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.NET Forum / .NET Framework / New Users / December 2004raising events from a windows service
I have a windows service that generates events when it finds certain database information. That is, the service runs, and on a timer event, it checks a few database tables for new records. If the new records have been created, it generates the NewRecord event. I would like to have a client application subscribe to the events generated by the windows service, but when I attempt to use the service as a reference in my VS.NET project, I get an error that says I can only reference DLL's. Can anyone recommend a way for me to "accomplish my mission"? Thanks, Trevor Braun I believe that with .Net you are going to have to use remoting to do this. My question for you is: If both these programs are running on the same system, why not have the client program check for the new records and handle them internallly? Chris >I have a windows service that generates events when it finds certain >database information. That is, the service runs, and on a timer event, it [quoted text clipped - 10 lines] > Thanks, > Trevor Braun Thanks for the thoughts. I didn't mean to give the impression that the client application would be running on the same machine, however at present (during development) that would be the case. I understand that remoting will be required for the eventual deployment, but for building and testing the windows service, I just wanted a simple client application that could monitor the events being raised by the service. How will remoting affect this? BTW, I haven't used .NET remoting before. Trevor >I believe that with .Net you are going to have to use remoting to do this. >My question for you is: [quoted text clipped - 18 lines] >> Thanks, >> Trevor Braun > I have a windows service that generates events when it finds certain > database information. That is, the service runs, and on a timer > event, it checks a few database tables for new records. If the new > records have been created, it generates the NewRecord event. > > I would like to have a client application subscribe to the events Stop there. Client *application*. Windows *service*. You have two processes. > generated by the windows service, but when I attempt to use the > service as a reference in my VS.NET project, I get an error that says > I can only reference DLL's. That's because the memory address for your event handler is only valid in the current process. Your service is another process. > Can anyone recommend a way for me to "accomplish my mission"? You have to use .NET remoting. .NET remoting is used to marshal calls between two AppDomains. Those AppDomains could be in the same process, could be in two different processes on the same machine (your situation) or in two different processes on two different machines. Note that if the service is running as LOCALSYSTEM it won't be able to raise an even to a client on another machine because LOCALSYSTEM does not network privileges. It depends on what sort of data you pass in the event, if its simple data then I would favour use a lightweight solution like a socket to pass the data to the client. (If you have some database rowobject, you just need to serialize the object, send it through the socket and deserialize it on the client side.) Richard  Signaturewww.richardgrimes.com my email evpuneqt@zicf.bet is encrypted with ROT13 (www.rot13.org) Thanks for the terrific explanation. I'll do some more research on the remoting before I continue. Trevor >> I have a windows service that generates events when it finds certain >> database information. That is, the service runs, and on a timer [quoted text clipped - 31 lines] > > Richard I've looked at the remoting stuff, and gotten a sample (remote DLL) working, but still not across applications. I'm lost as to how I can have the client application reference events in the server application. Do you know of any examples where I can see a server application raising an event in a client application? Thanks, Trevor Braun >> I have a windows service that generates events when it finds certain >> database information. That is, the service runs, and on a timer [quoted text clipped - 31 lines] > > Richard I've looked at the remoting stuff, and gotten a sample (remote DLL) working, but still not across applications. I'm lost as to how I can have the client application reference events in the server application. Do you know of any examples where I can see a server application raising an event in a client application? Thanks, Trevor Braun >> I have a windows service that generates events when it finds certain >> database information. That is, the service runs, and on a timer [quoted text clipped - 31 lines] > > Richard This is where things can get complicated, depending on your application needs. If you are using remoting, it may be better to forget about using events in the conventional sense. Instead, I would suggest designing the system so clients call a "subscribe" method on the service. The service will then maintain a list of subscribed clients, and when needed, sends a "message" to the client. Problems may occur if for some reason, sending this message fails (e.g. network problem, or client machine crashes, etc.) Then server would need to handle exception and remove client from list. But what if the client is still reachable, and the problem was temporary. Now they are not receiveing the events? Probably a few solutions, one would be to have the client send a mesasge to the server if it had not received a message for X intervals. Enough rambling though. If you are looking for specific remoting answers, I would recommend you post a mesage on the microsoft.public.dotnet.framework.remoting group, as Ken Kolda and Sam Santiago generally give very good advice there on a lot of remoting queries. HTH Dan > I've looked at the remoting stuff, and gotten a sample (remote DLL) working, > but still not across applications. [quoted text clipped - 41 lines] > > > > Richard
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