PropertyHandle :: GetStringKey - Get identifying key value for selected instance

The function returns the key value of the selected instance in string format, i.e. key components are separated by '|' and numbers and other non-text components are converted into characters. When no key name is passed, the function returns the identifying key value.

The result is stored in an intermediate property handle buffer and might be destroyed when calling the next property handle function.

When passing an invalid key name, when no identifying key is defined for the structure or when no instance is selected, the function returns NULL.