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Why can't I connect to my server using my public IP?

I have a server program and a client program. While developing the program I run the server and the client on the same machine for convenience. The server starts listening to incoming connections using these lines:
var listener = new TcpListener(IPAddress.Any, 7070);
listener.Start();
The client connects to the server using these lines (simplified):
var client = new TcpClient(AddressFamily.InterNetwork);
client.Connect(IPAddress.Loopback, 7070);
I use IPAddress.Loopback because I run the programs on the same machine. But, knowing that the server and the client won't be necessarily run on the same machine in the future, I changed it to my public IP from http://icanhazip.com (IPAddress.Parse(...)). Because of that the client was unable to connect to the server on the same machine with the exception No connection could be made because the target machine actively refused it <my public ip:7070>.
I tried disabling my firewall but it's still not working. Why is the server refusing the connection? Didn't I specifically tell it to listen to all interfaces with IPAddress.Any?
Why does that happen and how do I fix it?

Here's an answer built from my comments on the question, which are hopefully correct:
Your public IP is provided by your ISP and is actually the address of your router. The router does network address translation (NAT) for outgoing connections from computers within your local network. These requests look to the Internet like they're all coming from one IP, and your router sends responses to the right local computer based on an address translation table. This works for outgoing connections but not incoming connections.
If something tries to open a TCP connection from the Internet to your router, the router has no idea what local computer it might be trying to connect to unless you specifically configure it to forward that traffic to a particular computer on your local network. That's where port forwarding comes in. If you haven't configured port forwarding, the router just says, "sorry, I'm not handling incoming requests on port 7070."

Is your development machine behind a router?
Network traffic sent to you via your public IP address reaches your router on a given port via a specific networking protocol. Your router needs to know where to send this traffic internally on your network. Traffic is coming from the Internet to your machine, and your router either cannot or will not forward the traffic to your computer's machine.
You don't notice this in your day-to-day life thanks to the power of Network Address Translation (NAT) and Universal Plug and Play (UPnP). Glossing over some details here, Network Address Translation allows traffic headers to be modified to route traffic from your public IP to your actual machine's IP on the network. When incoming traffic attempts to open a port for connectivity on your network, the router needs to be configured to forward that traffic appropriately. Universal Plug and Play is a protocol supported on many modern routers to allow software and devices to seamlessly route traffic without the need to forward ports.
This leaves you with two options:
For development purposes, access your router and forward the desired port to your development machine
For a more robust application, especially if you're going to be running this on different machines or different networks, consider adding UPnP support to your application while also understanding that UPnP may not be supported or enabled by some users, in which case port forwarding is still necessary.

.Net classes to detect machines connected via cable

I need to connect two machines via cable and do some work with these once they are connected. Are there any .Net classes which would let me detect two machines connected via cable (when I say cable I do not mean computers on same network but machines which are physically connected)?
I did come across some links on stackoverflow but none seem to have been answered.
Hence any links to previous relevant posts are welcome.

There isn't a .NET class that will just give you this but there are probably some techniques you could use, to varying degrees of success:
If you have access to both machines as part of your process, set a pre-defined static IP address on each device, then if you make sure that only the Ethernet port is active on both devices just wait until you can ping your predefined static IP address and you know they're connected.
Something like https://stackoverflow.com/a/12659133/1742551, running packet sniffing code on one machine to detect when activity is happening on the other end of the cable... not sure how you'd then tell what the IP address of the other machine is though and I guess you'd need this in order to do the work you describe.
You could use the TcpListener class to listen for connections on a specific port number, then from the other machine scan all other available IPs on the same subnet as itself for a device which is listening on this port (assuming that you can ensure both machines are on the same subnet). Assuming this connection is the only connection available, and both devices have no static ip address set, Windows will autoconfigure the IP address due to the lack of DHCP server then they'll both be assigned 169.254.x.y ip addresses as part of the 255.255.0.0 subnet, so this should work.
Probably some other option that I haven't thought of yet, but you get the point...

C# TcpListener external IP

I'm creating a TcpListener, and I want clients from other computers to be able to join my listener.
I've read and understood that I have to do Port Forwarding, but it doesn't make any sense to me - when I publish my app, I want other people to create this Listener, and I can't tell them to do Port Forwarding.
Is there any possibility to create a TcpListener that clients will be able to join without Port Forwarding?
Thank you.

Well, lets try to clear somethings out first.
The main reason to use port forwarding is because you have a NAT router in front of an internal network. To setup a port forward is to instruct the NAT router to forward traffic to a certain port on the public interface to a port on an internal computer.
If you don't have a NAT router you don't need port forwarding.
Many routers today support UPnP, a technique to kindly ask the router to create a specific port forward. A suitable library to use might be ManagedUPnP.
However you still need to figure out the public IP of the router and what port you have opened and communicate that to your other applications.
If your router does not allow UPnP or there are other fire wall rules in place you can not set up a port forwarding correctly.

