If your client opens a socket to the server, it can be kept open indefinitely. Just make sure every once in a while send a few keepalive packets through. Otherwise smart routers might kill your connection.

So YES, it can be done!

You first need to ask yourself a few questions:

• Is my client a web browser or an executable?
• How many clients will be connecting to my server?
• How often are the clients going to be sent data?
• How important is it that the clients receive the data immediately (as opposed to a second or two delay)?

If your client is a web browser then your options are more limited, look at Comet for some guidance. As someone else mentioned, here s a good blog post explaining how to do Comet with ASP.NET.

If you are not expecting many clients then opening connections from the client to the server can be a viable solution, take a look at these MSDN pages for information and a basic example: - TcpListener - TcpClient

If you re expecting many clients (more than 50 depending on your server hardware) then your options depend on the answers to the other questions:

If your clients get data frequently and short (seconds) delays are acceptable then polling is an easy solution, you can hide the implementation from the client so it appears that the data is being pushed, but in reality there s a thread in the client that is polling every few seconds.

If your clients don t get data very frequently then something like Comet can be a good option, do a search for "Long Polling WCF" if you re looking for a .NET solution.

Update: Since you ve now stated that your client is Silverlight I would strongly recommend these blog posts, talks about something like what you re after:

• Sending data from the server using sockets (here, here and here)
• Sending data using WCF Duplex Service (here and here)

The WCF Duplex Service is nicer in that it uses set messages and responses, so there s no need to parse text or binary data yourself. It s basically a version of Comet for Silverlight.

The short answer is to use polling.

This question is very similar:

asp.net http server push to client

I suggest you use Comet. Check out the following article and example.

I needed to do this earlier this year, but I can t find the code on this computer. Does this SO link help?

UPDATE: I found the article that I used when I needed to get this. Warning it uses 3.5 and I think WCF. Code copied from Keyvan Nayyeri s article here:

        OperationContext context = OperationContext.Current;
        MessageProperties messageProperties = context.IncomingMessageProperties;
        RemoteEndpointMessageProperty endpointProperty =
            messageProperties[RemoteEndpointMessageProperty.Name]
            as RemoteEndpointMessageProperty;

        return string.Format("Hello {0}! Your IP address is {1} and your port is {2}",
            value, endpointProperty.Address, endpointProperty.Port);

Your best solution is to use polling like Kragen suggest s, it ensures your server doesn t get bogged down with idle sessions that only exist to send data in rare occasions. Polling gives you the same functionality however you don t have that constant load on your server, that in addition to needing one thread per client (assuming your server spawns a thread for each client like apache, etc). With a thin client (running within a browser) you could do it using AJAX to prevent constant reloading of the page. If within a thick client you could do the same thing but use a background timed event to send requests.

If you still need to push data to your client from the server in a Fat Client scenario the best solution is to have the client open a listening socket, when a request from the server needs to be pushed to the client, the server connects to the client and sends the request (upon authentication the server records that users IP to initiate the connection). In a thin client scenario (via web browser) this is extremely harder to do if not impossible (when proper security policies are used).

We would suggest a more detailed explanation of why you are trying to do this, that would allow us to give you a more targeted solution that fits your needs.

there is a lot of ways - best way - to have the client check the server for updates, and have it pull the data over (using standard ftp protocol - for files or other communication protocols for anything else - like WCF, Web Services ect.) this way you don t need to worry about firewalls and nats.

Viewing you comment - it will be hard, as if the client is behind nat it is not possible without static routing - so if this might be your case, you can t push... you have to open the connection from the client side.