ASP.NET Core - Swashbuckle.AspNetCore.SwaggerGen.SwaggerGeneratorException: Ambiguous HTTP method for action

I’m currently rewriting an existing ASP.NET Web API application to ASP.NET Core.

I ported a controller from ASP.NET Web API to ASP.NET Core. Here is the end result:

	[ApiController]
	[Route("[controller]")]
	public class AuthTypeController : ControllerBase
	{
	private readonly IAuthTypeService _authTypeService;
	public AuthTypeController(IAuthTypeService authTypeService)
	{
	_authTypeService = authTypeService;
	}
	[HttpGet("{id}")]
	[ResponseCache(Duration =3600, Location = ResponseCacheLocation.Any)]
	public async Task<ActionResult<AuthType>> GetAuthTypeById(int ID)
	{
	var authType = await _authTypeService.FindById(ID);
	if (authType == null)
	{
	return NotFound();
	}
	return authType;
	}
	public async Task<ActionResult<IList<AuthType>>> GetAll()
	{
	var authTypes = await _authTypeService.FindAll();
	return new List<AuthType>(authTypes);
	}
	}

view raw AuthTypeController.cs hosted with ❤ by GitHub

Everything looked OK at first sight, but when I tried to run the application I got the following error message:

Swashbuckle.AspNetCore.SwaggerGen.SwaggerGeneratorException: Ambiguous HTTP method for action - Controllers.AuthTypeController.GetAll (VLM.IAM.Services). Actions require an explicit HttpMethod binding for Swagger/OpenAPI 3.0

   at Swashbuckle.AspNetCore.SwaggerGen.SwaggerGenerator.GenerateOperations(IEnumerable`1 apiDescriptions, SchemaRepository schemaRepository)

   at Swashbuckle.AspNetCore.SwaggerGen.SwaggerGenerator.GeneratePaths(IEnumerable`1 apiDescriptions, SchemaRepository schemaRepository)

   at Swashbuckle.AspNetCore.SwaggerGen.SwaggerGenerator.GetSwagger(String documentName, String host, String basePath)

   at Swashbuckle.AspNetCore.Swagger.SwaggerMiddleware.Invoke(HttpContext httpContext, ISwaggerProvider swaggerProvider)

   at Microsoft.AspNetCore.Server.IIS.Core.IISHttpContextOfT`1.ProcessRequestAsync()

The Swagger/OpenAPI middleware requires that you explicitly annotate all your action methods. So I had to update the GetAll action method and annotate it with a [HttpGet] attribute:

	[HttpGet]
	public async Task<ActionResult<IList<AuthType>>> GetAll()
	{
	var authTypes = await _authTypeService.FindAll();
	return new List<AuthType>(authTypes);
	}

view raw GetAll.cs hosted with ❤ by GitHub