NSwag.MSBuild fails under NetCore_31: System.InvalidOperationException: Unable to resolve service for type 'Microsoft.Extensions.Configuration.IConfiguration' while attempting to activate 'Startup'

I get the following error using NSwag.MSBuild to generate clients. The project should have been set up to

System.InvalidOperationException: Unable to resolve service for type 'Microsoft.Extensions.Configuration.IConfiguration' while attempting to activate 'Startup'.
   at Microsoft.Extensions.DependencyInjection.ActivatorUtilities.ConstructorMatcher.CreateInstance(IServiceProvider provider)
   at Microsoft.Extensions.DependencyInjection.ActivatorUtilities.CreateInstance(IServiceProvider provider, Type instanceType, Object[] parameters)
   at Microsoft.Extensions.DependencyInjection.ActivatorUtilities.GetServiceOrCreateInstance(IServiceProvider provider, Type type)
   at Microsoft.AspNetCore.Hosting.StartupLoader.LoadMethods(IServiceProvider hostingServiceProvider, Type startupType, String environmentName)
   at Microsoft.AspNetCore.Hosting.WebHostBuilderExtensions.<>c__DisplayClass3_0.<UseStartup>b__1(IServiceProvider sp)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitFactory(FactoryCallSite factoryCallSite, RuntimeResolverContext context)
...

The .csproj has this set

<Project Sdk="Microsoft.NET.Sdk.Web">
  <PropertyGroup>
    <TargetFramework>netcoreapp3.1</TargetFramework>
    <CopyLocalLockFileAssemblies>true</CopyLocalLockFileAssemblies>
  </PropertyGroup>

  <Target Name="NSwag" AfterTargets="Build">
    <Copy SourceFiles="@(ReferencePath)" DestinationFolder="$(OutDir)References" />
    <Exec Command="$(NSwagExe_Core31) run nswag.json /variables:FilePath=&quot;$(OutDir)$(AssemblyName).dll&quot;" />
    <RemoveDir Directories="$(OutDir)References" />
  </Target>
</Project>

nswag.json looks like so:

{
    "runtime": "NetCore31",
    "defaultVariables": null,
    "documentGenerator": {
        "aspNetCoreToOpenApi": {
            "project": null,
            "msBuildProjectExtensionsPath": null,
            "configuration": null,
            "runtime": null,
            "targetFramework": null,
            "noBuild": false,
            "verbose": true,
            "workingDirectory": null,
            "requireParametersWithoutDefault": true,
            "apiGroupNames": null,
            "defaultPropertyNameHandling": "Default",
            "defaultReferenceTypeNullHandling": "NotNull",
            "defaultDictionaryValueReferenceTypeNullHandling": "NotNull",
            "defaultResponseReferenceTypeNullHandling": "NotNull",
            "defaultEnumHandling": "Integer",
            "flattenInheritanceHierarchy": false,
            "generateKnownTypes": true,
            "generateEnumMappingDescription": false,
            "generateXmlObjects": false,
            "generateAbstractProperties": false,
            "generateAbstractSchemas": true,
            "ignoreObsoleteProperties": false,
            "allowReferencesWithProperties": false,
            "excludedTypeNames": [],
            "serviceHost": null,
            "serviceBasePath": null,
            "serviceSchemes": [],
            "infoTitle": "My Title",
            "infoDescription": null,
            "infoVersion": "1.0.0",
            "documentTemplate": null,
            "documentProcessorTypes": [],
            "operationProcessorTypes": [],
            "typeNameGeneratorType": null,
            "schemaNameGeneratorType": null,
            "contractResolverType": null,
            "serializerSettingsType": null,
            "useDocumentProvider": true,
            "documentName": "v1",
            "aspNetCoreEnvironment": null,
            "createWebHostBuilderMethod": null,
            "startupType": null,
            "allowNullableBodyParameters": true,
            "output": "swagger.json",
            "outputType": "Swagger2",
            "assemblyPaths": [
                "$(FilePath)"
            ],
            "assemblyConfig": null,
            "referencePaths": [],
            "useNuGetCache": false
        }
    },
    "codeGenerators": {
        "openApiToTypeScriptClient": {
            "className": "{controller}Client",
            "moduleName": "",
            "namespace": "",
            "typeScriptVersion": 2.7,
            "template": "Axios",
            "promiseType": "Promise",
            "httpClass": "HttpClient",
            "withCredentials": false,
            "useSingletonProvider": false,
            "injectionTokenType": "OpaqueToken",
            "rxJsVersion": 6.0,
            "dateTimeType": "Date",
            "nullValue": "Undefined",
            "generateClientClasses": true,
            "generateClientInterfaces": false,
            "generateOptionalParameters": false,
            "exportTypes": true,
            "wrapDtoExceptions": false,
            "exceptionClass": "ApiException",
            "clientBaseClass": null,
            "wrapResponses": false,
            "wrapResponseMethods": [],
            "generateResponseClasses": true,
            "responseClass": "SwaggerResponse",
            "protectedMethods": [],
            "configurationClass": null,
            "useTransformOptionsMethod": false,
            "useTransformResultMethod": false,
            "generateDtoTypes": true,
            "operationGenerationMode": "MultipleClientsFromOperationId",
            "markOptionalProperties": true,
            "generateCloneMethod": false,
            "typeStyle": "Class",
            "classTypes": [],
            "extendedClasses": [],
            "extensionCode": null,
            "generateDefaultValues": true,
            "excludedTypeNames": [],
            "excludedParameterNames": [],
            "handleReferences": false,
            "generateConstructorInterface": true,
            "convertConstructorInterfaceData": false,
            "importRequiredTypes": true,
            "useGetBaseUrlMethod": false,
            "baseUrlTokenName": "API_BASE_URL",
            "queryNullValue": "",
            "inlineNamedDictionaries": false,
            "inlineNamedAny": false,
            "templateDirectory": null,
            "typeNameGeneratorType": null,
            "propertyNameGeneratorType": null,
            "enumNameGeneratorType": null,
            "serviceHost": null,
            "serviceSchemes": null,
            "output": "../@company/product-api/ApiClient.generated.ts"
        }
    }
}

[RicoSuter]

Have you tried using “project” parameter (with path to .csproj) and “noBuild”: true instead of “assemblyPaths”? This is the preferred and more tested way to load the project from CLI.

[shivanaru]

Thank you for your reply - @RicoSuter . I figured out the issue was the csproj file had <platform>x64 </platform>. Once I removed it and saved the csproj file, Lamar worked fine and then NSwag had no issue. It worked with either AddOpenApiDocument or AddSwaggerDocument. Either one was fine. The swagger output was different based on which option. I went to OpenApi spec and that was what I needed. So switched to that. NSwag is fantastic!! Thank you again for all your work on it!