使用Visual Studio开发ASP.NET Core MVC and Entity Framework Core初学者教程

原文地址：https://docs.asp.net/en/latest/data/ef-mvc/intro.html

The Contoso University sample web application demonstrates how to create ASP.NET Core 1.0 MVC web applications using Entity Framework Core 1.0 and Visual Studio 2015.

Contoso University网络应用的案例，演示了如何使用Entity Framework Core 1.0 and Visual Studio 2015来创建ASP.NET Core 1.0 MVC网络应用。

The sample application is a web site for a fictional Contoso University. It includes functionality such as student admission, course creation, and instructor assignments. This tutorial series explains how to build the Contoso University sample application from scratch. You can download the completed application.

EF Core 1.0 is the latest version of EF but does not yet have all the features of EF 6.x. For information about how to choose between EF 6.x and EF Core 1.0, see EF Core vs. EF6.x. If you choose EF 6.x, see the previous version of this tutorial series.

EF Core 1.0是EF的最新版本，但是还没有包括所有EF6.X的所有功能。关于如何在EF 6.x和 EF Core 1.0进行选择的信息，请参看EF Core vs. EF6.x。如果你选择EF 6.X，请参看the previous version of this tutorial series。

Sections:

章节：

Prerequisites 前提

Troubleshooting

If you run into a problem you can’t resolve, you can generally find the solution by comparing your code to the completed project that you can download. For some common errors and how to solve them, see the Troubleshooting section of the last tutorial in the series. If you don’t find what you need there, you can post questions to the ASP.NET Entity Framework forum, the Entity Framework forum, or StackOverflow.com for ASP.NET Core or EF Core.

The Contoso University Web Application

The application you’ll be building in these tutorials is a simple university web site.

Users can view and update student, course, and instructor information. Here are a few of the screens you’ll create.

The UI style of this site has been kept close to what’s generated by the built-in templates, so that the tutorial can focus mainly on how to use the Entity Framework.

Create an ASP.NET Core MVC web application

Open Visual Studio 2015 and create a new ASP.NET Core C# web project named “ContosoUniversity”.

  • From the File menu, select New > Project.
  • From the left pane, select Templates > Visual C# > Web.
  • Select the ASP.NET Core Web Application (.NET Core) project template.
  • Enter ContosoUniversity as the name and click OK.

  • Wait for the New ASP.NET Core Web Application (.NET Core) dialog to appear
  • Select the Web Application template and ensure that Authentication is set to Individual User Accounts.
  • Clear the Host in the cloud check box.
  • Click OK

Note

Don’t miss setting authentication to Individual User Accounts. You won’t be using authentication in this tutorial, but you need to enable it because of a limitation of .NET Core Preview 2 Visual Studio tooling. Scaffolding for MVC controllers and views only works when Individual User Accounts authentication is enabled.

Set up the site style

A few simple changes will set up the site menu, layout, and home page.

Open Views/Shared/_Layout.cshtml and make the following changes:

  • Change each occurrence of “ContosoUniversity” to “Contoso University”. There are three occurrences.
  • Add menu entries for Students, Courses, Instructors, and Departments, and delete the Contact menu entry.

The changes are highlighted.

