How to check EXE file in C#

What you are describing I think is usually called a previous instance; it sounds like you want to limit your application to a single instance. There are very many articles about how to do that. Usually when an application is limited to a single instance, the application will first send to the previous instance a copy of the parameters, if any, that were provided for the new instance and then it would acctivate the previous instance and then the new instance will end.

If the only thing you want to do is to determine if a previous instance exists, then there is a simple and reliable way to do that; you can use a Mutex. A Mutex is the most common solution for that.

There is sample code for a Mutex in Check running processes on a mobile device programmatically. I assume the reason it did not work for Nora is that (probably) Windows Mobile does not support use of Mutex like that; it should work for you.

You can't check anything programatically until the application starts.

The best thing to do is to place Rafeeq's code at the top of your Main() method. If an instance of the application is already running, then you can display a message to the user and exit the second instance gracefully by returning immediately (with the 'return' statement) from the Main() method.

if i want to check before running the application...
which event should be used for it...