Port Mapping in C# is not working

See more:

C#

port-mapping

UPnP

i need to perform port forwarding in a desktop application written in C#.
i used this code :

using System;
using System.Threading;
using NATUPNPLib;

namespace iSpyApplication
{
    public static class NATControl
    {
        public static UPnPNAT NAT = new UPnPNAT();
        private static IStaticPortMappingCollection _mappings;

        public static IStaticPortMappingCollection Mappings
        {
            get
            {
               if (_mappings==null)
               {

                   try
                   {
                       if (NAT.NATEventManager != null)
                        _mappings = NAT.StaticPortMappingCollection;
                   }
                   catch
                   {
                   }
               }

               return _mappings;
            }
         }

        public static bool SetPorts(int wanPort, int lanPort)
        {
           bool b = false;
           int i = 3;
           while (Mappings == null && i > 0)
           {
               Thread.Sleep(2000);
               i--;
           }

           if (Mappings != null)
           {
               try
               {
                   Mappings.Remove(wanPort, "TCP");
               }
               catch (Exception ex)
               {
                   // do something
               }
               try
               {
                   Mappings.Add(wanPort, "TCP", lanPort, internalIP, true, "iSpy");
                   b = true;
               }
               catch (Exception ex)
               {
                   // do something
               }
           }

           return b;

        } // method

  } // class    

} // namespace

UPnP is enabled in my linksys router

the code is running and not giving any errors or exceptions, however, mapping is simply not happening.

here is how i am testing it:


1. i have an application that listens to an internal port e.g. 8080.

2. i manually add a mapping entry to my router:
application : HTTP , external port 8080 , internal port 8080, protocol: TCP , internal IP , enabled

3. i request the following URL in my browser:
http : // publicIP : externalport /
4. the application receives a socket on the internal port and replies with a message.

i manually remove the mapping entry from the router and try to add it programmatically:

1. the same
2. i run a code that performs mapping
3. the same
4. browser displays the following message :

Unable to connect

Firefox can't establish a connection to the server a publicIP:externalPort

i have searched for C# code that performs mapping, and they all look the same. Why would mapping fail ?
Thanks in advance