 1 <!DOCTYPE html>
 2 <html>
 3 <head>
 4     <meta charset="utf-8" />
 5     <meta name="viewport" content="width=device-width, initial-scale=1.0" />
 6     <title>@ViewData["Title"] - Contoso University</title>
 7
 8     <environment names="Development">
 9         <link rel="stylesheet" href="~/lib/bootstrap/dist/css/bootstrap.css" />
10         <link rel="stylesheet" href="~/css/site.css" />
11     </environment>
12     <environment names="Staging,Production">
13         <link rel="stylesheet" href="https://ajax.aspnetcdn.com/ajax/bootstrap/3.3.6/css/bootstrap.min.css"
14               asp-fallback-href="~/lib/bootstrap/dist/css/bootstrap.min.css"
15               asp-fallback-test-class="sr-only" asp-fallback-test-property="position" asp-fallback-test-value="absolute" />
16         <link rel="stylesheet" href="~/css/site.min.css" asp-append-version="true" />
17     </environment>
18 </head>
19 <body>
20     <div class="navbar navbar-inverse navbar-fixed-top">
21         <div class="container">
22             <div class="navbar-header">
23                 <button type="button" class="navbar-toggle" data-toggle="collapse" data-target=".navbar-collapse">
24                     <span class="sr-only">Toggle navigation</span>
25                     <span class="icon-bar"></span>
26                     <span class="icon-bar"></span>
27                     <span class="icon-bar"></span>
28                 </button>
29                 <a asp-area="" asp-controller="Home" asp-action="Index" class="navbar-brand">Contoso University</a>
30             </div>
31             <div class="navbar-collapse collapse">
32                 <ul class="nav navbar-nav">
33                     <li><a asp-area="" asp-controller="Home" asp-action="Index">Home</a></li>
34                     <li><a asp-area="" asp-controller="Home" asp-action="About">About</a></li>
35                     <li><a asp-area="" asp-controller="Students" asp-action="Index">Students</a></li>
36                     <li><a asp-area="" asp-controller="Courses" asp-action="Index">Courses</a></li>
37                     <li><a asp-area="" asp-controller="Instructors" asp-action="Index">Instructors</a></li>
38                     <li><a asp-area="" asp-controller="Departments" asp-action="Index">Departments</a></li>
39                 </ul>
40                 @await Html.PartialAsync("_LoginPartial")
41             </div>
42         </div>
43     </div>
44     <div class="container body-content">
45         @RenderBody()
46         <hr />
47         <footer>
48             <p>&copy; 2016 - Contoso University</p>
49         </footer>
50     </div>
51
52     <environment names="Development">
53         <script src="~/lib/jquery/dist/jquery.js"></script>
54         <script src="~/lib/bootstrap/dist/js/bootstrap.js"></script>
55         <script src="~/js/site.js" asp-append-version="true"></script>
56     </environment>
57     <environment names="Staging,Production">
58         <script src="https://ajax.aspnetcdn.com/ajax/jquery/jquery-2.2.0.min.js"
59                 asp-fallback-src="~/lib/jquery/dist/jquery.min.js"
60                 asp-fallback-test="window.jQuery">
61         </script>
62         <script src="https://ajax.aspnetcdn.com/ajax/bootstrap/3.3.6/bootstrap.min.js"
63                 asp-fallback-src="~/lib/bootstrap/dist/js/bootstrap.min.js"
64                 asp-fallback-test="window.jQuery && window.jQuery.fn && window.jQuery.fn.modal">
65         </script>
66         <script src="~/js/site.min.js" asp-append-version="true"></script>
67     </environment>
68
69     @RenderSection("scripts", required: false)
70 </body>
71 </html>

In Views/Home/Index.cshtml, replace the contents of the file with the following code to replace the text about ASP.NET and MVC with text about this application:

 1 @{
 2     ViewData["Title"] = "Home Page";
 3 }
 4
 5 <div class="jumbotron">
 6     <h1>Contoso University</h1>
 7 </div>
 8 <div class="row">
 9     <div class="col-md-4">
10         <h2>Welcome to Contoso University</h2>
11         <p>
12             Contoso University is a sample application that
13             demonstrates how to use Entity Framework Core 1.0 in an
14             ASP.NET Core MVC 1.0 web application.
15         </p>
16     </div>
17     <div class="col-md-4">
18         <h2>Build it from scratch</h2>
19         <p>You can build the application by following the steps in a series of tutorials.</p>
20         <p><a class="btn btn-default" href="https://docs.asp.net/en/latest/data/ef-mvc/intro.html">See the tutorial &raquo;</a></p>
21     </div>
22     <div class="col-md-4">
23         <h2>Download it</h2>
24         <p>You can download the completed project from GitHub.</p>
25         <p><a class="btn btn-default" href="https://github.com/aspnet/Docs/tree/master/aspnet/data/ef-mvc/intro/samples/cu-final">See project source code &raquo;</a></p>
26     </div>
27 </div>

Press CTRL+F5 to run the project or choose Debug > Start Without Debugging from the menu. You see the home page with tabs for the pages you’ll create in these tutorials.

Entity Framework Core NuGet packages

Because you used the Individual User Accounts option when you created the project, support for EF Core has already been installed.

