如何在类库项目中正确定义 DBContext？

我正在使用实体框架 7 并创建了两个项目。一个项目是 ASP.NET 5 Web API 项目，另一个是类库项目（包），我想在其中存储所有数据访问层逻辑。这样我就可以将此包用于另一个报告项目，并且我可能提供的其他附加服务。

基本上，我在 Web api 项目的控制器中有一个简单的帖子，它调用我的数据库项目中的函数。当函数启动数据库时，它说数据库未定义，即使它在两个项目中都定义了。

ERROR

{"No database providers are configured. Configure a database provider by overriding OnConfiguring in your DbContext class or in the AddDbContext method when setting up services."}

CODE

数据库项目：InsertPerson

public int InsertPerson(tbl_Person person)
{
    using (var db = new AppContext())
    {
        try
        {
            db.tbl_Person.Add(person);
            db.SaveChanges();
            return person.PersonID;
        }

        catch
        {
            return 0;
        }
    }
}

配置文件

Web API 项目：Startup.cs

public void ConfigureServices(IServiceCollection services)
{
    services.AddEntityFramework()
        .AddSqlServer()
        .AddDbContext<AppContext>(options =>
            options.UseSqlServer(Configuration["Data:DefaultConnection:ConnectionString"]));

    services.AddIdentity<ApplicationUser, IdentityRole>()
        .AddEntityFrameworkStores<AppContext>();

    services.AddMvc();
}

Web API 项目：appsettings.json

{
  "Data": {
    "DefaultConnection": {
      "ConnectionString": "Server=localhost;Database=testDB;Trusted_Connection=True;MultipleActiveResultSets=true"
    }
  },
  "Logging": {
    "IncludeScopes": false,
    "LogLevel": {
      "Default": "Verbose",
      "System": "Information",
      "Microsoft": "Information"
    }
  }
}

Web API 项目：project.json

{
  "userSecretsId": "blah,
  "version": "1.0.0-rc1-final",
  "compilationOptions": {
    "emitEntryPoint": true
  },

  "dependencies": {
    "DataLibrary": "",
    "Microsoft.AspNet.Authentication.OAuthBearer": "1.0.0-beta7",
    "Microsoft.AspNet.Http.Abstractions": "1.0.0-rc1-final",
    "Microsoft.AspNet.Identity.EntityFramework": "3.0.0-rc1-final",
    "Microsoft.AspNet.IISPlatformHandler": "1.0.0-rc1-final",
    "Microsoft.AspNet.Mvc": "6.0.0-rc1-final",
    "Microsoft.AspNet.Server.Kestrel": "1.0.0-rc1-final",
    "Microsoft.AspNet.StaticFiles": "1.0.0-rc1-final",
    "Microsoft.Extensions.Configuration.FileProviderExtensions": "1.0.0-rc1-final",
    "Microsoft.Extensions.Configuration.Json": "1.0.0-rc1-final",
    "Microsoft.Extensions.Configuration.UserSecrets": "1.0.0-rc1-final",
    "Microsoft.Extensions.Logging": "1.0.0-rc1-final",
    "Microsoft.Extensions.Logging.Console": "1.0.0-rc1-final",
    "Microsoft.Extensions.Logging.Debug": "1.0.0-rc1-final",
    "Remotion.Linq": "2.0.1"
  },

  "commands": {
    "web": "Microsoft.AspNet.Server.Kestrel"
  },

  "frameworks": {
    "dnx451": { },
    "dnxcore50": { }
  },

  "exclude": [
    "wwwroot",
    "node_modules"
  ],
  "publishExclude": [
    "**.user",
    "**.vspscc"
  ]
}

数据库项目：Startup.cs

public void ConfigureServices(IServiceCollection services)
{
    services.AddEntityFramework()
        .AddSqlServer()
        .AddDbContext<AppContext>(options =>
            options.UseSqlServer(Configuration["Data:DefaultConnection:ConnectionString"]));

    services.AddIdentity<ApplicationUser, IdentityRole>()
        .AddEntityFrameworkStores<AppContext>();

    services.AddMvc();
}

数据库项目：appsettings.json

{
  "Data": {
    "DefaultConnection": {
      "ConnectionString": "Server=localhost;Database=testDB;Trusted_Connection=True;MultipleActiveResultSets=true"
    }
  },
  "Logging": {
    "IncludeScopes": false,
    "LogLevel": {
      "Default": "Verbose",
      "System": "Information",
      "Microsoft": "Information"
    }
  }
}

数据库项目：AppContext.cs

namespace DataLibrary
{
    public class AppContext : IdentityDbContext<ApplicationUser>
    {
        public DbSet<tbl_Person> tbl_Person { get; set; }

        public static AppContext Create()
        {
            return new AppContext();
        }

        protected override void OnModelCreating(ModelBuilder builder)
        {
            new tbl_PersonMap(builder.Entity<tbl_Person>());
            base.OnModelCreating(builder);
        }
    }
}

示例代码中的问题似乎是您正在更新 AppContext 实例，而不是从 ServiceProvider 解析它。当您以这种方式初始化 DbContext 时，会为上下文隐式创建一个新的 ServiceProvider，并且该上下文不会考虑 Startup.cs 中的任何配置。如果您在 OnConfiguring 中配置上下文并且对应用程序其余部分中的依赖项注入不感兴趣，则可以使用此模式。

在这种情况下，我希望您通过构造函数注入来获取 AppContext。对于这种解决方案，您还需要在 ServiceCollection 中注册数据访问层类。结果应该看起来更像这样：

数据库项目：AppContext.cs

namespace DataLibrary
{
    public class AppContext : IdentityDbContext<ApplicationUser>
    {
        public DbSet<tbl_Person> tbl_Person { get; set; }

        protected override void OnModelCreating(ModelBuilder builder)
        {
            new tbl_PersonMap(builder.Entity<tbl_Person>());
            base.OnModelCreating(builder);
        }
    }
}

数据库项目：PersonH​​elper.cs

namespace DataLibrary
{
    class PersonHelper
    {
        private readonly AppContext db;

        public PersonHelper(AppContext context)
        {
            db = context;
        }

        public int InsertPerson(tbl_Person person)
        {
            try
            {
                db.tbl_Person.Add(person);
                db.SaveChanges();
                return person.PersonID;
            }
            catch
            {
                return 0;
            }
        }
    }
}

Web API 项目：Startup.cs

public void ConfigureServices(IServiceCollection services)
{
    services.AddEntityFramework()
        .AddSqlServer()
        .AddDbContext<AppContext>(options =>
            options.UseSqlServer(Configuration["Data:DefaultConnection:ConnectionString"]));

    services.AddIdentity<ApplicationUser, IdentityRole>()
        .AddEntityFrameworkStores<AppContext>();

    services.AddScoped<PersonHelper>();

    services.AddMvc();
}

Web API 项目：MyController.cs

[Route("api/[controller]")]
public class MyController : Controller
{
    private readonly personHelper helper;

    public MyController(PersonHelper helper)
    {
        this.helper = helper;
    }

    // POST api/my
    [HttpPost]
    public void Post([FromBody]string value)
    {
        var person = new tbl_Person
        {
          // ...
        }

        return helper.InsertPerson(person);
    }
}

您甚至可以考虑在 IServiceCollection 上为数据访问层类添加扩展方法，以帮助减少配置中的重复，尤其是在添加更多常用服务时。