Error: "Not supported by Swagger 2.0: Multiple operations with path 'api/Products' and method 'GET'"

[ChristianWeyer]

Hm... I see in the README that Swagger 2.0 does not include the query string component when mapping a URL to an action.
As a result, Swashbuckle will raise an exception if it encounters multiple actions with the same path (sans query string) and HTTP method.

Phew. Is there any generic solution for that? We (and others, I figure) do have a lot of APIs with query strings and a standard route like
config.Routes.MapHttpRoute("DefaultApi", "api/{controller}");

Any ideas?
Thanks!

[domaindrivendev]

Unfortunately this is a constraint imposed by the Swagger specification https://github.com/swagger-api/swagger-spec/blob/master/versions/2.0.md.

I had some late involvement with the Swagger 2.0 working group and pushed hard to have this constraint removed but to no avail. As you pointed out, this does cause an issue for certain WebApi implementations. I'll describe the workaround below but first I want to play devil's advocate and look at it from an API perspective, agnostic of implementation frameworks or even specific language constructs.

In it's essence, the constraint really just says the following paths have to be described as one operation.

GET api/products
GET api/products?producType={productType}

Behind the scenes these may be implemented as separate C# or separate Java methods, but in the context of describing a REST API, would you describe them separately? Swagger 2.0 says "No ... it's one operation with an optional productType parameter".

Technically, I believe they are two different resources (hence why I opposed the constraint) but I do see some sense in the Swagger 2.0 approach. For example, I don't think I've ever seen any API docs broken down this way - it's invariably just by path with additional information about query parameters included.

Anyway, philosophy aside - breaking the Swagger 2.0 spec is not an option and so I can only provide some workarounds.

The most straight forward would be to consolidate your multiple, "overloaded", actions with a single action with optional parameters. You could even delegate internally to the private overloaded versions.

If a change to the implementation isn't an option - Swashbuckle 5.0 provides a config setting ResolveConflictingActions. This takes a function of the form - Func<<IEnumerable<ApiDescription>, ApiDescription> which you can provide to consolidate the actions into one ApiDescription, and therefore one Operation, at the documentation level only.

This "merge" process will get trickier if the response type and errors codes also differ but at that point you'd have to ask questions about the API design.

Hope this helps - let me know if it makes sense?

[ChristianWeyer]

Hmm...
I think having a method for list of products like this:
public IEnumerable GetProducts()

and one method for getting product details like this:
public ProductDto GetProduct(int id)

is a very common scenario.
Different return types, at least.

So yes: changing the implementation is not an option.
Hm...

[domaindrivendev]

True, it's extremely common to have a "collection" resource and corresponding "item" resource. However, the standard approach here would be to use a "path" parameter for the id:

GET /api/products
GET /api/products/{id}

If you're early in your API design, I would strongly advice this approach. It's a widely accepted standard in REST design. In fact it's reflected by the default route template when you create a new WebApi project:

routes.MapHttpRoute(
    name: "API Default",
    routeTemplate: "api/{controller}/{id}",
    defaults: new { id = RouteParameter.Optional }
);

Swagger will have no problem with this because the two different operations can be distinguished by path alone. If you want to go against the grain and represent the two different operations as follows:

GET /api/products
GET /api/products?id={id}

Then you will find it very difficult to describe your API with Swagger. Although it does represent best practice, I do think Swagger 2.0 is a little too opinionated in this regard. Swashbuckle provides good workarounds and frankly, can't do much more. If you feel very strongly about this, I would recommend posting an issue here - https://github.com/swagger-api/swagger-spec

[OnamChilwan]

@domaindrivendev you mention workarounds via Swashbuckle what are these? The only one I have comes across is using the first action in the list which hides implementation in my opinion and isn't ideal. Is there a way of exposing your example above?

[domaindrivendev]

True, it's not ideal! However, I still maintain that it's due to an overly opinionated constraint in the Swagger specification and with that being the root cause, maybe worth posting an issue there.

The bottom line is this ... because of this constraint, you CANNOT describe the actions as separate Swagger Operations and so, the corresponding ApiDescriptions have to be merged into one for the Swagger document to be generated. This is what the ResolveConflictingActions option is for.

It's also worth noting that you're not limited to just taking the first description, you're free to implement any merge strategy you like. For example, if the actions share the same response type and only differ in parameters, you could return a completely new ApiDescription that includes a union of parameters, marking the non-common ones as optional.

Finally, if they don't share the same response type ... then, at an API level (independent of SB, C# or any server-side frameworks) you simply won't be able to describe your API with Swagger.

[decoder318]

@domaindrivendev

The workaround specified in the documentation won't work.

httpConfiguration
.EnableSwagger((c) =>
{
c.SingleApiVersion("v1", "A title for your API"));
c.ResolveConflictingActions(apiDescriptions => apiDescriptions.First());
});

cuz apiDescription is of type IEnumerable and not an ICollection or IList
Is there a better way of handling this? Or did you forget updating the documentation?

[decoder318]

Also, for some reason, the rendered swagger lists about 30 controllers, and ignores the rest.

[jbongaarts]

@decoder318 The work around does work. First() is an extension method for IEnumerable. Add 'using System.Linq;' to make it available.

[gjj]

@jbongaarts can confirm this works! Have been trying to look for a solution for ages.

[bernardbr]

@jiajian this works but not as well. In fact it suppresses the methods that conflict leaving only one in the documentation.

[caodaiming]

action added route("api/getstudent")