OFTYPE
Returns a collection of objects from a query expression that is of a specific type.
OFTYPE ( expression, [ONLY] test_type )
Arguments
- expression
Any valid query expression that returns a collection of objects.
- test_type
The type to test each object returned by expression against. The type must be qualified by a namespace.
Return Value
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.
Remarks
An 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.
An 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:
OfType(employees, NamespaceName.Manager)
It is also possible to up cast a collection using the type filter:
OfType(executives, NamespaceName.Manager)
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:
| Pattern | Behavior |
|---|---|
OFTYPE(Collection(EntityType), EntityType) |
Collection(EntityType) |
OFTYPE(Collection(ComplexType), ComplexType) |
Throws |
OFTYPE(Collection(RowType), RowType) |
Throws |
Example
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.
SELECT onsiteCourse.Location FROM
OFTYPE(SchoolEntities.Courses, SchoolModel.OnsiteCourse)
AS onsiteCourse