运用模型绑定和web窗体显示和检索数据（Retrieving and displaying data with model binding and web forms）

原文 http://www.asp.net/web-forms/overview/presenting-and-managing-data/model-binding/retrieving-data

此文是我翻译的中午版本。

      本系列教程演示了使用ASP.NET Web窗体项目的模型绑定的基本方面. 模型绑定使数据交互更简单明了，而不是处理数据源对象(如数据绑定控件或控件上的)。本文是入门级的，以后的教程会有更高级的。

     该模型绑定模式适用于任何数据访问技术。在本教程中，您将使用实体框架，但是您可以使用你最熟悉的数据访问技术。如   GridView, ListView, DetailsView, or FormView control, ,您可以指定要使用的方法的名称选择、更新、删除和创建数据。在本教程中，将以 SelectMethod为例。

     在该方法内，提供检索数据的逻辑。在接下来的教程中，您将设置值UpdateMethod，DeleteMethod和InsertMethod。

     你可以下载完整的C#或VB项目。可下载的代码使用Visual Studio 2012或Visual Studio 2013。 使用Visual Studio 2012中的模板 比Visual Studio 2013本教程中所示的模板 略有不同。

     在本教程中，您在Visual Studio中运行程序。 您也可以使其在互联网上可用的应用程序部署到一个托管服务提供商。微软Azure的免费试用帐户提供多达10个网站托管 。 有关如何将Visual Studio web项目部署到Azure ，请参见使用Visual Studio系列部署ASP.NET Web 项目 。 该教程还展示如何使用EF 首先 将SQL Server数据库部署到Azure的SQL数据库。

     

在本教程中，你将

1. 创建数据对象反应大学生注册课程
2. 从对象中创建数据库表
3. 用测试数据填充数据库
4. 在web 窗体显示数据

Set up project

In Visual Studio 2013, 新建 ASP.NET Web Application 称作ContosoUniversityModelBinding.

选择  Web Forms 模板,   点确定创建项目。

First, you will做一些小的改动去改变你网站的面貌. 打开 Site.Master 文件,  改变标题为 Contoso University  代替原来的

<title><%: Page.Title %> - Contoso University</title>
Then, change the header text from Application name to Contoso University.

<div class="navbar-header">
    <button type="button" class="navbar-toggle" data-toggle="collapse" data-target=".navbar-collapse">
        <span class="icon-bar"></span>
        <span class="icon-bar"></span>
        <span class="icon-bar"></span>
    </button>
    <a class="navbar-brand" runat="server" href="~/">Contoso University</a>
</div>

Also in Site.Master, change the navigation links that appear in the header to reflect the pages that are relevant to this site. You will not need either the About page or the Contact page so those links can be removed. Instead, you will need a link to a page called Students. This page has not been created yet.

<ul class="nav navbar-nav">
    <li><a runat="server" href="~/">Home</a></li>
    <li><a runat="server" href="~/Students">Students</a></li>
</ul>

Save and close Site.Master.

Now, you’ll create the web form for displaying student data. Right-click your project, and Add a New Item. Select the Web Form with Master Page template, and name it Students.aspx.

Select Site.Master as the master page for the new web form.

Create the data models and database

You will use Code First Migrations to create objects and the corresponding database tables. These tables will store information about the students and their courses.

In the Models folder, add a new class named UniversityModels.cs.

In this file, define the SchoolContext, Student, Enrollment, and Course classes as follows:

using System;
using System.Collections.Generic;
using System.Data.Entity;
using System.ComponentModel.DataAnnotations;

namespace ContosoUniversityModelBinding.Models
{
    public class SchoolContext : DbContext
    {
        public DbSet<Student> Students { get; set; }
        public DbSet<Enrollment> Enrollments { get; set; }
        public DbSet<Course> Courses { get; set; }
    }

    public class Student
    {
        [Key, Display(Name = "ID")]
        [ScaffoldColumn(false)]
        public int StudentID { get; set; }

        [Required, StringLength(), Display(Name="Last Name")]
        public string LastName { get; set; }

        [Required, StringLength(), Display(Name = "First Name")]
        public string FirstName { get; set; }

        [EnumDataType(typeof(AcademicYear)), Display(Name = "Academic Year")]
        public AcademicYear Year { get; set; }

        public virtual ICollection<Enrollment> Enrollments { get; set; }
    }

    public class Enrollment
    {
        [Key]
        public int EnrollmentID { get; set; }
        public int CourseID { get; set; }
        public int StudentID { get; set; }
        public decimal? Grade { get; set; }
        public virtual Course Course { get; set; }
        public virtual Student Student { get; set; }
    }

    public class Course
    {
        [Key]
        public int CourseID { get; set; }
        public string Title { get; set; }
        public int Credits { get; set; }
        public virtual ICollection<Enrollment> Enrollments { get; set; }
    } 

    public enum AcademicYear
    {
        Freshman,
        Sophomore,
        Junior,
        Senior
    }
}

The SchoolContext class derives from DbContext, which manages the database connection and changes in the data.

