From which interface/class does the Count method come from

Question

$HashTable.Values.GetEnumerator().Count | Measure-Object -Sum | Select-Object Sum

in this small peice of powershell code, where is the .Count coming from?

Where I have searched:

1. I searched through the documentation for GetEnumerator GetEnumerator Documentation

2. this method returns an object of type IDictionaryEnumerator IDictionaryEnumerator

and in IdictionaryEnumerator I cannot see any method like GetCount() for which Count can be an aliase.

summary: Where is the Count() method defined?