Develop Reference

Detect Active RDP Session

I need to implement a dead man's switch in my application. If the application is running from RDP, I need to act if it loses connection to the remote client.
I know when I am running in RDP by using
GetSystemMetrics(SystemMetric.SM_REMOTESESSION)
But when the client closes without signing out, the session will continue. This is the scenario I want to react to, but I don't know how to detect a client disconnecting. I need to know if there is an active RDP user or not.
I could potentially find the remote endpoint by watching the RDP port, but as think could potentially be setup on a non-default port, I'd like to avoid this solution if a better one exists.
I'd prefer a solution that was not specific to WinForms, WPF, UWP etc. Bonus points if it works with .NET Core as well.

Not a .NET Core solution, but a windows API one. There are session change notifications that you can opt into via WTSRegisterSessionNotification (and similarly unregister later).
These notifications are then delivered in your windows message loop, so you need to be running one. (WinForms and WPF do, and there are specific mechanisms to allow you to perform custom message handling)
You'll then get notified when the session becomes locked or disconnected.

How would I run a continuous task on an ASP.Net Server?

I've tried researching this, but haven't found much that sounds similar to something I'm needing to implement. In short, we'll be running an ASP Website on a server that will be accessed by clients. Ideally, we have a function that we want to initialize upon the start of a user's session, and stop when the session ends. While the session is happening, this function sends and receives messages via socket communication, meaning we need to access the send/receive functions of this class from pages in order to move information. What's the best way to go about this?

Look into SignalR. That's probably what you're wanting. Its "hubs" are effectively what you're looking for to spin up on session initiation, and spin down when the user disappears. It has a client-side JS library that automatically chooses the best connection method available (e.g., websockets > server-sent-events > long-polling), and it allows you to send messages both from the client to the server, and from the server to the client.
http://www.asp.net/signalr
Another alternative that I've played around with in the past is XSockets:
https://xsockets.net/
It's similar to SignalR in many respects, but it's not free.

It's hard to tell from you description, are you looking to communicate with the client browser via sockets? Or are you trying to communicate with some other service via sockets?
Web applications are not ideally suited for deterministic types of actions. It's difficult for the web server to know whether or not the client has actually closed their browser or not. In most cases, sessions simply time out after a period of inactivity (20+ minutes in most cases). So you cannot reliably know when the users session has actually ended.
To top it off, there are certain edge cases where Session_End will not fire. For instance, if the app pool recycles, then no Session_End event will fire. This may not be an issue, since if the app pool recycles your other connections would also recycle, but it's still an issue to keep in mind.
Finally, Web apps are not intended to be long running.

Not lock up the GUI on thread.sleep

I'm writing a multiple user server\client application.
Essentially, it will implement a chat room and allow users to communicate with each other. I've gotten the application to work between the server\client so far by sending a request to the server, which is always checking for an incoming network connection, and responding to it immidiately.
However, for the client to receive chat messages from the server, the only thing I can think of is running a server on the client. If I were to do this, however, the client would freeze up and not be able to do anything. Plus, the client is not designed for opening ports to connect to the server.
What would be the best recommendation on waiting on data from the server to come to the client, without causing the client to lock up?
Thanks!
(and also, I'm not a \professional\ c# programmer, more of an amateur, so please don't give me very complicated answers)

If I were you, I would use either a background worker, or a second thread. If you don't want to do that, you can use thread.suspend.
To start a new thread:
Using System.Threading;
Thread t = new Thread()
t.Start;
Note: this is not recommended.

Communication between C# clients which are behind router and Java server which is on public internet

I have a C# applications which acts like a client and it can be installed on any system which is directly connected to public internet (through data cards or port forwarding) or they can be behind router also (without port forwarding).
The other application which is developed using java acts like a server application which is on the public internet. Now, my java application wants to push a message to C# application which is behind router. Java application has the clients public and private (192.168.x.x) IP address. Java application is supposed to run 24x7.
So, now there are two options for me:
Whenever c# application starts it will establish a socket connection with java application and this socket connection will remain open till C# application gets closed.
Whenever Java application has something for C# application it will create a socket connection with C# application then it will push the message and then close the connection.
Now, with 1st option there is a problem that there will be lots of unnecessary connection since there can be thousands of client application and it may happen that on some day there will be nothing to push for some clients. and I don't know how to go for 2nd option.
What will be the right way to accomplish this task (option 1 or 2)?
Is UPnP protocol right for 2nd option? What are the open source UPnP libraries which has both the API's (C# and Java). I found one such called ohnet. Will it be a right thing for me? I didn't found a single small example for OhNet to test.

