A route named "x" is already in the route collection. Route names must be unique. Exception with ASP.NET MVC 3

I'm doing an ASP.NET MVC 3 web service and I keep getting this exception intermittently.

Stack trace:

Server Error in '/' Application.

A route named 'ListTables' is already in the route collection. Route names must be unique.
Parameter name: name

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. 

Exception Details: System.ArgumentException: A route named 'ListTables' is already in the route collection. Route names must be unique.
Parameter name: name

Source Error: 


Line 24:            //     }
Line 25:            // );
Line 26:             context.MapRoute(
Line 27:                 "ListTables",
Line 28:                 // example: 

Source File: C:\inetpub\wwwroot\SchemaBrowserService\Website\Areas\Api\ApiAreaRegistration.cs    Line: 26 

Stack Trace: 


[ArgumentException: A route named 'ListTables' is already in the route collection. Route names must be unique.
Parameter name: name]
   System.Web.Routing.RouteCollection.Add(String name, RouteBase item) +2329682
   System.Web.Mvc.RouteCollectionExtensions.MapRoute(RouteCollection routes, String name, String url, Object defaults, Object constraints, String[] namespaces) +236
   System.Web.Mvc.AreaRegistrationContext.MapRoute(String name, String url, Object defaults, Object constraints, String[] namespaces) +59
   System.Web.Mvc.AreaRegistrationContext.MapRoute(String name, String url, Object defaults) +17
   SchemaBrowserService.Areas.Api.ApiAreaRegistration.RegisterArea(AreaRegistrationContext context) in C:\inetpub\wwwroot\SchemaBrowserService\Website\Areas\Api\ApiAreaRegistration.cs:26
   System.Web.Mvc.AreaRegistration.CreateContextAndRegister(RouteCollection routes, Object state) +105
   System.Web.Mvc.AreaRegistration.RegisterAllAreas(RouteCollection routes, IBuildManager buildManager, Object state) +199
   System.Web.Mvc.AreaRegistration.RegisterAllAreas(Object state) +45
   System.Web.Mvc.AreaRegistration.RegisterAllAreas() +6
   Website.MvcApplication.Application_Start() in C:\Users\djackson\Downloads\RestApiMvc3\Website\Website\Global.asax.cs:35

Version Information: Microsoft .NET Framework Version:4.0.30319; ASP.NET Version:4.0.30319.272

It's probably related to the fact that Route Debugger shows that I have some old routes that I've modified or deleted and won't go away (even after rebooting my machine). The stack trace also refers to a source file that has long since been deleted and my app has been moved to a new location, cleaned and rebuilt since then. What am I missing?

Here is all of my route registration code:

// in Global.asax.cs:
public static void RegisterRoutes(RouteCollection routes)
{
    routes.IgnoreRoute("{resource}.axd/{*pathInfo}");
    routes.MapRoute(
        "Default2", // Route name
        "Api/{controller}/{action}/{id}", // URL with parameters
        new { controller = "DataSource", action = "Index", area = "Api", id = UrlParameter.Optional } // Parameter defaults
        );

        routes.MapRoute(
            "Default", // Route name
            "{controller}/{action}/{id}", // URL with parameters
            new { controller = "Home", action = "Index", id = UrlParameter.Optional } // Parameter defaults
        );

    }

protected void Application_Start()
{
    AreaRegistration.RegisterAllAreas();
    RegisterRoutes(RouteTable.Routes);
}

// in ApiAreaRegistration.cs:
public class ApiAreaRegistration : AreaRegistration
{
    public override string AreaName { get { return "Api"; } }

    public override void RegisterArea(AreaRegistrationContext context)
    {
        // DataSources
        
        // Tables
        context.MapRoute(
            "ListTables",
            // example: 
            // /api/DataSources/DataSource/1/schemata/schema/dbo/tables
               "Api/DataSources/DataSource/{dataSourceId}/schemata/{schemaName}/tables",
            new
            {
                controller = "Tables",
                action = "TableList",
                schemaName = "dbo",
                dataSourceId = "DefaultId"
            }
        );


        // Schemata
        context.MapRoute(
          "Schema",
            // example: 
            // /api/DataSources/DataSource/1/schemata/schema/dbo
              "Api/DataSources/DataSource/{dataSourceId}/schemata/{schemaName}",
          new
          {
              controller = "Schema",
              action = "Schema",
              dataSourceId = "DefaultId",
              schemaName = UrlParameter.Optional
          }
       );
                  
       // // DataSources
        context.MapRoute(
            "SingleDataSource",
            "Api/DataSources/DataSource/{dataSourceId}",
            new
            {
                controller = "DataSource",
                action = "DataSource",
                dataSourceId = UrlParameter.Optional
            }
        );
        context.MapRoute(
            "ListDataSources",
            "Api/DataSources",
            new
            {
                controller = "DataSource",
                action = "DataSourceList",
                dataSourceId = "DefaultId"
            }
        );
        context.MapRoute(
             "Api_default",
             "Api/{controller}/{action}/{id}",
             new { action = "Index", id = UrlParameter.Optional }
        );
       
    }
}

To fix this problem I had to go into the bin folder on my project, delete all DLL files and then rebuild and this fixed the problem.

This error can occur due to multiple causes, I had the same error and solved it by modifying the Global.asax class.

