Check if port can be mapped with upnp

I have a function in C# that finds an open port :

    var listener = new TcpListener(IPAddress.Loopback, 0);
        listener.Start();
        int port = ((IPEndPoint)listener.LocalEndpoint).Port;
        listener.Stop();

        return port;

Is it possible, once I have opened that port, to check if this port can be mapped with UPnP? Because I'm about to try to forward it and I want to know in advance if it's going to fail.

Yes you certainly can, however I must warn you that unfortunately many home internet routers and gateways have a poor implementation of the UPnP protocol and it doesn't always work very well. It sucks but that's the world we live in. :)

How to map or open a port using UPnP

The UPnP protocol specification is very big and complex so I strongly suggest you use a framework. There are a couple of great open source C# frameworks to use:

Mono.NAT
Open.NAT

I personally recommend Open.NAT as it has a more modern async implementation.

Here is an example of using Open.NAT to attempt to open or map UDP port 1600:

public static void Main(string[] args) {
    // Setup and initialize Open.NAT:
    NatUtility.PortMapper = Upnp; // Search only for Upnp devices
    NatUtility.TraceSource.Switch.Level = SourceLevels.Information;
    NatUtility.DeviceFound += DeviceFound;
    NatUtility.Initialize();

    // Start looking for UPnP devices:
    NatUtility.StartDiscovery();

    while(true) {}
}

// This method is called every time a new UPnP device is found:
private static async void DeviceFound (object sender, DeviceEventArgs args) {
    var deviceIP = await device.GetExternalIPAsync();
    Console.WriteLine("Found a UPnP device with IP " + deviceIP);

    Console.WriteLine("Attempting to map UDP port 1600...");
    await device.CreatePortMapAsync(new Mapping(Protocol.Udp, 1600, 1600, 10000, "Open.Nat Testing"));
}

Simple! You can of course do a lot more such as asking the router for all the UPnP ports it currently has mapped. You should check the Open.NAT documentation for that.