如何：执行多态查询 (EntityClient)

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

运行本示例中的代码

1. 生成 CourseManager 应用程序。有关更多信息和说明，请参见实体框架快速入门。

2. 执行演练：映射继承 - 每个层次结构一个表的“实现每个层次结构一个表继承”一节中的指令以修改其 EDM。

示例

OFTYPE 表达式指定一个类型表达式，此表达式旨在针对集合的每个元素执行类型测试。OFTYPE 表达式生成指定类型的新集合，其中仅包含等效于该类型或其子类型的那些元素。例如，如果 Manager 是 Employee 的子类型，则以下表达式将从员工集合生成仅包含经理的集合：

OfType(employees, Manager)

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

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"]);
            }
        }
    }
}

另请参见

任务

如何：使用每个层次结构一个表继承创建和执行对象查询（实体框架）

概念

Entity SQL 语言

其他资源

使用 EntityClient（实体框架任务）
How to: Execute an Entity SQL Query Using EntityCommand