Can't find BasePath property in OpenApiDocument

[Oceania2018]

'OpenApiDocument' does not contain a definition for 'BasePath' and no accessible extension method 'BasePath' accepting a first argument of type 'OpenApiDocument' could be found (are you missing a using directive or an assembly reference?)

This should work for doc.BasePath.

            app.UseSwagger(c =>
            {
                if (info.RouteTemplate != null)
                    c.RouteTemplate = info.RouteTemplate;
                c.PreSerializeFilters.Add((doc, req) =>
                {
                    doc.BasePath = info.BasePath ?? "/" + serviceName;
                });
            });

https://swagger.io/docs/specification/2-0/api-host-and-base-path/