You can create server application and forward ports on your pc. Client application (this one you will publish) will just connect to your pc so they can be on the NAT. You can also combine your application with some php/asp pages but it depends on data you would like to send. If it's some kind of PC statistics like uptime, hardware etc. you would just use http query in client app to website script you've created (for instance mypage.com/?uptime=100&ram=2gb&hash=xxxx etc.)

Only the server (the computer which is accepting TCP requests) needs to have the port forwarded.
The common model is that you (the developer/producer of the service) host the server. Then customers (people who subscribe to your service) connect to your service using either an IP or a URL. If your service is behind a firewall (you have a router between your computer and your internet modem) then you will have to forward the port. You will probably also have to open the port in Window's firewall, but I expect you have done this already. In this model the customer does not have to do anything with their router (it is like using a web browser).
If you are making a product where your customers are hosting the service then they will have to deal with the port issues. In which case you could try ManagedUPnP like Albin Sunnanbo suggests or redirect them to one of the many sites explaining how to setup port forwarding.

How to scan for a port waiting for a connection on a network

I am currently working on a little chat utility in C#.
The problem is I can't figure out how to scan the network for a specific port on all machines. I am currently using this method
IPGlobalProperties network = IPGlobalProperties.GetIPGlobalProperties();
IEnumerable<IPEndPoint> connections = network.GetActiveTcpListeners()
.Where(x => x.Port == ConstParams.iPort);
where ConstParams.iPort is the port I want to scan (6910 here).
The problem is that the returned values are only local ports and the "0.0.0.0" ip address...
How can I scan for all open ports (6910) on the current network?

Rather than using port scanning, I suggest you to implement a simple discovery mechanism based on multicast/broadcast communication.
During the startup the application should broadcast/multicast its IP/Port information. All running instances should respond to this message with their IP/Port information. This kind of a discovery mechanism is easy to implement, and it is both faster in runtime and more dynamic than the port scanning approach.

You should consider multicast, but rather than rolling your own, rely on an existing standard with library support, like mDNS:
http://en.wikipedia.org/wiki/Multicast_DNS
Or, since you said C#, using one of its native solutions:
http://msdn.microsoft.com/en-us/library/system.net.peertopeer.aspx

Scanning ports is a poor choice, you will most likely trigger firewalls on machines in the network to display your machine as an attacker. Any Intrusion detection systems on the networks could potentially be triggered as well. It's a lot of overhead for what you need.
I would recommend doing a broadcast using UDP or a multicast to discover other clients
http://www.codeproject.com/Articles/1705/IP-Multicasting-in-C
Another option would be to have a centralized server, either on a web server (php script, asp.net page, etc) or a web service (REST) which the chat client would connect to on start up, POSTing it's listening IP/Port, and then in turn would receive a list of all recently announced IP/Ports of the other clients on the network. You'd probably want some keep alive here, IE: the client would POST to the page every 5 minutes, if an IP does not POST for 10 minutes, it would be removed from the list.
To get the public IP of the machine, you could check out this page:
http://www.whatismyip.com/faq/automation.asp
You'd just need to send a web request to it to retrieve the IP. If you want to get the non 0.0.0.0/127.0.0.1 IP of the local interface, you can check out these posts:
Get local IP address
How do I get the Local Network IP address of a computer programmatically? (C#)

GetIPGlobalProperties only returns info about your local machine (see http://msdn.microsoft.com/en-us/library/system.net.networkinformation.ipglobalproperties.getipglobalproperties.aspx ).
To find out which other machines on the network have that port open, you'd have to iterate through the a range of IPs, attempting to connect on that port. There is no central repository to query on this.
This article describes an approach: http://www.dijksterhuis.org/building-a-simple-portscanner-in-c/

why cant I see UDP traffic generated by c# application in wireshark?

I am using the UDPClient class to send and receive messages on my loopback address. The executables are also interacting with each other. But why doesnt the traffic appear in wireshark?
BTW I am running windows inside parallels on OSX and can select only 1 interface that is my intel pro net network card in wireshark.

Here's the key phrase:
on my loopback address
The loopback address is a complete additional interface, not just an address. Wireshark is configured to listen on a specific interface, and I'm guessing the loopback interface is not it.

The loopback traffic is not captured by the Net Packet Filter driver.
One workaround is to send it to the IP address of your Intel Pro NIC. In my experience, this is enough to hit the NPF capture driver and show up in Wireshark. (Well, to be accurate, my experience in that matter doesn't involve a VM so YMMV).
Of course, the listener should be bound to that NIC IP to receive the packets (and not only to localhost).