The Application_Start method at Global.asax.cs was like:

protected void Application_Start()
{
    AreaRegistration.RegisterAllAreas();
    RouteConfig.RegisterRoutes(RouteTable.Routes);
    RouteConfig.RegisterRoutes(RouteTable.Routes);
    FilterConfig.RegisterGlobalFilters(GlobalFilters.Filters);
    BundleConfig.RegisterBundles(BundleTable.Bundles);
}

The following line occurs twice in this method:

RouteConfig.RegisterRoutes(RouteTable.Routes);

This ensured that the route was twice added to the route list and at the same time causing the error.

I changed the Application_Start method as follows and the error disappeared:

protected void Application_Start()
{
    AreaRegistration.RegisterAllAreas();
    RouteConfig.RegisterRoutes(RouteTable.Routes);
    AreaRegistration.RegisterAllAreas();
    FilterConfig.RegisterGlobalFilters(GlobalFilters.Filters);
    BundleConfig.RegisterBundles(BundleTable.Bundles);
}

This may not be the answer for your problem, but can perhaps help others in the future. I didn't see this answer between the others, so I decided to add this.

I found out that Global.asax was referring to an old version of the site's DLL file before I renamed it. The DLL was not being cleaned up when I did Build > Clean up because the VS project/solution didn't refer to it any more. It seems that sometimes only the newer version of the DLL was being used, allowing the site to work correctly, but eventually both of them would be loaded causing the route conflicts.

The routes get loaded from all assemblies within AppDomain.CurrentDomain, so if your old assemblies are still part of that, you might be still getting old/duplicate routes.

In my case, I faced with this issue, when I added reference to another project from solution, which also was MVC and use the same names in area (I didn't want to added this project, I don't know how it happened). When I removed this DLL, project started to work.

Deleting the DLLs alone didn't work for me (in VS2013), but deleting the entire 'bin' and 'obj' folders and then building the solution worked perfectly! Makes me wish I hadn't spent so long trying to fix it...

None of the suggestions worked for me. Went ahead and restarted the web server (IIS in this case) and that cleared the error after I had fixed the code. DLL must have been cached in IIS.

try this code, only change name

routes.MapRoute(
            name: "Default",
            url: "{controller}/{action}/{id}",
            defaults: new { controller = "Home", action = "Index", id = UrlParameter.Optional }
        );
        routes.MapHttpRoute(
          name: "API",
          routeTemplate: "api/{controller}/{action}",
          defaults: new { action = "GetAgentId" }
      );

I am getting same error. But finally I have got solution. Scenario: I am adding different(mvc4 application) dll in my web api mvc4 application. When try to run. I am getting same error. Root Cause- When my web api application run .Application register all area from self and start loading to current application domain dll references. When application load dll(MVC4 application) that time getting error because current maproute already add key for "HelpPage_Default".

Solution. 1.Change key for RegisterArea in maproute either current application or existing application(Refer dll). 2.Move code dll(mvc4 application) code to different liberary and refer to new dll.

I was manually calling AttributeRoutingHttpConfig.Start() in my Global.asax. Did not notice this auto-generated line at the top of the file which automatically calls it.

[assembly: WebActivator.PreApplicationStartMethod(typeof(Mev.Events.Web.AttributeRoutingHttpConfig), "Start")]

I had an application that was a Forms app migrated to MVC with a third party component used for authentication which redirected to another site. The component would start a session twice if the user wasn't already logged in (once for initial connection to site and once for return). So I solved this with the following code:

if (routes.Count < 3)
            {
                routes.IgnoreRoute("login.aspx");
                routes.IgnoreRoute("default.aspx");
                routes.IgnoreRoute("{resource}.axd/{*pathInfo}");

                routes.MapRoute(
                    name: "Default",
                    url: "{controller}/{action}/{id}",
                    defaults: new {action = "Index", id = UrlParameter.Optional}
                    );
            }

Deleting the dlls in the bin folder did work 100%, I still had dlls my project needed to rebuild. Rather make a copy of the bin folder. then delete the original.rebuild the project. if it fails, place the missing dlls into the bin folder.

I was running an old MVC2 website and I got this issue because the IIS 'Managed Pipeline Mode' was set on 'Integrated' by default (press F4 on the project). Changing it to 'Classic' fixed the issue

When publishing to an Azure App Service I had to check the Publish Dialog's "Settings"->"File Publish Options"->"Remove additional files at destination" to get the old project DLL and symbol files removed. Then the site would load.

This is essentially the current answers (Fleas's) solution at the core. Delete the offending DLL.

What caused this old DLL to be retained was I was loading an older version of the website (MVC 3~5 templates but different web project with colliding namespaces, since the newer version was a copy of this project made some point in the recent past.) The newer project's DLLs just needed to be deleted. There are various ways to achieve this. I found using a dialog to be the easiest atm. Logging into the file system and axing the files by hand certainly works too.

If you are versioning, and you use two APIs with the same name, you will get this error. If you need the same Get, try changing the Name attribute of the route:

TestsController.cs:

      [MapToApiVersion("1.0")]
      [Route("{moniker}", Name = "GetTest")]
      public async Task<IHttpActionResult> Get(string moniker, bool param1 = false)

  [MapToApiVersion("1.1")]
  [Route("{moniker}", Name = "GetTest11")] // Change name here for a different version
  public async Task<IHttpActionResult> Get(string moniker)

And pass in the version in the URL:

http://localhost:6600/api/tests/2020?api-version=1.1

faced this issue. the problem was occurred after adding an area to my project. there was a call for MapMvcAttributeRoutes() in RegisterArea() method. therefore, you should not look for a duplicate route names, just look for duplicate MapMvcAttributeRoutes() calls.