How to show existing process instead of creating new process in c# .net

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C#

hello everybody
i have two problems and i am using c#
1. How to show existing (running )process instead of create new process in menu click event

2. how to close all running process in my application

I am calling the chm file to show help window in my application.

But My application doing create new process for every menu click event and it close only the one process not all process

this is my code please correct this.

         ProcessStartInfo psi;
         Process p;
         string prcpath = @"D:\Myproject\testhelp.chm";

          private void mnuOnScreen_Click(object sender, EventArgs e)
         {
              psi = new ProcessStartInfo(prcpath);
                 p = Process.Start(psi);
                 MessageBox.Show("New process created");
                
             }

//code for close process

private void FrmTRMS_FormClosing(object sender, FormClosingEventArgs e)
         {
            
             try
             {
                 if (p.HasExited)
                   {

                       p.CloseMainWindow();

                   }
                   else
                   {
                       p.Kill();
                   }
                
             }
             catch (Exception ex)
             {
                 MessageBox.Show(ex.Message);
             }
            
         }