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Editorial Team
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Asked: 2026-05-17T21:24:49+00:00

Here is the C# code I’m using to launch a subprocess and monitor its

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Here is the C# code I’m using to launch a subprocess and monitor its output:

using (process = new Process()) {
    process.StartInfo.FileName = executable;
    process.StartInfo.Arguments = args;
    process.StartInfo.UseShellExecute = false;
    process.StartInfo.RedirectStandardOutput = true;
    process.StartInfo.RedirectStandardInput = true;
    process.StartInfo.CreateNoWindow = true;
    process.Start();

    using (StreamReader sr = process.StandardOutput) {
        string line = null;
        while ((line = sr.ReadLine()) != null) {
            processOutput(line);
        }
    }

    if (process.ExitCode == 0) {
        jobStatus.State = ActionState.CompletedNormally;
        jobStatus.Progress = 100;
    } else {
        jobStatus.State = ActionState.CompletedAbnormally;
    }
    OnStatusUpdated(jobStatus);
}

I am launching multiple subprocesses in separate ThreadPool threads (but no more than four at a time, on a quad-core machine). This all works fine.

The problem I am having is that one of my subprocesses will exit, but the corresponding call to sr.ReadLine() will block until ANOTHER one of my subprocesses exits. I’m not sure what it returns, but this should NOT be happening unless there is something I am missing.

There’s nothing about my subprocess that would cause them to be “linked” in any way – they don’t communicate with each other. I can even look in Task Manager / Process Explorer when this is happening, and see that my subprocess has actually exited, but the call to ReadLine() on its standard output is still blocking!

I’ve been able to work around it by spinning the output monitoring code out into a new thread and doing a process.WaitForExit(), but this seems like very odd behavior. Anyone know what’s going on here?

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1 Answer

  1. Editorial Team

    I think it’s not your code that’s the issue. Blocking calls can unblock for a number of reasons, not only because their task was accomplished.

    I don’t know about Windows, I must admit, but in the Unix world, when a child finishes, a signal is sent to the parent process and this wakes him from any blocking calls. This would unblock a read on whatever input the parent was expecting.

    It wouldn’t surprise me if Windows worked similarly. In any case, read up on the reasons why a blocking call may unblock.

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