2) is not feasible if you don't have control over network configuration at the client end. It won't in general be possible for the server to make connections to the client if the client is behind any moderately secure firewall / router.
So you will in general have have to go for some variant of 1) where the client creates a connection to the server.
You don't necessarily have to keep the connection open though - it's always possible to get the client to poll the server periodically to check if there are any new updates.
If you want realtime updates to the client from the server then you will still need to keep a connection open. This isn't necessarily a problem if you use Java NIO you should be able to handle tens of thousands of simultaneous incoming connections relatively easily.

Using option 2, will you have to queue messages for your C# client until it connects? That could make your Java application run into out of memory problems if the C# application doesn't connect.

I would definitely use method 2 by adding a static route in the router (port forward). You should - however - ensure that the server behind the router is protected from the rest of your network (DMZ).
UPDATE:
Perhaps I have missed something here (method 1 or 2) :-) - but just to make it absolutely clear: It is always the client that should initiate the connection to the server. And yes, you could allow the client to request the server for updates on a regular basis.

Creating a heavily restricted server discovery application in c#

I am writing you because of a new problem I need to solve, and I have now been banging my head against a wall for too long now.
Basically, I need to create an application that can take care of the following:
A user starts an app, which sends a broadcast to the subnet, and recieves a response of all servers there with their IP (and some additional info). The user can then select what server he wants to connect to.
Making it work is simple enough, with identifying the subnet, and broadcasting with UDP, and then having a different server application recieving it and sending back a response . The problem lies with these restrictions, that I need to take into consideration:
There will most likely also be clients on the server machines in the network, meaning that we can assume that the application is present on all machines. Every machine needs to have the listener running, and every machine can launch the GUI for selecting a server.
I am only allowed to add one exception to the firewall - an exception that handles both sending out the broadcasts, recieving broadcasts, sending answers and recieving answers.
I should also only be adding one Windows Service
on a server machine, the listener should run as a windows service, so the user won't notice it. Nor will the user notice, that the response is sent back to the client.
On the client machine, the user can start an application, which will notify the application to emmit the broadcast, and will get all the server responses, so the user can choose one to connect to.
Besides from the application that the user launches in order to select a server, there should be no interaction with the user whatsoever. Not even a popup, requesting the user to allow traffic trough the firewall - it should all be automatically
It needs to work on and in between Win XP, Win Vista and Win 7.
I don't know if I am putting too many constrains on myself, but I really hope that I can make the application with these requirements.
I have a few ideas - I just need to figure out how to do it:
Should i make everything into one application, that I add to the firewall exception list, so it will take care of the traffic on both the server and the client machines?
Should I add a custom exception to the firewall, allowing UDP traffic on a specific port, and then have all traffic flow trough that?
Is there a third and better option for managing that?
It is OK to have the service running on both client and server machines. But can it take care of everything for me - like it handling both the broadcast send/recieve and answer send/recieve? And is there any way to extract the information about servers on the network from a service?
I know it is a lot, but I really hope that you will be able to help me out.
let me know if I wasn't clear enough, or if you need further explanations.
I am coding in C# .Net, and I can utilize all I want from the .Net framework. As soon as I have this functionality implemented
All the best
/Sagi

The kind of peer-to-peer networking problems become simple to the point of being trivial if you designate one machine as the master server. It should have a well-known name that all sub-servers can connect to so they can publish (and withdraw) their availability. A client can then send a query request to the same server and get a list of known servers in return.
This can also solve your firewall problem, the master server could be listening on port 80.
Look into the System.Net.PeerToPeer namespace for a p2p solution supported by the framework.

Maybe a UPnP server and client may be a solution to your problem?