ASP.NET Web API - The requested resource does not support http method 'GET'

Question

I am make POST call into my Web API using Postman. I get the error: "The requested resource does not support http method 'GET'." I am not making a GET call. If I do call one of my GET methods, it works fine and returns the expected result.

My controller class:

[RoutePrefix("api/login")]
    public class LoginController : ApiController
    {
        ModelContext db = new ModelContext();

        [HttpPost]
        [Route("validate")]
        public HttpResponseMessage Validate([FromBody] LoginViewModel login)
        {
            try
            {

                    var message = Request.CreateResponse(HttpStatusCode.OK);
                    return message;
            }
            catch (Exception ex)
            {
                var message = Request.CreateErrorResponse(HttpStatusCode.BadRequest, ex.Message);
                return message;
            }
        }        

    }

I have the Web API running locally and call with this URL:

http://localhost:44303/api/login/validate

This url returns:

<Error>
<Message>
The requested resource does not support http method 'GET'.
</Message>
</Error>

My routing in WebApiConfig.cs

config.MapHttpAttributeRoutes();

            config.Routes.MapHttpRoute(
                name: "DefaultApi",
                routeTemplate: "api/{controller}/{id}",
                defaults: new { id = RouteParameter.Optional }
            );

While this controller message returns HttpResponseMessage, I have tested by changing the response to "string" and just returning a string value, but I get the same error. I have read through so many SO posts but none appear to fix my issue. I am at a total loss to explain this behavior. I appreciate any ideas.

EDIT I have tested GETs in other controllers and they are returning data as expected.

EDIT FOR CONTEXT 6/3/2020 This method in the default ValuesController works:

[Route("api/values")]
public IEnumerable<string> Get()
{
   return new string[] { "value1", "value2" };
}

These 2 methods in the SAME controller do not work:

// GET api/values/5
public string Get(int id)
{
  return "value";
}

// POST api/values
public void Post([FromBody]string value)
{
}

EDIT #2 6/3/2020 Now all of my api methods are working for the default ValuesController. I do not know why. My custom POST methods in other controllers such as the Post method above are still not working. Here is my current WebApiConfig.cs:

public static void Register(HttpConfiguration config)
        {
            // Web API configuration and services

            // Web API routes
            config.MapHttpAttributeRoutes();

            config.Routes.MapHttpRoute(
                name: "Api_Get",
                routeTemplate: "api/{controller}/{action}/{id}",
                defaults: new { id = RouteParameter.Optional, action = "Get" },
                constraints: new { httpMethod = new HttpMethodConstraint(HttpMethod.Get) }
            );

            config.Routes.MapHttpRoute(
                name: "Api_Post",
                routeTemplate: "api/{controller}/{action}/{id}",
                defaults: new { id = RouteParameter.Optional, action = "Post" },
                constraints: new { httpMethod = new HttpMethodConstraint(HttpMethod.Post) }
            );
        }

I do not have any middleware or special routing that I know of. Any exception handling would be simple try-catch.

I am trying to use attribute routing vs convention but that seems to be an issue.

Answer 1

You don't need to create a route table by HttpMethods because you have [HttpPost] attribute.

Replace all config.Routes.MapHttpRoute by

config.Routes.MapHttpRoute(
    name: "DefaultApi",
    routeTemplate: "api/{controller}/{id}",
    defaults: new { id = RouteParameter.Optional }
);

It's preferable to use Attribute Routing for Web API project. It will reduce the chances of errors because in RouteConfig class there can be any mistake while creating a new custom route. Also, you don't have to take care of the routing flow ie, from most specific to most general. All here is the attribute you use the action method.