If you want to add EF Core support to a new project that you create without the Individual User Accounts option, install the following NuGet packages:

  • The package for the database provider you want to target. To use SQL Server, the package is Microsoft.EntityFrameworkCore.SqlServer. For a list of available providers see Database Providers.
  • The package for the EF command-line tools: Microsoft.EntityFrameworkCore.Tools. This package is a preview release, so to install it you have to enable preview release installation. After installing the package, you also have to add a reference to it in the tools collection in the project.json file.

If you open the project.json file, you’ll see that these packages are already installed.

 1 {
 2   "dependencies": {
 3     "Microsoft.NETCore.App": {
 4       "version": "1.0.0",
 5       "type": "platform"
 6     },
 7     "Microsoft.EntityFrameworkCore.SqlServer": "1.0.0",
 8     "Microsoft.EntityFrameworkCore.Tools": {
 9       "version": "1.0.0-preview2-final",
10       "type": "build"
11     }
12     // other dependencies not shown
13   },
14
15   "tools": {
16     "Microsoft.EntityFrameworkCore.Tools": "1.0.0-preview2-final"
17     // other tools not shown
18   }
19 }

Create the data model 创建数据模型

Next you’ll create entity classes for the Contoso University application. You’ll start with the following three entities.

接下来，你将创建Contoso大学应用的实体类。你将从以下三个实体开始。

There’s a one-to-many relationship between Student and Enrollment entities, and there’s a one-to-many relationship between Course and Enrollment entities. In other words, a student can be enrolled in any number of courses, and a course can have any number of students enrolled in it.

在Student和Enrollment实体间有一个“一对多”关系，在Course和Enrollment实体间有一个“一对多”关系。话句话说，一个学生可以在任何数量的课程里注册，一门课程可以有任何数量的学生注册。

In the following sections you’ll create a class for each one of these entities.

后面，你将逐一创建这些实体。

The Student entity 学生实体

In the Models folder, create a class file named Student.cs and replace the template code with the following code.

在Models文件夹中，创建一个名为Stduent.cs的类文件，再用下面的代码替换模板中的代码。

 1 using System;
 2 using System.Collections.Generic;
 3
 4 namespace ContosoUniversity.Models
 5 {
 6     public class Student
 7     {
 8         public int ID { get; set; }
 9         public string LastName { get; set; }
10         public string FirstMidName { get; set; }
11         public DateTime EnrollmentDate { get; set; }
12
13         public ICollection<Enrollment> Enrollments { get; set; }
14     }
15 }

The ID property will become the primary key column of the database table that corresponds to this class. By default, the Entity Framework interprets a property that’s named ID or classnameID as the primary key.

ID属性将会成为与该类相应的数据库表的主键列。缺省情况下，EF将名称为“ID”或者“类名ID”的属性解释为主键。

The Enrollments property is a navigation property. Navigation properties hold other entities that are related to this entity. In this case, the Enrollments property of a Student entity will hold all of the Enrollment entities that are related to that Student entity. In other words, if a given Student row in the database has two related Enrollment rows (rows that contain that student’s primary key value in their StudentID foreign key column), that Student entity’s Enrollments navigation property will contain those two Enrollment entities.

Enrollments属性是导航属性。导航属性控制着与该类相关的实体。在这种情况下，Student实体的enrollments属性持有所有与Student实体相关的Enrollment实体。话句话说，如果给定了一个数据库中的Student行，该行与两个Enrollment行相关（），这个Student实体的Enrollments导航属性将会包含两个Enrollment实体。

If a navigation property can hold multiple entities (as in many-to-many or one-to-many relationships), its type must be a list in which entries can be added, deleted, and updated, such as ICollection<T>. You can specify ICollection<T> or a type such as List<T> or HashSet<T>. If you specify ICollection<T>, EF creates a HashSet<T> collection by default.

