Question

I wrote a program in c#
now I would like to know what is the proper way to prevent the program from starting if it is already running?

so if it is already running, and double-click on the program it will not start because it is already running.

I can do that, but I was thinking of a standard and proper way.

Answer 1

The recommended way to do this is with a system mutex.

bool createdNew;
using(var mutex = new System.Threading.Mutex(true, "MyAppName", out createdNew))
{
    if (createdNew)
        // first instance
        Application.Run();
    else
        MessageBox.Show("There is already an instace running");
}

The first parameter to the Mutex ctor tells it to give create a system wide mutex for this thread. If the Mutex already exists it will return out false through the 3rd parameter.

Update
Where to put this?
I’d put this in program.cs. If you put it in form_load you’ll need to keep the mutex for the life time of the application (have the mutex as a member on the form), and manually release it in the form unload.
The earlier you call this the better, before the other app opens DB connections etc. and before resources are put created for forms / controlls etc.