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作法：執行多型查詢

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此主題示範如何使用 OFTYPE 運算子來執行多型 Entity SQL 查詢。

執行此範例中的程式碼

  1. 將 School 模型新增至您的專案，並設定專案使用 Entity Framework。 如需詳細資訊，請參閱操作說明：使用實體資料模型精靈。

  2. 在應用程式的代碼頁中，新增下列 using 指示詞 （Imports 在 Visual Basic 中）：

    using System;
using System.Collections.Generic;
using System.Data;
using System.Data.Common;
using System.Data.EntityClient;
using System.Data.Metadata.Edm;

    Imports System.Collections.Generic
Imports System.Collections
Imports System.Data.Common
Imports System.Data
Imports System.IO
Imports System.Data.SqlClient
Imports System.Data.EntityClient
Imports System.Data.Metadata.Edm

  3. 遵循逐步解說：對應繼承 - 每個階層的資料表中的步驟，修改概念模型以擁有每個階層的資料表繼承。

範例

下列範例會使用 OFTYPE 運算子，從 OnsiteCourses 的集合中取得並顯示只有 Courses 的集合。

using (EntityConnection conn = new EntityConnection("name=SchoolEntities"))
{
    conn.Open();
    // Create a query that specifies to
    // get a collection of only OnsiteCourses.

    string esqlQuery = @"SELECT VAlUE onsiteCourse FROM
        OFTYPE(SchoolEntities.Courses, SchoolModel.OnsiteCourse)
        AS onsiteCourse";
    using (EntityCommand cmd = new EntityCommand(esqlQuery, conn))
    {
        // Execute the command.
        using (DbDataReader rdr = cmd.ExecuteReader(CommandBehavior.SequentialAccess))
        {
            // Start reading.
            while (rdr.Read())
            {
                // Display OnsiteCourse's location.
                Console.WriteLine("CourseID: {0} ", rdr["CourseID"]);
                Console.WriteLine("Location: {0} ", rdr["Location"]);
            }
        }
    }
}

Using conn As New EntityConnection("name=SchoolEntities")
    conn.Open()
    ' Create a query that specifies to 
    ' get a collection of only OnsiteCourses. 

    Dim esqlQuery As String = "SELECT VAlUE onsiteCourse FROM " & _
        "OFTYPE(SchoolEntities.Courses, SchoolModel.OnsiteCourse) AS onsiteCourse"
    Using cmd As New EntityCommand(esqlQuery, conn)
        ' Execute the command. 
        Using rdr As DbDataReader = cmd.ExecuteReader(CommandBehavior.SequentialAccess)
            ' Start reading. 
            While rdr.Read()
                ' Display OnsiteCourse's location. 
                Console.WriteLine("CourseID: {0} ", rdr("CourseID"))
                Console.WriteLine("Location: {0} ", rdr("Location"))
            End While
        End Using
    End Using
End Using

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