如果导航属性持有多个实体（如在多对多或者1对多关系中），其类型必须是一个list，以便实现实体的增加、删除和更新，例如ICollection<T>。你可以指定ICollection<T>或者诸如List<T>、HashSet<T>的类型。如果你指定了ICollection<T>，EF会缺省创建一个HashSet<T>集合。

The Enrollment entity Enrollment实体

In the Models folder, create Enrollment.cs and replace the existing code with the following code:

namespace ContosoUniversity.Models
{
    public enum Grade
    {
        A, B, C, D, F
    }

    public class Enrollment
    {
        public int EnrollmentID { get; set; }
        public int CourseID { get; set; }
        public int StudentID { get; set; }
        public Grade? Grade { get; set; }

        public Course Course { get; set; }
        public Student Student { get; set; }
    }
}

The EnrollmentID property will be the primary key; this entity uses the classnameID pattern instead of ID by itself as you saw in the Student entity. Ordinarily you would choose one pattern and use it throughout your data model. Here, the variation illustrates that you can use either pattern. In a later tutorial, you’ll see how using ID without classname makes it easier to implement inheritance in the data model.

EnrollmentID属性将会成为主键；该实体使用classnameID属性而不是ID自己。通常，你将选择一个模型，并在整个数据模型中使用。在此，用图解说明遇到的这些变化。在后续的教程中，你将看到如何使用ID而不是classname，这样会使数据模型中更容易实施继承。

The Grade property is an enum. The question mark after the Grade type declaration indicates that the Grade property is nullable. A grade that’s null is different from a zero grade – null means a grade isn’t known or hasn’t been assigned yet.

Grade属性是枚举型。Grade后面的问号表明Grade属性是可为null的。值为Null的Grade与值为0的Grade是不同的，null意味着这个Grade为未知或者还没有指定。

The StudentID property is a foreign key, and the corresponding navigation property is Student. An Enrollment entity is associated with one Student entity, so the property can only hold a single Student entity (unlike the Student.Enrollments navigation property you saw earlier, which can hold multiple Enrollment entities).

StduentID属性是外键，对应的导航属性是Student。Enrollment实体与Stduent实体相关联，所以该属性仅能持有单独的一个Stduent实体（不像前面看到的Stduent.Enrollments导航属性可控制多个Enrollment实体）。

The CourseID property is a foreign key, and the corresponding navigation property is Course. An Enrollment entity is associated with one Course entity.

CourseID属性是一个外键，相应的的导航实体是Course。一个Enrollment实体与一个Course实体项关联。

Entity Framework interprets a property as a foreign key property if it’s named <navigation property name><primary key property name> (for example, StudentID for the Student navigation property since the Student entity’s primary key is ID). Foreign key properties can also be named simply <primary key property name> (for example, CourseID since the Course entity’s primary key is CourseID).

如果一个属性的名称是<导航属性名><主键属性名>（例如：StduentID对于Stduent导航属性，因为Student实体的主键是ID），Entity Framework将这个属性解释为外键属性。外键属性也可被命名为相同的简单的<主键属性名>（例如：CourseID，因为Course实体的主键是CourseID。

The Course entity

In the Models folder, create Course.cs and replace the existing code with the following code:

using System.Collections.Generic;
using System.ComponentModel.DataAnnotations.Schema;

namespace ContosoUniversity.Models
{
    public class Course
    {
        [DatabaseGenerated(DatabaseGeneratedOption.None)]
        public int CourseID { get; set; }
        public string Title { get; set; }
        public int Credits { get; set; }

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

The Enrollments property is a navigation property. A Course entity can be related to any number of Enrollment entities.

Enrollments属性是导航属性。Course实体可与任意数量的Enrollment实体相关联。

We’ll say more about the DatabaseGenerated attribute in a later tutorial in this series. Basically, this attribute lets you enter the primary key for the course rather than having the database generate it.

Create the Database Context 创建数据库上下文

The main class that coordinates Entity Framework functionality for a given data model is the database context class. You create this class by deriving from the System.Data.Entity.DbContext class. In your code you specify which entities are included in the data model. You can also customize certain Entity Framework behavior. In this project, the class is named SchoolContext.

