuntimeResolverLock lockType)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitRootCache(ServiceCallSite singletonCallSite, RuntimeResolverContext context)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor`2.VisitCallSite(ServiceCallSite callSite, TArgument argument)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.Resolve(ServiceCallSite callSite, ServiceProviderEngineScope scope)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.DynamicServiceProviderEngine.<>c__DisplayClass1_0.<RealizeService>b__0(ServiceProviderEngineScope scope)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.ServiceProviderEngine.GetService(Type serviceType, ServiceProviderEngineScope serviceProviderEngineScope)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.ServiceProviderEngine.GetService(Type serviceType)
   at Microsoft.Extensions.DependencyInjection.ServiceProvider.GetService(Type serviceType)
   at Microsoft.Extensions.DependencyInjection.ServiceProviderServiceExtensions.GetService[T](IServiceProvider provider)
   at Microsoft.AspNetCore.Hosting.WebHost.EnsureStartup()
   at Microsoft.AspNetCore.Hosting.WebHost.EnsureApplicationServices()
   at Microsoft.AspNetCore.Hosting.WebHost.Initialize()
   at Microsoft.AspNetCore.Hosting.WebHostBuilder.Build()

Expected:
Swagger JSON/YAML succesfully written to <file path>

Assumption:
Called code only checks with Startup name and name of generic class will be different.

aspnetcore/src/Hosting/Hosting/src/Internal/StartupLoader.cs

Line 243 in 371a26f

var startupNameWithoutEnv = "Startup";

aspnetcore/src/Hosting/Hosting/src/Internal/StartupLoader.cs

Line 269 in 371a26f

throw new InvalidOperationException(String.Format("A type named '{0}' or '{1}' could not be found in assembly '{2}'.",

[Tratcher]

Does dotnet run work with a generic startup?

[sviatoslav-vilkovych]

Does dotnet run work with a generic startup?

Yes, without any problems.

[Tratcher]

Is there any more to that stack trace? Who's calling WebHostBuilder.Build in this tool?

I ask because the templates haven't used WebHostBuilder since 2.2, it was replaced by HostBuilder.

aspnetcore/src/ProjectTemplates/Web.ProjectTemplates/content/WebApi-CSharp/Program.cs

Lines 20 to 21 in 139bd5c

	Host.CreateDefaultBuilder(args)
	.ConfigureWebHostDefaults(webBuilder =>

[Tratcher]

Ah:
https://github.com/domaindrivendev/Swashbuckle.AspNetCore/blob/cc13950b6bdba54c233851a26a42153f7f89e9ee/src/Swashbuckle.AspNetCore.Cli/Program.cs#L143-L144

[adityamandaleeka]

Triage: this particular issue with Swashbuckle will likely go away now that domaindrivendev/Swashbuckle.AspNetCore#2147 is being done which updates Swashbuckle to use the new host.

[adityamandaleeka]

As a workaround, you could add a class that derives from Startup, or use SwaggerHostFactory as described here: https://github.com/domaindrivendev/Swashbuckle.AspNetCore/blob/master/README.md#swashbuckleaspnetcorecli.

Closing the issue.