OFTYPE (Entity SQL)

Returns a collection of objects from a query expression that is of a specific type.

Syntax

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            

See Also

Entity SQL Reference