Get local IP address

In the internet there are several places that show you how to get an IP address. And a lot of them look like this example:

String strHostName = string.Empty;
// Getting Ip address of local machine...
// First get the host name of local machine.
strHostName = Dns.GetHostName();
Console.WriteLine("Local Machine's Host Name: " + strHostName);
// Then using host name, get the IP address list..
IPHostEntry ipEntry = Dns.GetHostEntry(strHostName);
IPAddress[] addr = ipEntry.AddressList;

for (int i = 0; i < addr.Length; i++)
{
    Console.WriteLine("IP Address {0}: {1} ", i, addr[i].ToString());
}
Console.ReadLine();

With this example I get several IP addresses, but I’m only interested in getting the one that the router assigns to the computer running the program: the IP that I would give to someone if he wishes to access a shared folder in my computer for instance.

If I am not connected to a network and I am connected to the internet directly via a modem with no router then I would like to get an error. How can I see if my computer is connected to a network with C# and if it is then to get the LAN IP address.