与给定的数据模型Entity Framework同等的主类是数据库上下文类。通过导出System.Data.Entity.DbContext类可创建该类。在代码中，你可指定哪一个实体包含在数据模型中。你也可客户化某个Entity Framework行为。在该项目中，该类被命名为SchoolContext。

In the Data folder create a new class file named SchoolContext.cs, and replace the template code with the following code:

在Data文件夹中创建一个名为SchoolContext.cs的新类文件，然后用下列代码替换模板代码。

using ContosoUniversity.Models;
using Microsoft.EntityFrameworkCore;

namespace ContosoUniversity.Data
{
    public class SchoolContext : DbContext
    {
        public SchoolContext(DbContextOptions<SchoolContext> options) : base(options)
        {
        }

        public DbSet<Course> Courses { get; set; }
        public DbSet<Enrollment> Enrollments { get; set; }
        public DbSet<Student> Students { get; set; }
    }
}

This code creates a DbSet property for each entity set. In Entity Framework terminology, an entity set typically corresponds to a database table, and an entity corresponds to a row in the table.

代码为每个实体集合创建了一个DbSet。在Entity Framework术语中，一个实体集合通常与一个数据库表相关，一个实体与表中的一个行相关。

You could have omitted the DbSet<Enrollment> and DbSet<Course> statements and it would work the same. The Entity Framework would include them implicitly because the Student entity references the Enrollment entity and the Enrollment entity references the Course entity.

你可以省略DbSet<Enrollment>和DbSet<Course>声明，这仍将同样工作。Entity Framework将暗含他们，因为Student实体引用了Enrollment实体，Enrollment实体引用了Course实体。

When the database is created, EF creates tables that have names the same as the DbSet property names. Property names for collections are typically plural (Students rather than Student), but developers disagree about whether table names should be pluralized or not. For these tutorials you’ll override the default behavior by specifying singular table names in the DbContext. To do that, add the following highlighted code after the last DbSet property.

当创建数据库时，EF创建了与DbSet属性名相同的表。

using ContosoUniversity.Models;
using Microsoft.EntityFrameworkCore;

namespace ContosoUniversity.Data
{
    public class SchoolContext : DbContext
    {
        public SchoolContext(DbContextOptions<SchoolContext> options) : base(options)
        {
        }

        public DbSet<Course> Courses { get; set; }
        public DbSet<Enrollment> Enrollments { get; set; }
        public DbSet<Student> Students { get; set; }

        protected override void OnModelCreating(ModelBuilder modelBuilder)
        {
            modelBuilder.Entity<Course>().ToTable("Course");
            modelBuilder.Entity<Enrollment>().ToTable("Enrollment");
            modelBuilder.Entity<Student>().ToTable("Student");
        }
    }
}

Register the context with dependency injection

ASP.NET Core implements dependency injection by default. Services (such as the EF database context) are registered with dependency injection during application startup. Components that require these services (such as MVC controllers) are provided these services via constructor parameters. You’ll see the controller constructor code that gets a context instance later in this tutorial.

To register SchoolContext as a service, open Startup.cs, and add the highlighted lines to the ConfigureServices method.

services.AddDbContext<SchoolContext>(options =>
    options.UseSqlServer(Configuration.GetConnectionString("DefaultConnection")));

services.AddDbContext<ApplicationDbContext>(options =>
    options.UseSqlServer(Configuration.GetConnectionString("DefaultConnection")));

The name of the connection string is passed in to the context by calling a method on a DbContextOptionsBuilder object. For local development, the ASP.NET Core configuration system reads the connection string from the appsettings.json file. The connection string is highlighted in the following appsettings.json example.

{
  "ConnectionStrings": {
    "DefaultConnection": "Server=(localdb)\\mssqllocaldb;Database=ContosoUniversity1;Trusted_Connection=True;MultipleActiveResultSets=true"
  },
  "Logging": {
    "IncludeScopes": false,
    "LogLevel": {
      "Default": "Debug",
      "System": "Information",
      "Microsoft": "Information"
    }
  }
}

The connection string created by the Visual Studio new-project template has a generated database name with a numeric suffix to guarantee uniqueness. You don’t have to change that name.

SQL Server Express LocalDB

The connection string specifies a SQL Server LocalDB database. LocalDB is a lightweight version of the SQL Server Express Database Engine and is intended for application development, not production use. LocalDB starts on demand and runs in user mode, so there is no complex configuration. By default, LocalDB creates .mdf database files in the C:/Users/<user> directory.

Add code to initialize the database with test data

The Entity Framework will create an empty database for you. In this section, you write a method that is called after the database is created in order to populate it with test data.

