如何在单独的类库.net core 中使用 DbContext？

Question

我正在尝试从类库中的 .net core 3.1 MVC 项目访问我的 dbcontext。目前我将我的数据库注入到startup.cs的服务集合中

public class AppDbContext : DbContext
{
    public DbSet<User> Users {get; set;}
    public AppDbContext(DbContextOptions<AppDbContext> options)
        : base(options)
    { }

    protected override void OnModelCreating(ModelBuilder builder)
    {
        // Handles duplicate entry for email in user table
        builder.Entity<User>()
            .HasIndex(u => u.Email)
            .IsUnique();
    }
}

但是，我不确定如何在我的类库中访问这个 AppDbContext。我尝试像访问控制器一样访问它，但显然它不知道 DI 容器。

更多信息： 此库用于发出的常见请求。它必须始终与主 Web 应用程序分开，并且用户永远不必在此类库中编写代码。因此，我需要能够从类库中访问主 Web 项目中的 dbcontext。

Answer 1

正如您所说，您正在开发类库以使用库客户端传递的任何DbContext，那么您必须执行以下操作：

首先考虑你的类库有以下接口和类，你的DbContext 将被使用：

public interface IUnitOfWork 
{
    IRepository<T> Repository<T>() where T : class;
    Task SaveChangesAsync();
}

internal class UnitOfWork : IUnitOfWork
{
    private readonly DbContext _dbContext;
    private Hashtable _repositories;
    public UnitOfWork(DbContext dbContext)
    {
        _dbContext = dbContext;
    }

    public IRepository<T> Repository<T>() where T : class
    {
        if (_repositories == null)
            _repositories = new Hashtable();

        var type = typeof(T).Name;

        if (!_repositories.ContainsKey(type))
        {
            var repositoryType = typeof(Repository<>);

            var repositoryInstance =
                Activator.CreateInstance(repositoryType.MakeGenericType(typeof(T)), _dbContext);

            _repositories.Add(type, repositoryInstance);
        }

        return (IRepository<T>)_repositories[type];
    }

    public async Task SaveChangesAsync()
    {
        await _dbContext.SaveChangesAsync();
    }
}

public interface IRepository<TEntity> where TEntity : class
{
     Task InsertEntityAsync(TEntity entity);
}

internal class Repository<TEntity> : IRepository<TEntity> where TEntity : class
{
    private readonly DbContext _dbContext;
    public Repository(DbContext dbContext)
    {
        _dbContext = dbContext;
    }

    public async Task InsertEntityAsync(TEntity entity)
    {
        await _dbContext.Set<TEntity>().AddAsync(entity);
    }
 }

现在在你的类库中编写一个服务集合扩展方法如下：

public static class ServiceCollectionExtensions
{
   
    public static void RegisterYourLibrary(this IServiceCollection services, DbContext dbContext)
    {
        if (dbContext == null)
        {
            throw new ArgumentNullException(nameof(dbContext));
        }

        services.AddScoped<IUnitOfWork, UnitOfWork>(uow => new UnitOfWork(dbContext));
    }
}

现在在您的客户端应用程序的Startup.ConfigureServices 中如下：

public void ConfigureServices(IServiceCollection services)
{
    string connectionString = Configuration.GetConnectionString("ConnectionStringName");
    services.AddDbContext<AppDbContext>(option => option.UseSqlServer(connectionString));

    ServiceProvider serviceProvider = services.BuildServiceProvider();
    AppDbContext appDbContext = serviceProvider.GetService<AppDbContext>();

    services.RegisterYourLibrary(appDbContext); // <-- Here passing the DbConext instance to the class library

    .......
}

用法：

public class EmployeeController : Controller
{
    private readonly IUnitOfWork _unitOfWork;
 
    public EmployeeController(IUnitOfWork unitOfWork)
    {
        _unitOfWork = unitOfWork;
    }

    public async Task<IActionResult> Insert()
    {
        Employee employee = new Employee();
        await _unitOfWork.Repository<Employee>().InsertEntityAsync(employee);
        await _unitOfWork.SaveChangesAsync();
        return View();
    }
}

Answer 2

在收到 PMC 命令Add-Migration InitialCreate 的错误后，我就这样解决了：

Unable to create an object of type 'ApplicationDbContext'. For the different patterns supported at design time, see https://go.microsoft.com/fwlink/?linkid=851728

将IDesignTimeDbContextFactory 添加为@JoseGonzalez，但我不想在IDesignTimeDbContextFactory 的实现中硬编码我的连接字符串，而是从我的应用程序appsettings.json 中获取它。

最终解决方案如下所示：

using Microsoft.EntityFrameworkCore;
using Microsoft.EntityFrameworkCore.Design;
using Microsoft.Extensions.Configuration;
using System;
using System.Collections.Generic;
using System.IO;
using System.Text;

namespace MyNamespace
{
    public class ApplicationDbContextFactory : IDesignTimeDbContextFactory<ApplicationDbContext>
    {
        public ApplicationDbContext CreateDbContext(string[] args)
        {
            var configuration = new ConfigurationBuilder()
                    .SetBasePath(Directory.GetCurrentDirectory())
                    .AddJsonFile("appsettings.json")
                    .Build();

            var optionsBuilder = new DbContextOptionsBuilder();

            var connectionString = configuration
                        .GetConnectionString("DefaultConnection");

            optionsBuilder.UseSqlServer(connectionString);

            return new ApplicationDbContext(optionsBuilder.Options);
        }
    }
}

然后我可以像这样在我的其他项目中使用它：

var applicationDbContextFactory = new ApplicationDbContextFactory();

using (var dbContext = applicationDbContextFactory.CreateDbContext(args))
{
    
}

Answer 3

如果您在另一个项目/lib 中创建了 dbcontext，您首先需要迁移它，然后更新它。 EF 使用 IDesignTimeDbContextFactory 通知有关该库的实体框架。

 public class ContextFactoryNeededForMigrations : IDesignTimeDbContextFactory<AppDbContext >
    {
        private const string ConnectionString =
            "Server=(localdb)\\mssqllocaldb;Database=EfCoreInActionDb;Trusted_Connection=True;MultipleActiveResultSets=true";

        public EfCoreContext CreateDbContext(string[] args)
        {
            var optionsBuilder = new DbContextOptionsBuilder<EfCoreContext>();
            optionsBuilder.UseSqlServer(ConnectionString,
                b => b.MigrationsAssembly("DataLayer"));

            return new EfCoreContext(optionsBuilder.Options);
        }

然后你可以像这样将它添加到你的 startup.cs DI 容器中。

 services.AddDbContextPool<AppDbContext >( 
                options => options.UseSqlServer(connection,
                b => b.MigrationsAssembly("DataLayer"))); 

这是关于这个主题的好教程。 enter link description here