Nswag: AspNetCoreToSwaggerGenerator and TestServer (CLI): Cannot resolve IApiDescriptionGroupCollectionProvider (.NET Core 2.0)

[RicoSuter]

When executing the new AspNetCoreToSwaggerGenerator against a .NET Core 2.0 via CLI and TestServer, it cannot resolve IApiDescriptionGroupCollectionProvider:

But examining the services object, it clearly has a IApiDescriptionGroupCollectionProvider registration, even with the correct type ($1):

Also this works for the .NET Core 2.1 sample project.

Steps to reproduce:

• Open NSwag.sln
• Select NSwag.ConsoleCore as startup project and select 2.0:

• In project debug settings, use the command line args: run "MyPath\NSwag\samples\WithoutMiddleware\Sample.AspNetCore20\nswag_assembly.nswag"
• Run project with all exceptions enabled (break on all exceptions)

The same works for MyPath\NSwag\samples\WithoutMiddleware\Sample.AspNetCore21\nswag_assembly.nswag

/cc @pranavkm @rynowak any idea why this is happening?

[pranavkm]

cc @Tratcher
Maybe it's accessing host services rather than request services which is where the value needs to come from?

[Tratcher]

No, Host.Services is replaced by app services after Startup. And that service is pretty clearly in the Realized Services list.

[Ghostbird]

Any news on / workarounds for this? I'm currently stuck at this issue while trying to generate documentation from my .NET Core 2.1 API.

[RicoSuter]

@Ghostbird I think this problem here is only an issue with .NET Core <= 2.0, with 2.1+ it should work. What is your exception?

[Ghostbird]

Exception

System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation.
---> System.InvalidOperationException: No service for type 'Microsoft.AspNetCore.Mvc.ApiExplorer.IApiDescriptionGroupCollectionProvider' has been registered.

.NET Core version

dotnet --version
2.1.500

System info

uname -a
Linux gijsbertdebian 4.9.0-8-amd64 #1 SMP Debian 4.9.130-2 (2018-10-27) x86_64 GNU/Linux

MsBuild configuration excerpt

<Project Sdk="Microsoft.NET.Sdk.Web">
  <PropertyGroup>
    <TargetFramework>netcoreapp2.1</TargetFramework>
  </PropertyGroup>
  <ItemGroup>
    <PackageReference Include="NSwag.MSBuild" Version="12.0.4"/>
    <PackageReference Include="NSwag.SwaggerGeneration.AspNetCore" Version="12.0.4"/>
  </ItemGroup>
  <Target Name="NSwag" AfterTargets="Build">
    <Copy SourceFiles="@(ReferencePath)" DestinationFolder="$(OutDir)References" />
    <Exec Command="$(NSwagExe_Core21) run nswag.json" />
    <RemoveDir Directories="$(OutDir)References" />
  </Target>
</Project>

nswag.json

{
  "runtime": "NetCore21",
  "defaultVariables": null,
  "swaggerGenerator": {
    "aspNetCoreToSwagger": {
      "project": "Api.csproj",
      "noBuild": true
    }
  },
  "codeGenerators": {}
}

[Ghostbird]

Found that it is the same issue solved here.

When using services.AddMvcCore() instead of services.AddMvC() in Startup.cs:ConfigureServices(), you must do:
C# services.AddMvcCore().AddApiExplorer()

[adam-ah]

Nice find, @Ghostbird!

For Google's sake, if you get

InvalidOperationException: API Explorer not registered in DI.

on startup, the solution is to add

services.AddMvcCore().AddApiExplorer();

in public void ConfigureServices(IServiceCollection services)