Here you’ll use the EnsureCreated method to automatically create the database. In a later tutorial you’ll see how to handle model changes by using Code First Migrations to change the database schema instead of dropping and re-creating the database.

In the Data folder, create a new class file named DbInitializer.cs and replace the template code with the following code, which causes a database to be created when needed and loads test data into the new database.

using System;
using System.Collections.Generic;
using System.Linq;
using Microsoft.EntityFrameworkCore;
using Microsoft.Extensions.DependencyInjection;
using ContosoUniversity.Data;

namespace ContosoUniversity.Models
{
    public static class DbInitializer
    {
        public static void Initialize(SchoolContext context)
        {
            context.Database.EnsureCreated();

            // Look for any students.
            if (context.Students.Any())
            {
                return;   // DB has been seeded
            }

            var students = new Student[]
            {
            new Student{FirstMidName="Carson",LastName="Alexander",EnrollmentDate=DateTime.Parse("2005-09-01")},
            new Student{FirstMidName="Meredith",LastName="Alonso",EnrollmentDate=DateTime.Parse("2002-09-01")},
            new Student{FirstMidName="Arturo",LastName="Anand",EnrollmentDate=DateTime.Parse("2003-09-01")},
            new Student{FirstMidName="Gytis",LastName="Barzdukas",EnrollmentDate=DateTime.Parse("2002-09-01")},
            new Student{FirstMidName="Yan",LastName="Li",EnrollmentDate=DateTime.Parse("2002-09-01")},
            new Student{FirstMidName="Peggy",LastName="Justice",EnrollmentDate=DateTime.Parse("2001-09-01")},
            new Student{FirstMidName="Laura",LastName="Norman",EnrollmentDate=DateTime.Parse("2003-09-01")},
            new Student{FirstMidName="Nino",LastName="Olivetto",EnrollmentDate=DateTime.Parse("2005-09-01")}
            };
            foreach (Student s in students)
            {
                context.Students.Add(s);
            }
            context.SaveChanges();

            var courses = new Course[]
            {
            new Course{CourseID=1050,Title="Chemistry",Credits=3,},
            new Course{CourseID=4022,Title="Microeconomics",Credits=3,},
            new Course{CourseID=4041,Title="Macroeconomics",Credits=3,},
            new Course{CourseID=1045,Title="Calculus",Credits=4,},
            new Course{CourseID=3141,Title="Trigonometry",Credits=4,},
            new Course{CourseID=2021,Title="Composition",Credits=3,},
            new Course{CourseID=2042,Title="Literature",Credits=4,}
            };
            foreach (Course c in courses)
            {
                context.Courses.Add(c);
            }
            context.SaveChanges();

            var enrollments = new Enrollment[]
            {
            new Enrollment{StudentID=1,CourseID=1050,Grade=Grade.A},
            new Enrollment{StudentID=1,CourseID=4022,Grade=Grade.C},
            new Enrollment{StudentID=1,CourseID=4041,Grade=Grade.B},
            new Enrollment{StudentID=2,CourseID=1045,Grade=Grade.B},
            new Enrollment{StudentID=2,CourseID=3141,Grade=Grade.F},
            new Enrollment{StudentID=2,CourseID=2021,Grade=Grade.F},
            new Enrollment{StudentID=3,CourseID=1050},
            new Enrollment{StudentID=4,CourseID=1050,},
            new Enrollment{StudentID=4,CourseID=4022,Grade=Grade.F},
            new Enrollment{StudentID=5,CourseID=4041,Grade=Grade.C},
            new Enrollment{StudentID=6,CourseID=1045},
            new Enrollment{StudentID=7,CourseID=3141,Grade=Grade.A},
            };
            foreach (Enrollment e in enrollments)
            {
                context.Enrollments.Add(e);
            }
            context.SaveChanges();
        }
    }
}

The code checks if there are any students in the database, and if not, it assumes the database is new and needs to be seeded with test data. It loads test data into arrays rather than List<T> collections to optimize performance.

