In some situations, categories are subdivided into instances, which track data about multiple occurrences of the object that a category relates to. Instances apply to the category as whole, rather than to individual counters. Every counter within a category has each instance defined for the category. For example, the Process category contains instances named Idle and System. Every counter within the Process category thus contains data for each instance, showing information about either idle processes or system processes.

Many categories do not contain multiple instances, so you can leave this property empty to indicate that no instance is associated with the category.

If this System.Diagnostics.PerformanceCounter instance points to a noncustom category, you can choose from only the existing category instances. You can create new category instances only in custom categories, which allow you to define as many counters and category instances as you need.

To create a performance category instance, specify an instanceName on the PerformanceCounter.#ctor constructor. If the category instance specified by instanceName already exists the new object will reference the existing category instance.

If the instance name is automatically generated and might contain the characters "(", ")", "#", "\", or "/", use the character mapping in the following table.

The AppDomain.FriendlyName property of the AppDomain object obtained from the AppDomain.CurrentDomain property is a common source of instance names that can contain invalid characters.