In the Student class, notice the attributes that have been applied to the FirstName, LastName, and Yearproperties. These attributes will be used for data validation in this tutorial. To simplify the code for this demonstration project, only these properties were marked with data-validation attributes. In a real project, you would apply validation attributes to all properties that need validated data, such as properties in the Enrollment and Course classes.

Save UniversityModels.cs.

You will use the tools for Code First Migrations to set up a database based on these classes.

In Package Manager Console, run the command:
enable-migrations -ContextTypeName ContosoUniversityModelBinding.Models.SchoolContext

If the command completes successfully you will receive a message stating migrations have been enabled,

Notice that a new file named Configuration.cs has been created. In Visual Studio, this file is automatically opened after it is created. The Configuration class contains a Seed method which enables you to pre-populate the database tables with test data.

Add the following code to the Seed method. You’ll need to add a using statement for theContosoUniversityModelBinding.Models namespace.

namespace ContosoUniversityModelBinding.Migrations
{
    using System;
    using System.Data.Entity;
    using System.Data.Entity.Migrations;
    using System.Linq;
    using ContosoUniversityModelBinding.Models;

    internal sealed class Configuration : DbMigrationsConfiguration<SchoolContext>
    {
        public Configuration()
        {
            AutomaticMigrationsEnabled = false;
        }

        protected override void Seed(SchoolContext context)
        {

            context.Students.AddOrUpdate(
                 new Student {
                     FirstName = "Carson",
                     LastName = "Alexander",
                     Year = AcademicYear.Freshman },
                 new Student {
                     FirstName = "Meredith",
                     LastName = "Alonso",
                     Year = AcademicYear.Freshman },
                 new Student {
                     FirstName = "Arturo",
                     LastName = "Anand",
                     Year = AcademicYear.Sophomore },
                 new Student {
                     FirstName = "Gytis",
                     LastName = "Barzdukas",
                     Year = AcademicYear.Sophomore },
                 new Student {
                     FirstName = "Yan",
                     LastName = "Li",
                     Year = AcademicYear.Junior },
                 new Student {
                     FirstName = "Peggy",
                     LastName = "Justice",
                     Year = AcademicYear.Junior },
                 new Student {
                     FirstName = "Laura",
                     LastName = "Norman",
                     Year = AcademicYear.Senior },
                 new Student {
                     FirstName = "Nino",
                     LastName = "Olivetto",
                     Year = AcademicYear.Senior }
                 );

            context.SaveChanges();

            context.Courses.AddOrUpdate(
                new Course { Title = "Chemistry", Credits =  },
                new Course { Title = "Microeconomics", Credits =  },
                new Course { Title = "Macroeconomics", Credits =  },
                new Course { Title = "Calculus", Credits =  },
                new Course { Title = "Trigonometry", Credits =  },
                new Course { Title = "Composition", Credits =  },
                new Course { Title = "Literature", Credits =  }
                );

            context.SaveChanges();

            context.Enrollments.AddOrUpdate(newEnrollment{StudentID=,CourseID=,Grade=},newEnrollment{StudentID=,CourseID=,Grade=},newEnrollment{StudentID=,CourseID=,Grade=},newEnrollment{StudentID=,CourseID=,Grade=},newEnrollment{StudentID=,CourseID=,Grade=},newEnrollment{StudentID=,CourseID=,Grade=},newEnrollment{StudentID=,CourseID=},newEnrollment{StudentID=,CourseID=},newEnrollment{StudentID=,CourseID=,Grade=},newEnrollment{StudentID=,CourseID=,Grade=},newEnrollment{StudentID=,CourseID=},newEnrollment{StudentID=,CourseID=,Grade=});

            context.SaveChanges();}}}

Save Configuration.cs.

In the Package Manager Console, run the command add-migration initial.

Then, run the command update-database.

If you receive an exception when running this command, it is possible that the values for StudentID and CourseID have varied from the values in the Seed method. Open those tables in the database and find existing values for StudentID and CourseID. Add those values in the code for seeding the Enrollments table.

The database file has been added, but it is currently hidden in the project. Click Show All Files to display the file.

Notice the .mdf file now appears in the App_Data folder.

Double-click the .mdf file and open the Server Explorer. The tables now exist and are populated with data.

Display data from Students and related tables

With data in the database, you are now ready to retrieve that data and display it in a web page. You will use aGridView control to display the data in columns and rows.