In Startup.cs, modify the Configure method to call this seed method on application startup. First, add the context to the method signature so that ASP.NET dependency injection can provide it to your DbInitializer class.

public void Configure(IApplicationBuilder app, IHostingEnvironment env, ILoggerFactory loggerFactory, SchoolContext context)
{
    loggerFactory.AddConsole(Configuration.GetSection("Logging"));
    loggerFactory.AddDebug();

Then call your DbInitializer.Initialize method at the end of the Configure method.

    app.UseMvc(routes =>
    {
        routes.MapRoute(
            name: "default",
            template: "{controller=Home}/{action=Index}/{id?}");
    });

    DbInitializer.Initialize(context);
}

Now the first time you run the application the database will be created and seeded with test data. Whenever you change your data model, you can delete the database, update your seed method, and start afresh with a new database the same way. In later tutorials you’ll see how to modify the database when the data model changes, without deleting and re-creating it.

Create a controller and views

Next, you’ll use the scaffolding engine in Visual Studio to add an MVC controller and views that will use EF to query and save data.

The automatic creation of CRUD action methods and views is known as scaffolding. Scaffolding differs from code generation in that the scaffolded code is a starting point that you can modify to suit your own requirements, whereas you typically don’t modify generated code. When you need to customize generated code, you use partial classes or you regenerate the code when things change.

  • Right-click the Controllers folder in Solution Explorer and select Add > New Scaffolded Item.
  • In the Add Scaffold dialog box:
    • Select MVC controller with views, using Entity Framework.
    • Click Add.
  • In the Add Controller dialog box:
    • In Model class select Student.
    • In Data context class select SchoolContext.
    • Accept the default StudentsController.cs as the name.
    • Click Add.

When you click Add, the Visual Studio scaffolding engine creates a StudentsController.cs file and a set of views (.cshtml files) that work with the controller.

(The scaffolding engine can also create the database context for you if you don’t create it manually first as you did earlier for this tutorial. You can specify a new context class in the Add Controller box by clicking the plus sign to the right of Data context class. Visual Studio will then create your DbContext class as well as the controller and views.)

You’ll notice that the controller takes a SchoolContext as a constructor parameter.

namespace ContosoUniversity.Controllers
{
    public class StudentsController : Controller
    {
        private readonly SchoolContext _context;

        public StudentsController(SchoolContext context)
        {
            _context = context;
        }

ASP.NET dependency injection will take care of passing an instance of SchoolContext into the controller. You configured that in the Startup.cs file earlier.

The controller contains an Index action method, which displays all students in the database. The method gets a list of students from the Students entity set by reading the Students property of the database context instance:

public async Task<IActionResult> Index()
{
    return View(await _context.Students.ToListAsync());
}

You’ll learn about the asynchronous programming elements in this code later in the tutorial.

The Views/Students/Index.cshtml view displays this list in a table:

@model IEnumerable<ContosoUniversity.Models.Student>

@{
    ViewData["Title"] = "Index";
}

<h2>Index</h2>

<p>
    <a asp-action="Create">Create New</a>
</p>
<table class="table">
    <thead>
        <tr>
            <th>
                @Html.DisplayNameFor(model => model.EnrollmentDate)
            </th>
            <th>
                @Html.DisplayNameFor(model => model.FirstMidName)
            </th>
            <th>
                @Html.DisplayNameFor(model => model.LastName)
            </th>
            <th></th>
        </tr>
    </thead>
    <tbody>
        @foreach (var item in Model)
        {
            <tr>
                <td>
                    @Html.DisplayFor(modelItem => item.EnrollmentDate)
                </td>
                <td>
                    @Html.DisplayFor(modelItem => item.FirstMidName)
                </td>
                <td>
                    @Html.DisplayFor(modelItem => item.LastName)
                </td>
                <td>
                    <a asp-action="Edit" asp-route-id="@item.ID">Edit</a> |
                    <a asp-action="Details" asp-route-id="@item.ID">Details</a> |
                    <a asp-action="Delete" asp-route-id="@item.ID">Delete</a>
                </td>
            </tr>
        }
    </tbody>
</table>

Press CTRL+F5 to run the project or choose Debug > Start Without Debugging from the menu.

Click the Students tab to see the test data that the DbInitializer.Initialize method inserted. Depending on how narrow your browser window is, you’ll see the Student tab link at the top of the page or you’ll have to click the navigation icon in the upper right corner to see the link.

