How to: Execute a Polymorphic Query (EntityClient)

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This topic shows how to execute a polymorphic Entity SQL query using the OFTYPE operator.

To run the code in this example

  1. Build the CourseManager application. For more information and instructions, see the Entity Framework Quickstart.

  2. Modify its EDM by completing the instructions in the Implementing Table-per-Hierarchy Inheritance section in the Walkthrough: Mapping Inheritance - Table-per-Hierarchy.

Example

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 that contains only those elements that were equivalent either to that type or to a sub-type of it. For example, 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, Manager)

The following example uses an OFTYPE operator to get and display a collection of only Students from a collection of People.

Using connection As EntityConnection = New EntityConnection("name= SchoolEntities ")
    connection.Open()

    ' Create a query that specifies to 
    ' get a collection of Students
    ' with enrollment dates from
    ' a collection of People.
    Dim esqlQuery As String = "SELECT Student.LastName, " & _
            " Student.EnrollmentDate FROM " & _
            " OFTYPE(SchoolEntities.Person, " & _
            " SchoolModel.Student) AS Student "

    ' Create an EntityCommand and pass the connection object 
    ' and Enity SQL query to its constructor.
    Using command As EntityCommand = New EntityCommand(esqlQuery, connection)
        ' Execute the command.
        Using reader As DbDataReader = command.ExecuteReader(CommandBehavior.SequentialAccess)
            ' Start reading.
            Do While reader.Read
                ' Display student's last name and enrollment date.
                Console.Write(reader.Item("LastName"))
                Console.WriteLine(reader.Item("EnrollmentDate"))
            Loop
        End Using
    End Using
End Using

using (EntityConnection conn = new EntityConnection("name=SchoolEntities"))
{
    conn.Open();
    // Create a query that specifies to 
    // get a collection of only Students
    // with enrollment dates from 
    // a collection of People.
    string esqlQuery = @"SELECT Student.LastName, 
        Student.EnrollmentDate FROM 
        OFTYPE(SchoolEntities.Person, 
        SchoolModel.Student) AS Student";

    using (EntityCommand cmd = new EntityCommand(esqlQuery, conn))
    {
        // Execute the command.
        using (DbDataReader rdr = cmd.ExecuteReader(CommandBehavior.SequentialAccess))
        {
            // Start reading.
            while (rdr.Read())
            {
                // Display student's last name and enrollment date.
                Console.Write(rdr["LastName"] + " ");
                Console.WriteLine(rdr["EnrollmentDate"]);
            }
        }
    }
}

See Also

Tasks

How to: Create and Execute Object Queries using Table-per-Hierarchy Inheritance (Entity Framework)

Concepts

Entity SQL Language

Other Resources

Working with EntityClient (Entity Framework Tasks)
How to: Execute an Entity SQL Query Using EntityCommand