OFTYPE (Entity SQL)
Returns a collection of objects from a query expression that is of a specific type.
OFTYPE ( expression, [ONLY] test_type )
Any valid query expression that returns a collection of objects.
The type to test each object returned by
expression against. The type must be qualified by a namespace.
A collection of objects that are of type
test_type, or a base type or derived type of
test_type. If ONLY is specified, only instances of the
test_type or an empty collection will be returned.
OFTYPE expression specifies a type expression that is issued to perform a type test against each element of a collection. The
OFTYPE expression produces a new collection of the specified type containing only those elements that were either equivalent to that type or a sub-type of it.
OFTYPE expression is an abbreviation of the following query expression:
select value treat(t as T) from ts as t where t is of (T)
Given that a Manager is a subtype of Employee, the following expression produces a collection of only managers from a collection of employees:
It is also possible to up cast a collection using the type filter:
Since all executives are managers, the resulting collection still contains all the original executives, though the collection is now typed as a collection of managers.
The following table shows the behavior of the
OFTYPE operator over some patterns. All exceptions are thrown from the client side before the provider is invoked:
The following Entity SQL query uses the OFTYPE operator to return a collection of OnsiteCourse objects from a collection of Course objects. The query is based on the School Model.
[!CODE [DP EntityServices Concepts 2#OFTYPE](../CodeSnippet/VS_Snippets_Data/dp entityservices concepts 2#oftype)]