Single instance C# window that repaints

Question

What I thought would be pretty easy is quickly defeating me. I'm not a native C# programmer, but was asked to create a WinForm application that has a single instance. I''ve seen the Mutex examples already on StackOverflow, but the one thing that eludes me is the ability to pass parameters to window on the command line, parse the values and repaint the form with the new values.

Anyone have an example of this? The main thing that seems to be tripping me up is the threading. I want to run my.exe and show the window. Each time the form is run, I don't want a new form -- just to get the new parameters and show them in the form.

Any/All replies are appreciated!

Answer 1

When you starting another instance of your application, you are running same code, but on different process. So, you need to look on passing data between processes. Something like Named Pipes or Remoting.

Answer 2

@lazyberezovsky is right. Invoking again the application from the command line will spawn a different, unrelated process and you would require inter-process communication to forward the new parameters to the previously running app instance, before quitting the new process being invoked.

IMHO, the easiest way (not the best certainly) to communicate between these two processes would be using the Windows Registry, as this is already thread-safe and the API is very simple.

First, when the application runs, before showing the main form, I would perform a check to see if another instance of the app is running.

If false, it is the first time the app runs and I would process the command line and show the form as regular. I would also clear the registry key used for inter-process communication (see below).

If true, then I would store the command line in the registry on a specific key that will serve for inter-process communication and then I would terminate the application without even showing the main form.

Your running application (the first instance) will require to start a polling mechanism (could be a Windows timer firing once each second) that regularly examines the registry key where a new command line is expected . It would normally find and empty string and do nothing, but if the retrieved value is not empty, then it would mean the user spawned again the application with a different set of parameters, then you can proceed to decode your command line and repaint the window as necessary. After this, make sure you clear the registry entry again, so the polling mechanism resumes and detects the next time the application is invoked by the user.

Named pipes, WCF, .remoting or TCP sockets are IPC mechanisms that can be used and won't require a polling mechanism, that may be frowned upon by some. ;)

Hope this helps!