如何：执行多态查询 (EntityClient)

本主题说明如何使用 OFTYPE (Entity SQL) 运算符执行多态 Entity SQL 查询。

运行本示例中的代码

1. 将 School 模型 添加到您的项目中并将项目配置为使用实体框架。 有关更多信息，请参见如何：使用实体数据模型向导（实体框架）。

2. 在应用程序的代码页中，添加以下 using 语句（在 Visual Basic 中为 Imports）：

   Imports System
   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
   

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

3. 按照Walkthrough: Mapping Inheritance - Table per Hierarchy中的步骤修改概念模型以实现每个层次结构一个表继承。

示例

下面的示例使用 OFTYPE 运算符从 Courses 的集合中获取和显示仅包含 OnsiteCourses 的集合。

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

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"]);
            }
        }
    }
}

另请参见

概念

用于实体框架的 EntityClient 提供程序
Entity SQL 语言