查询实体和关联（实体框架快速入门）

项目
02/28/2011

在本任务中，您将针对在 School 模型中表示实体和关联的 CLR 对象创建强类型查询，并将显示控件绑定到从这些查询中返回的对象集合。

在 School 数据库中查询系

在 CourseViewer 窗体的代码文件的开始处，添加以下 using (C#) 或 Imports (Visual Basic) 语句，以引用从 School 数据库和实体命名空间中创建的模型。
```
Imports System.Data.Objects
Imports System.Data.Objects.DataClasses
```
```
using System.Data.Objects;
using System.Data.Objects.DataClasses;
```

在 CourseViewer 窗体的分部类定义的顶部，添加以下用于创建 ObjectContext 实例的代码。

' Create an ObjectContext instance based on SchoolEntity.
Private schoolContext As SchoolEntities

//Create an ObjectContext instance based on SchoolEntity
private SchoolEntities schoolContext;

在 CourseViewer 窗体设计器中，双击 CourseViewer 窗体。

此时将打开窗体的代码页并创建 courseViewer _Load 事件处理程序方法。

在 courseViewer _Load 事件处理程序方法中，复制并粘贴以下用于定义 DataGridView 的代码，执行返回系科集合的查询（按 Name 排序），然后将 Department 对象的集合绑定到 departmentList 控件。

' Initialize the ObjectContext.
schoolContext = New SchoolEntities()

' Define a query that returns all Department objects
' and related Course objects, ordered by name.
Dim departmentQuery As ObjectQuery(Of Department) = _
    From d In schoolContext.Departments.Include("Courses") _
    Order By d.Name _
    Select d
Try
    ' Bind the ComboBox control to the query.
    ' To prevent the query from being executed multiple times during binding, 
    ' it is recommended to bind controls to the result of the Execute method. 
    Me.departmentList.DisplayMember = "Name"
    Me.departmentList.DataSource = CType(departmentQuery, ObjectQuery).Execute(MergeOption.AppendOnly)
Catch ex As Exception
    MessageBox.Show(ex.Message)
End Try

//Initialize the ObjectContext
schoolContext = new SchoolEntities();

// Define a query that returns all Department  
// objects and course objects, ordered by name.
var departmentQuery = from d in schoolContext.Departments.Include("Courses")
                      orderby d.Name
                      select d;
try
{
    // Bind the ComboBox control to the query, 
    // which is executed during data binding.
    // To prevent the query from being executed multiple times during binding, 
    // it is recommended to bind controls to the result of the Execute method. 
    this.departmentList.DisplayMember = "Name";
    this.departmentList.DataSource = ((ObjectQuery)departmentQuery).Execute(MergeOption.AppendOnly);
}
catch (Exception ex)
{
    MessageBox.Show(ex.Message);
}

显示所选系的课程

在 CourseViewer 窗体设计器中，双击 departmentList 控件。

此时将创建 departmentList_SelectedIndexChanged 事件处理程序方法。

粘贴以下用于加载与所选系相关的课程的代码。

Try
    ' Get the object for the selected department.
    Dim department As Department = _
        CType(Me.departmentList.SelectedItem, Department)

    ' Bind the grid view to the collection of Course objects 
    ' that are related to the selected Department object.
    courseGridView.DataSource = department.Courses

    ' Hide the columns that are bound to the navigation properties on Course.
    courseGridView.Columns("Department").Visible = False
    courseGridView.Columns("StudentGrades").Visible = False
    courseGridView.Columns("OnlineCourse").Visible = False
    courseGridView.Columns("OnsiteCourse").Visible = False
    courseGridView.Columns("People").Visible = False
    courseGridView.Columns("DepartmentId").Visible = False

    courseGridView.AllowUserToDeleteRows = False
courseGridView.AutoResizeColumns(DataGridViewAutoSizeColumnsMode.AllCells)
Catch ex As Exception
    MessageBox.Show(ex.Message)
End Try

    try
    {
        //Get the object for the selected department.
        Department department = (Department)this.departmentList.SelectedItem;

        //Bind the grid view to the collection of Course objects
        // that are related to the selected Department object.
        courseGridView.DataSource = department.Courses;

        // Hide the columns that are bound to the navigation properties on Course.
        courseGridView.Columns["Department"].Visible = false;
        courseGridView.Columns["StudentGrades"].Visible = false;
        courseGridView.Columns["OnlineCourse"].Visible = false;
        courseGridView.Columns["OnsiteCourse"].Visible = false;
        courseGridView.Columns["People"].Visible = false;
        courseGridView.Columns["DepartmentId"].Visible = false;

        courseGridView.AllowUserToDeleteRows = false;
courseGridView.AutoResizeColumns(DataGridViewAutoSizeColumnsMode.AllCells);
    }
    catch (Exception ex)
    {
        MessageBox.Show(ex.Message);
    }

后续步骤

您已成功地创建了返回 Department 和 Course 对象的查询并将这些对象绑定到控件。接下来，您需要将在数据网格中对 Course 对象所做的更改保存到数据库中：插入和更新数据（实体框架快速入门）。

查询实体和关联（实体框架快速入门）

在 School 数据库中查询系

显示所选系的课程

后续步骤

另请参见

概念

其他资源

其他资源