How do I launch a process with low priority? C#

Question

I want to execute a command line tool to process data. It does not need to be blocking. I want it to be low priority. So I wrote the below

 Process app = new Process();
 app.StartInfo.FileName = @"bin\convert.exe";
 app.StartInfo.Arguments = TheArgs;
 app.PriorityClass = ProcessPriorityClass.BelowNormal;
 app.Start();

However, I get a System.InvalidOperationException with the message "No process is associated with this object." Why? How do I properly launch this app in low priority?

Without the line app.PriorityClass = ProcessPriorityClass.BelowNormal; the app runs fine.

score 52 · Accepted Answer · answered Jun 18 '09 at 01:37

52

Try setting the PriorityClass AFTER you start the process. Task Manager works this way, allowing you to set priority on a process that is already running.

answered Jun 18 '09 at 01:37

Robert Harvey

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Turns out this is the only way. – Jun 23 '10 at 15:48
As pointed out by Alois Kraus this gives a race condition. The fact that this is the way it is designed to be done by Microsoft does not make it any less a very BAD idea. – Paul Childs Sep 15 '21 at 02:46
Race condition or not, I don't see the problem here. The process would *momentarily* be running in the parent process's priority. – Robert Harvey Sep 15 '21 at 03:02
2

It is actually a big problem. I get occasions when this moment is long enough for the process to complete. I thus end up with an intermittent run time crash when it tries to set the priority of a finished process. This is a serious design flaw. – Paul Childs Sep 24 '21 at 01:11

score 16 · Answer 2 · answered Oct 15 '15 at 21:06

16

You can create a process with lower priority by doing a little hack. You lower the parent process priority, create the new process and then revert back to the original process priority:

var parent   = Process.GetCurrentProcess();
var original = parent.PriorityClass;

parent.PriorityClass = ProcessPriorityClass.Idle;
var child            = Process.Start("cmd.exe");
parent.PriorityClass = original;

child will have the Idle process priority right at the start.

answered Oct 15 '15 at 21:06

Pablo Montilla

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That is by far the best solution with no races. The other solutions need to wait until the process is running which can be an issue if you start e.g. a script which starts other child processes which will then run with the initial priority and not the one you will set later. – Alois Kraus Nov 12 '19 at 12:55
I really don't like this, but I like debugging intermittent errors less. Turns out this is actually the only way. – Paul Childs Sep 24 '21 at 01:13

score 7 · Answer 3 · answered Sep 29 '11 at 15:46

7

If you're prepared to P/Invoke to CreateProcess, you can pass CREATE_SUSPENDED in the flags. Then you can tweak the process priority before resuming the process.

answered Sep 29 '11 at 15:46

Roger Lipscombe

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How do I launch a process with low priority? C#

3 Answers3

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