c# how to run code after process is completed?

Or change it up a little so you can subscribe to the Process.Exited() Event:

        private void button1_Click(object sender, EventArgs e)
        {
            Process p22 = new Process();
            p22.StartInfo.FileName = "c:\\3gffmpeg.exe";
            p22.StartInfo.WindowStyle = ProcessWindowStyle.Hidden;
            p22.StartInfo.Arguments = " -i c:\\" + realfilename + " -f 3gp -sameq -ar 22050 c:\\test.3gp";
            p22.EnableRaisingEvents = true;
            p22.Exited += new EventHandler(p22_Exited);
            p22.Start();
        }
 
        void p22_Exited(object sender, EventArgs e)
        {
            // ...do something in here...
        }
                                              

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getting a compilation error with idle mind's answer:

Server Error in '/' Application.
Compilation Error
Description: An error occurred during the compilation of a resource required to service this request. Please review the following specific error details and modify your source code appropriately.

Compiler Error Message: CS0165: Use of unassigned local variable 'p22'

in the page load i have exactly what i pasted in original question but i added this:

p22.Exited += new EventHandler(p2_Exited);

i added that right above p22 = Process.Start(info22);

then outside the page_load i have this:

void p22_Exited(object sender, EventArgs e)
    {
        MailMessage message22 = new MailMessage();
        message22.From = new MailAddress("");
        message22.Bcc.Add(new MailAddress(""));
        message22.Subject = "file finished";
        message22.Body = "file finished";
        SmtpClient client22 = new SmtpClient();
        client22.Send(message22);
    }

it highlights this line: Line 285:                            p22.Exited += new EventHandler(p22_Exited);

p22.Exited += new EventHandler(p2_Exited);  <--- typo, i have it correct in my code as (p22_Exited)

Um, p22 instance will be out of scope after the button click is executed and the page request is serviced. That was the reason that I did not propose this solution...
Also you should check if your process exit code  is 0 (It has finished processing OK)

Process p22;
ProcessStartInfo info22 = new ProcessStartInfo();
info22.FileName = "c:\\3gffmpeg.exe";
info22.WindowStyle = ProcessWindowStyle.Hidden;
info22.Arguments = " -i c:\\" + realfilename + " -f 3gp -sameq -ar 22050 c:\\test.3gp";
p22 = Process.Start(info22);
p22.WaitForExit();
MailMessage message22 = new MailMessage();
        message22.From = new MailAddress("");
        message22.Bcc.Add(new MailAddress(""));
if (p22.ExitCode == 0)
{
        message22.Subject = "file finished OK";
        message22.Body = "file finished OK";
}
else
{
        message22.Subject = "file finished with errors";
        message22.Body = "file finishedwith errors"+ p22.ExitCode ;
}
        SmtpClient client22 = new SmtpClient();
        client22.Send(message22);

Sorry...didn't see that this was ASP.net.  My solution is geared for WinForms.

Regarding the error, though, I changed the way the Process() class was being used.

FIRST I instantiated p22:

    Process p22 = new Process();

Then I DIRECTLY populated the StartInfo() structure instead of creating a separate one:

    p22.StartInfo.FileName = "c:\\3gffmpeg.exe";
    p22.StartInfo.WindowStyle = ProcessWindowStyle.Hidden;
    p22.StartInfo.Arguments = " -i c:\\" + realfilename + " -f 3gp -sameq -ar 22050 c:\\test.3gp";

Then I enabled raising events and subscribed to the Exited()...BEFORE calling Start():

    p22.EnableRaisingEvents = true;
    p22.Exited += new EventHandler(p22_Exited);

Finally, we call the Start() method on the INSTANCE we have already created:

    p22.Start();

Again, though, I was thinking of WinForms so I don't know how this applies to Web stuff...

mas, does "waitforexit" mean the user experience will be a hanging page until the ffmpeg conversion is complete, and before any of the rest of the code can finish? cus if so that seems like a problem. i'd like the process and after-code to be 'queued' up or whatever, to fire in background so user can move on with his life.

idle, so i can't do what you're proposing if i'm trying to throw all this into an ASPX's page_load?

I have no idea as I'm not a web guy...  =\

I was just explaining how it was being setup differently.