Open Students.aspx, and locate the MainContent placeholder. Within that placeholder, add a GridView control that includes the following code.

<asp:Content ID="Content1" ContentPlaceHolderID="MainContent" runat="server">
    <asp:GridView runat="server" ID="studentsGrid"
        ItemType="ContosoUniversityModelBinding.Models.Student" DataKeyNames="StudentID"
        SelectMethod="studentsGrid_GetData"
        AutoGenerateColumns="false">
        <Columns>
            <asp:DynamicField DataField="StudentID" />
            <asp:DynamicField DataField="LastName" />
            <asp:DynamicField DataField="FirstName" />
            <asp:DynamicField DataField="Year" />
            <asp:TemplateField HeaderText="Total Credits">
              <ItemTemplate>
                <asp:Label Text="<%# Item.Enrollments.Sum(en => en.Course.Credits) %>"
                    runat="server" />
              </ItemTemplate>
            </asp:TemplateField>
        </Columns>
    </asp:GridView>
</asp:Content>

There are a couple of important concepts in this markup code for you to notice. First, notice that a value is set for the SelectMethod property in the GridView element. This value specifies the name of the method that is used for retrieving data for the GridView. You will create this method in the next step. Second, notice that the ItemTypeproperty is set to the Student class that you created earlier. By setting this value, you can refer to properties of that class in the markup code. For example, the Student class contains a collection named Enrollments. You can useItem.Enrollments to retrieve that collection and then use LINQ syntax to retrieve the sum of enrolled credits for each student.

In the code-behind file, you need to add the method that is specified for the SelectMethod value. OpenStudents.aspx.cs, and add using statements for the ContosoUniversityModelBinding.Models andSystem.Data.Entity namespaces.

using ContosoUniversityModelBinding.Models;
using System.Data.Entity;
Then, add the following method. Notice that the name of this method matches the name you provided for SelectMethod.

public IQueryable<Student> studentsGrid_GetData()
{
    SchoolContext db = new SchoolContext();
    var query = db.Students.Include(s => s.Enrollments.Select(e => e.Course));
    return query;
}

The Include clause improves the performance of this query but is not essential for the query to work. Without the Include clause, the data would be retrieved using lazy loading, which involves sending a separate query to the database each time related data is retrieved. By providing the Include clause, the data is retrieved using eager loading, which means all of the related data is retrieved through a single query of the database. In cases where most of the related data is not be used, eager loading can be less efficient because more data is retrieved. However, in this case, eager loading provides the best performance because the related data is displayed for each record.

For more information about performance consideration when loading related data, see the section titled Lazy, Eager, and Explicit Loading of Related Data in the Reading Related Data with the Entity Framework in an ASP.NET MVC Application topic.

By default, the data is sorted by the values of the property marked as the key. You can add an OrderBy clause to specify a different value for sorting. In this example, the default StudentID property is used for sorting. In theSorting, Paging, and Filtering Data topic, you will enable the user to select a column for sorting.

Run your web application, and navigate to the Students page. The Students page displays the following student information.

Automatic generation of model binding methods

When working through this tutorial series, you can simply copy the code from the tutorial to your project. However, one disadvantage of this approach is that you may not become aware of the feature provided by Visual Studio to automatically generate code for model binding methods. When working on your own projects, automatic code generation can save you time and help you gain a sense of how to implement an operation. This section describes the automatic code generation feature. This section is only informational and does not contain any code you need to implement in your project.

When setting a value for the SelectMethod, UpdateMethod, InsertMethod, or DeleteMethod properties in the markup code, you can select the Create New Method option.

Visual Studio not only creates a method in the code-behind with the proper signature, but also generates implementation code to assist you with performing the operation. If you first set the ItemType property before using the automatic code generation feature, the generated code will use that type for the operations. For example, when setting the UpdateMethod property, the following code is automatically generated:

// The id parameter name should match the DataKeyNames value set on the control
public void studentsGrid_UpdateItem(int id)
{
    ContosoUniversityModelBinding.Models.Student item = null;
    // Load the item here, e.g. item = MyDataLayer.Find(id);
    if (item == null)
    {
        // The item wasn't found
        ModelState.AddModelError("", String.Format("Item with id {0} was not found", id));
        return;
    }
    TryUpdateModel(item);
    if (ModelState.IsValid)
    {
        // Save changes here, e.g. MyDataLayer.SaveChanges();

    }
}

Again, the above code does not need to be added to your project. In the next tutorial, you will implement methods for updating, deleting and adding new data.

Conclusion

In this tutorial, you created data model classes and generated a database from those classes. You filled the database tables with test data. You used model binding to retrieve data from the database, and then displayed the data in a GridView.

In the next tutorial in this series, you will enable updating, deleting, and creating data.