如何在EF Core中实例化DbContext

Question

我也设置了.net核心项目和数据库上下文.但我不能开始使用dbContext但由于此错误 -

"没有给出符合所需形式参数'选项'的论据"

控制器:

public IActionResult Index()
{
    using (var db = new BlexzWebDb())
    {

    }
    return View();
}

Dbcontext代码:

public class BlexzWebDb : DbContext
{
    public BlexzWebDb(DbContextOptions<BlexzWebDb> options)
       : base(options)
    { }

    public DbSet<User> Users { get; set; }
    public DbSet<Role> Roles { get; set; }
    public DbSet<AssignedRole> AssignedRoles { get; set; }

}

错误图片附.是什么可能解决这个问题？提前致谢

Answer 1

在EF Core中,将一些DbContextOptions传递给构造函数是很常见的.

所以一般来说,构造函数看起来像这样:

public BlexzWebDb(DbContextOptions<BlexzWebDb> options) : base(options)

正如你在那里看到的那样,没有参数构造函数形式的有效重载:

因此,这不起作用:

using (var db = new BlexzWebDb())

代替

.Net Core已经在其中实现了IoC.好的,这意味着; 你没有创建一个上下文,你要求框架根据你之前定义的一些规则给你一个.

示例:在某处您将注册dbcontext,(Startup.cs):

//typical configuration part of .net core
public void ConfigureServices(IServiceCollection services)
{
    //some mvc 
    services.AddMvc();

    //hey, options! 
    services.AddDbContextPool<BlexzWebDb>(options => 
           options.UseSqlServer(Configuration.GetConnectionString("BlexzWebConnection")));
    //...etc

现在注册部分已完成,您可以从框架中检索您的上下文.例如:通过控制器中的构造函数反转控制:

public class SomeController : Controller
{
    private readonly BlexzWebDb _db;

    //the framework handles this
    public SomeController(BlexzWebDb db)
    {
        _db = db;
    }

    //etc.

Answer 2

从ConnectionString实例化DbContext的新对象

var connectionstring = "Connection string";

var optionsBuilder = new DbContextOptionsBuilder<ApplicationDbContext>();
    optionsBuilder.UseSqlServer(connectionstring);


ApplicationDbContext dbContext = new ApplicationDbContext(optionsBuilder.Options);

// Or you can also instantiate inside using

using(ApplicationDbContext dbContext = new ApplicationDbContext(optionsBuilder.Options))
{
   //...do stuff
}

Answer 3

除了@Stefan的回答之外,还有另一种方法可以实现这一目标.您可以在DbContext类的OnCongfiguring方法中设置db连接字符串,而无需在startup.cs中添加DbContext服务.

Setting.cs

public static class Setting
{
    public static string ConnectionString { get; set; }
}

Startup.cs

Setting.ConnectionString = Configuration.GetSection("ConnectionStrings:BlexzDbConnection").Value;

BlexzWebDb.cs

public class BlexzWebDb : DbContext 
{
   protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
   {
       if (!optionsBuilder.IsConfigured)
       {
           optionsBuilder.UseSqlServer(Setting.ConnectionString);
       }
    }
}

HomeController.cs

public class HomeController : Controller
{
    private readonly BlexzWebDb db;

    public HomeController()
    {
        this.db = new BlexzWebDb();
    }

    //etc.

Answer 4

EF Core 3.1 的代码示例：

public class Test
{
    private readonly IServiceProvider _serviceProvider;

    public Test(IServiceProvider serviceProvider)
    {
        _serviceProvider = serviceProvider;
    }

    public async Task<RequestResult> Handle(...)
    {
        await using var context = CreateContext();
        ...
    }

    private DocumentContext CreateContext()
    {
        var options = _serviceProvider.GetService<IOptions<DocumentContextOptions>>();
        return new DocumentContext(options);
    }
}