StaticPortMappingCollection is null

问题

my StaticPortMappingCollection is remaining null. I am trying to do port forwarding application. Below is my code:

NATUPNPLib.UPnPNATClass upnpnat = new NATUPNPLib.UPnPNATClass();
NATUPNPLib.IStaticPortMappingCollection mappings = upnpnat.StaticPortMappingCollection;
int port = Convert.ToInt32(txtPort.Text);
mappings.Add(port, "TCP", port, lblLocalIP.Text, true, this.Name);

来源：https://stackoverflow.com/questions/21262410/staticportmappingcollection-is-null