Create a performance counter with an instance name

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I use the following two lines of code to create a performance counter with an
instance name.

PerformanceCounterCategory.Create(m_CategoryName, "",
PerformanceCounterCategoryType.MultiInstance, m_CounterName, "");

PerformanceCounter counter = new PerformanceCounter(m_CategoryName,
m_CounterName, "test", false);

But I get this exception:
"Category 'cat' is marked as single-instance. Performance counters in this
category can only be created without instance names."

Any idea?

Thanks.

Kai