NSwag error - System.InvalidOperationException: No service for type 'Microsoft.Extensions.DependencyInjection.IServiceProviderFactory`1[Autofac.ContainerBuilder]' has been registered.

In one of our projects we are using NSwag to generate our TypeScript DTO’s and services used in our Angular frontend. In this project we are using Autofac as our IoC container and have created a few extension methods that hook into the HostBuilder bootstrapping.

Unfortunately our custom logic brought NSwag into trouble and caused our build to fail with the following error message:

System.InvalidOperationException: No service for type 'Microsoft.Extensions.DependencyInjection.IServiceProviderFactory`1[Autofac.ContainerBuilder]' has been registered.

NSwag adds an extra build target in your csproj file and uses that to run the NSwag codegenerator tool:

	<Target Name="NSwag" AfterTargets="Build">
	<Exec Command="$(NSwagExe_Net50) run /variables:Configuration=$(Configuration)" />
	</Target>

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While investigating the root cause of this issue, we introduced a small workaround where we used a separate program.cs and startup.cs specifically for the NSwag codegenerator.

We added a minimal Program.cs file:

	public class NSwagProgram
	{
	/// <summary>
	/// Workaround Fake WebHostBuilder for NSwag code generator.
	/// While NSwag scans Assembly for Controllers, this throws an error because of SOFA Core setup.
	/// </summary>
	/// <param name="args"></param>
	/// <returns></returns>
	public static IWebHostBuilder FakeNSwagHostBuilder(string[] args) => WebHost.CreateDefaultBuilder(args)
	.ConfigureServices(services => services.AddAutofac())
	.UseStartup<NSwagStartup>();
	}

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And a minimal Startup.cs file:

	public class NSwagStartup
	{
	public NSwagStartup(IConfiguration configuration)
	{
	Configuration = configuration;
	}
	public IConfiguration Configuration { get; }
	public void ConfigureServices(IServiceCollection services)
	{
	services.AddControllers();
	services.AddOpenApiDocument(configure =>
	{
	configure.Title = "Example API";
	configure.DefaultReferenceTypeNullHandling = NJsonSchema.Generation.ReferenceTypeNullHandling.NotNull;
	configure.DefaultResponseReferenceTypeNullHandling = NJsonSchema.Generation.ReferenceTypeNullHandling.NotNull;
	configure.RequireParametersWithoutDefault = true;
	configure.SchemaType = NJsonSchema.SchemaType.Swagger2;
	});
	}
	public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
	{
	}
	}

view raw NSwagStartup.cs hosted with ❤ by GitHub

The magic to make this work is to change the nswag.json configuration file in the root of our project and point to the alternative files we’ve created above:

"createWebHostBuilderMethod": "Example.App.Web:Example.App.Web.NSwag.NSwagProgram.FakeNSwagHostBuilder",

"startupType": "Example.App.Web:Example.App.Web.NSwag.NSwagStartup",

	{
	"runtime": "Net50",
	"defaultVariables": null,
	"documentGenerator": {
	"aspNetCoreToOpenApi": {
	"project": "",
	"msBuildProjectExtensionsPath": null,
	"configuration": null,
	"runtime": null,
	"targetFramework": null,
	"noBuild": true,
	"verbose": false,
	"workingDirectory": null,
	"requireParametersWithoutDefault": true,
	"apiGroupNames": null,
	"defaultPropertyNameHandling": "CamelCase",
	"defaultReferenceTypeNullHandling": "Null",
	"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": "Example API",
	"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": "Example.App.Web:Example.App.Web.NSwag.NSwagProgram.FakeNSwagHostBuilder",
	"startupType": "Example.App.Web:Example.App.Web.NSwag.NSwagStartup",
	"allowNullableBodyParameters": true,
	"output": "wwwroot/api/specification.json",
	"outputType": "OpenApi3",
	"newLineBehavior": "Auto",
	"assemblyPaths": [
	"bin/$(Configuration)/net5.0/Example.App.Web.dll"
	],
	"assemblyConfig": null,
	"referencePaths": [],
	"useNuGetCache": true
	}
	},
	"codeGenerators": {
	"openApiToTypeScriptClient": {
	"className": "{controller}Client",
	"moduleName": "",
	"namespace": "",
	"typeScriptVersion": 2.7,
	"template": "Angular",
	"promiseType": "Promise",
	"httpClass": "HttpClient",
	"withCredentials": false,
	"useSingletonProvider": true,
	"injectionTokenType": "InjectionToken",
	"rxJsVersion": 6.0,
	"dateTimeType": "Date",
	"nullValue": "Undefined",
	"generateClientClasses": true,
	"generateClientInterfaces": true,
	"generateOptionalParameters": false,
	"exportTypes": true,
	"wrapDtoExceptions": false,
	"exceptionClass": "SwaggerException",
	"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",
	"enumStyle": "Enum",
	"useLeafType": false,
	"classTypes": [],
	"extendedClasses": [],
	"extensionCode": null,
	"generateDefaultValues": false,
	"excludedTypeNames": [],
	"excludedParameterNames": [],
	"handleReferences": false,
	"generateConstructorInterface": true,
	"convertConstructorInterfaceData": false,
	"importRequiredTypes": true,
	"useGetBaseUrlMethod": false,
	"baseUrlTokenName": "API_BASE_URL",
	"queryNullValue": "",
	"useAbortSignal": false,
	"inlineNamedDictionaries": false,
	"inlineNamedAny": false,
	"templateDirectory": null,
	"typeNameGeneratorType": null,
	"propertyNameGeneratorType": null,
	"enumNameGeneratorType": null,
	"serviceHost": null,
	"serviceSchemes": null,
	"output": "Client/src/app/shared/api/example-api.ts",
	"newLineBehavior": "Auto"
	}
	}
	}

view raw nswag.json hosted with ❤ by GitHub

Although this is not a final solution, until I found the exact root cause, this will do the trick...