View the Database

When you started the application, the DbInitializer.Initialize method calls EnsureCreated. EF saw that there was no database and so it created one, then the remainder of the Initialize method code populated the database with data. You can use SQL Server Object Explorer (SSOX) to view the database in Visual Studio.

Close the browser.

If the SSOX window isn’t already open, select it from the View menu in Visual Studio.

In SSOX, click (localdb)\MSSQLLocalDB > Databases, and then click the entry for the database name that is in the connection string in your appsettings.json file.

Expand the Tables node to see the tables in your database.

Right-click the Student table and click View Data to see the columns that were created and the rows that were inserted into the table.

The .mdf and .ldf database files are in the C:Users<yourusername> folder.

Because you’re calling EnsureCreated in the initializer method that runs on app start, you could now make a change to the Student class, delete the database, run the application again, and the database would automatically be re-created to match your change. For example, if you add an EmailAddress property to the Student class, you’ll see a new EmailAddress column in the re-created table.

Conventions

The amount of code you had to write in order for the Entity Framework to be able to create a complete database for you is minimal because of the use of conventions, or assumptions that the Entity Framework makes.

  • The names of DbSet properties are used as table names. For entities not referenced by a DbSet property, entity class names are used as table names.
  • Entity property names are used for column names.
  • Entity properties that are named ID or classnameID are recognized as primary key properties.
  • A property is interpreted as a foreign key property if it’s named <navigation property name><primary key property name> (for example, StudentID for the Student navigation property since the Student entity’s primary key is ID). Foreign key properties can also be named simply <primary key property name> (for example, EnrollmentID since the Enrollment entity’s primary key is EnrollmentID).

Conventional behavior can be overridden. For example, you can explicitly specify table names, as you saw earlier in this tutorial. And you can set column names and set any property as primary key or foreign key, as you’ll see in a later tutorial in this series.

Asynchronous code

Asynchronous programming is the default mode for ASP.NET Core and EF Core.

A web server has a limited number of threads available, and in high load situations all of the available threads might be in use. When that happens, the server can’t process new requests until the threads are freed up. With synchronous code, many threads may be tied up while they aren’t actually doing any work because they’re waiting for I/O to complete. With asynchronous code, when a process is waiting for I/O to complete, its thread is freed up for the server to use for processing other requests. As a result, asynchronous code enables server resources to be use more efficiently, and the server is enabled to handle more traffic without delays.

Asynchronous code does introduce a small amount of overhead at run time, but for low traffic situations the performance hit is negligible, while for high traffic situations, the potential performance improvement is substantial.

In the following code, the async keyword, Task<T> return value, await keyword, and ToListAsync method make the code execute asynchronously.

public async Task<IActionResult> Index()
{
    return View(await _context.Students.ToListAsync());
}
  • The async keyword tells the compiler to generate callbacks for parts of the method body and to automatically create the Task<IActionResult> object that is returned.
  • The return type Task<IActionResult> represents ongoing work with a result of type IActionResult.
  • The await keyword causes the compiler to split the method into two parts. The first part ends with the operation that is started asynchronously. The second part is put into a callback method that is called when the operation completes.
  • ToListAsync is the asynchronous version of the ToList extension method.

Some things to be aware of when you are writing asynchronous code that uses the Entity Framework:

  • Only statements that cause queries or commands to be sent to the database are executed asynchronously. That includes, for example, ToListAsync, SingleOrDefaultAsync, and SaveChangesAsync. It does not include, for example, statements that just change an IQueryable, such as var students = *context.Students.Where(s => s.LastName = "Davolio").
  • An EF context is not thread safe: don’t try to do multiple operations in parallel. When you call any async EF method, always use the await keyword.
  • If you want to take advantage of the performance benefits of async code, make sure that any library packages that you’re using (such as for paging), also use async if they call any Entity Framework methods that cause queries to be sent to the database.

For more information about asynchronous programming in .NET, see Async Overview.

Summary

You’ve now created a simple application that uses the Entity Framework Core and SQL Server Express LocalDB to store and display data. In the following tutorial, you’ll learn how to perform basic CRUD (create, read, update, delete) operations.

时间： 2024-10-12 21:16:50

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