Can the performance counter instance name of a process change even if the process has not exited

I am using this class as a base class for a category of tests that launch a process and give it some input and wait for it to become idle before giving it more input.

public abstract class TestProcessLaunchingBase
{
    protected PerformanceCounter PerfCounter { get; set; }

    protected void WaitForProcessIdle()
    {
        while (true)
        {
            float oldValue = PerfCounter.NextValue();

            Thread.Sleep(1000);

            float nextValue = PerfCounter.NextValue();

            if (nextValue == 0)
                break;
        }
    }

    protected void FindSpawnedProcessPerfCounter(int processId)
    {
        PerformanceCounterCategory cat = new PerformanceCounterCategory("Process");
        string[] instances = cat.GetInstanceNames();
        foreach (string instance in instances)
        {
            using (PerformanceCounter cnt = new PerformanceCounter("Process", "ID Process", instance, true))
            {
                int val = (int)cnt.RawValue;
                if (val == processId)
                {
                    PerfCounter = new PerformanceCounter("Process", "% Processor Time", instance);
                    break;
                }
            }

        }

        Assert.IsNotNull(PerfCounter, "Failed to perf counter");
    }
}

These tests occasionally fail because PerfCounter.NextValue() throws an

System.InvalidOperationException Instance 'foobar#2' does not exist in the specified Category

It seems like the instance name of the performance counter is not persistent.

If there are three foobar processes they might have instance names

• foobar pid 5331
• foobar #1 pid 5332
• foobar #2 pid 5333

It seems like if pid 5332 exits foobar #2 becomes foobar #1.

Questions:

1. Is this a documented behavior ? Can you not persistent a performance counter ? Do you have to look it up every time ?

2. Alternatively, is there a performance counter that can give Processor Time for all processes named foobar