Core EF Code First-黑色主题效果

电脑版发表于：2020/12/18 16:27

[TOC]

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####创建好实体

Student类：
```c#
public class Student
{
    public int ID { get; set; }
    public string LastName { get; set; }
    public string FirstMidName { get; set; }
    public DateTime EnrollmentDate { get; set; }

    public ICollection<Enrollment> Enrollments { get; set; }
}
```
Course类：
```c
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; }
}
```
Enrollment类：
```
    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; }
    }
```


####创建数据库上下文
使得给定的数据模型与 Entity Framework 功能相协调的主类是数据库上下文类。 可以通过继承 Microsoft.EntityFrameworkCore.DbContext 类的方式创建此类。 在该类中你可以指定数据模型中包含哪些实体
```
    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; }
    }
```

当数据库创建完成后， EF 创建一系列数据表，表名默认和 DbSet 属性名相同。 集合属性的名称一般使用复数形式，但不同的开发人员的命名习惯可能不一样，开发人员根据自己的情况确定是否使用复数形式
```
    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");
        }
    }
```

####注册 SchoolContext
ASP.NET Core 默认实现 依赖注入。 在应用程序启动过程通过依赖注入注册相关服务 （例如 EF 数据库上下文）
```
services.AddDbContext<SchoolContext>(options =>       
options.UseSqlServer(Configuration.GetConnectionString("DefaultConnection")));
```
打开 appsettings.json 文件，并按以下所示添加连接字符串。
```
{
  "ConnectionStrings": {
    "SchoolContext": "Server=.;Database=SchoolContext;uid=sa;password=123456",
    "SchoolContext2": "Server=(localdb)\\mssqllocaldb;Database=SchoolContext;Trusted_Connection=True;
	MultipleActiveResultSets=true"
  }
}
```
数据库连接字符串可以使用指定使用 SQL Server LocalDB 数据库。 LocalDB 是 SQL Server Express 数据库引擎的轻量级版本，用于应用程序开发，不在生产环境中使用。 LocalDB 作为按需启动并在用户模式下运行的轻量级数据库没有复杂的配置。 默认情况下， LocalDB 在 C:/Users/目录下创建 .mdf 数据库文件。

####初始化数据库
```
public static class DbInitializer
{
    public static void Initialize(SchoolContext context)
    {
        var isscuess = 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();
    }
}
```
在 Program.cs，修改 Main 方法，使得在应用程序启动时能执行以下操作：
```
using ContosoUniversity.Data;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.AspNetCore.Hosting;
using Microsoft.Extensions.Hosting;
using Microsoft.Extensions.Logging;
using System;

namespace ContosoUniversity
{
    public class Program
    {
        public static void Main(string[] args)
        {
            var host = CreateHostBuilder(args).Build();

            CreateDbIfNotExists(host);

            host.Run();
        }

        private static void CreateDbIfNotExists(IHost host)
        {
            using (var scope = host.Services.CreateScope())
            {
                var services = scope.ServiceProvider;
                try
                {
                    var context = services.GetRequiredService<SchoolContext>();
                    DbInitializer.Initialize(context);
                }
                catch (Exception ex)
                {
                    var logger = services.GetRequiredService<ILogger<Program>>();
                    logger.LogError(ex, "An error occurred creating the DB.");
                }
            }
        }

        public static IHostBuilder CreateHostBuilder(string[] args) =>
            Host.CreateDefaultBuilder(args)
                .ConfigureWebHostDefaults(webBuilder =>
                {
                    webBuilder.UseStartup<Startup>();
                });
    }
}
```

####EF CORE读取数据
我们这里可以使用异步的方式来读取与返回数据
```
public async Task<IActionResult> Index()
 {
     return View(await _context.Students.ToListAsync());
}
```
view页面中razor解析数据
```
@model IEnumerable<Learn.Models.Student>

@{
    ViewData["Title"] = "Index";
}

<h1>Index</h1>

<form>
    <p>
        <a asp-action="Create">Create New</a>
        <input type="text" name="firstMidName" value="@ViewBag.firstMidName" />
        <input type="submit" value="查询" />
    </p>
</form>

<table class="table">
    <thead>
        <tr>
            <th>
                @Html.DisplayNameFor(model => model.LastName)
            </th>
            <th>
                @Html.DisplayNameFor(model => model.FirstMidName)
            </th>
            <th>
                @Html.DisplayNameFor(model => model.EnrollmentDate)
            </th>
            <th></th>
        </tr>
    </thead>
    <tbody>
        @foreach (var item in Model)
        {
            <tr>
                <td>
                    @Html.DisplayFor(modelItem => item.LastName)
                </td>
                <td>
                    @Html.DisplayFor(modelItem => item.FirstMidName)
                </td>
                <td>
                    @Html.DisplayFor(modelItem => item.EnrollmentDate)
                </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>
```

剑轩

Core EF Code First-黑色主题效果

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