How can I know if a process is running?

This is a way to do it with the name:

Process[] pname = Process.GetProcessesByName("notepad");
if (pname.Length == 0)
  MessageBox.Show("nothing");
else
  MessageBox.Show("run");

You can loop all process to get the ID for later manipulation:

Process[] processlist = Process.GetProcesses();
foreach(Process theprocess in processlist){
   Console.WriteLine("Process: {0} ID: {1}", theprocess.ProcessName, theprocess.Id);
}

This is the simplest way I found after using reflector. I created an extension method for that:

public static class ProcessExtensions
{
    public static bool IsRunning(this Process process)
    {
        if (process == null) 
            throw new ArgumentNullException("process");

        try
        {
            Process.GetProcessById(process.Id);
        }
        catch (ArgumentException)
        {
            return false;
        }
        return true;
    }
}

The Process.GetProcessById(processId) method calls the ProcessManager.IsProcessRunning(processId) method and throws ArgumentException in case the process does not exist. For some reason the ProcessManager class is internal...

reshefm had a pretty nice answer; however, it does not account for a situation in which the process was never started to begin with.

Here is a a modified version of what he posted.

    public static bool IsRunning(this Process process)
    {
        try  {Process.GetProcessById(process.Id);}
        catch (InvalidOperationException) { return false; }
        catch (ArgumentException){return false;}
        return true;
    }

I removed his ArgumentNullException because its actually suppose to be a null reference exception and it gets thrown by the system anyway and I also accounted for the situation in which the process was never started to begin with or the close() method was used to close the process.

Synchronous solution :

void DisplayProcessStatus(Process process)
{
    process.Refresh();  // Important


    if(process.HasExited)
    {
        Console.WriteLine("Exited.");
    }
    else
    {
        Console.WriteLine("Running.");
    } 
}

Asynchronous solution:

void RegisterProcessExit(Process process)
{
    // NOTE there will be a race condition with the caller here
    //   how to fix it is left as an exercise
    process.Exited += process_Exited;
}

static void process_Exited(object sender, EventArgs e)
{
   Console.WriteLine("Process has exited.");
}