Terminating operations

In simple terms, a Terminating Operation is one that stops you from adding further query operations.

In the following query:

Query.EndsWith('e') 

EndsWith is not a Terminating Operation, as we can continue on to specify further operations, like so:

Query.EndsWith('e').StartsWith('a')

However, Predicate is a Terminating Operation, as it is not possible to specify further query operations. It effectively marks the end of your query.

Query.EndsWith('e').Predicate

In more strict terms, a Terminating Operation is one that does not return a Query Enumerator (Bound or Unbound) as Non-terminating Operations do. Sticking to our string query example above:

Query.EndsWith('e') 

returns an IUnboundStringQuery which contains further query operations.

Query.From(Listbox1.Items).EndsWith('e')

returns an IBoundStringQuery, which also contains further query operations.

Query.EndsWith('e').Predicate

returns a TPredicate<String>, which does not contain any query operations.

Terminating Operations and for..in loops

It follows that as for..in loops operate on Enumerators, only Unterminated Queries (ie. queries containing no Terminating Operations) can be used in a for..in loop.