无法解析“Microsoft.EntityFrameworkCore.DbContextOptions”类型的服务

c#

我正在尝试在 ASP.NET Core 项目上使用 EF Core 5.0 访问数据库。对于第一次迁移，我覆盖OnConfiguring()了 DBContext 上的方法并成功更新了数据库。

对于第二次迁移，我决定按照指南在 ASP.NET Core 中使用依赖项注入。这是我所做的更改。

1. 加入services.AddDbContext我的Startup.cs。
2. OnConfiguring()从 中删除了该方法DBContext。

运行后dotnet ef migrations add Posts，我收到以下错误：

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

如果我添加 --verbose 标志，我会得到这个输出：

Build started...
dotnet build blog/app/app.csproj /verbosity:quiet /nologo

Build succeeded.
    0 Warning(s)
    0 Error(s)

Time Elapsed 00:00:02.50
Build succeeded.

Finding DbContext classes...
Finding IDesignTimeDbContextFactory implementations...
Finding application service provider in assembly 'app'...
Finding Microsoft.Extensions.Hosting service provider...
No static method 'CreateHostBuilder(string[])' was found on class 'Program'.
No application service provider was found.
Finding DbContext classes in the project...
Found DbContext 'BlogContext'.
Microsoft.EntityFrameworkCore.Design.OperationException: Unable to create an object of type 'BlogContext'. For the different patterns supported at design time, see https://go.microsoft.com/fwlink/?linkid=851728
 ---> System.InvalidOperationException: Unable to resolve service for type 'Microsoft.EntityFrameworkCore.DbContextOptions`1[app.Data.BlogContext]' while attempting to activate 'app.Data.BlogContext'.
   at Microsoft.Extensions.DependencyInjection.ActivatorUtilities.ConstructorMatcher.CreateInstance(IServiceProvider provider)
   at Microsoft.Extensions.DependencyInjection.ActivatorUtilities.CreateInstance(IServiceProvider provider, Type instanceType, Object[] parameters)
   at Microsoft.Extensions.DependencyInjection.ActivatorUtilities.GetServiceOrCreateInstance(IServiceProvider provider, Type type)
   at Microsoft.EntityFrameworkCore.Design.Internal.DbContextOperations.<>c__DisplayClass13_4.<FindContextTypes>b__13()
   
Unable to create an object of type 'BlogContext'. For the different patterns supported at design time, see https://go.microsoft.com/fwlink/?linkid=851728
 

但是，当我运行 Web 应用程序时，代码按预期工作，因为在BlogContextDI 层创建并注入到我的类中，并且我可以访问数据库。

因此，我猜测 DI 层在运行dotnet ef migrations add命令时没有按预期运行。

这是我的代码。

// Program.cs

public class Program
{
    public static void Main(string[] args)
    {
        IHostBuilder builder = Host.CreateDefaultBuilder(args)
                                        .UseSerilog()
                                        .ConfigureWebHostDefaults(webBuilder =>
                                        {
                                            webBuilder.UseStartup<Startup>();
                                        });

        IHost host = builder.Build();

        host.Run();
    }
}

// BlogContext

using app.Models;
using Microsoft.EntityFrameworkCore;

namespace app.Data
{
    public class BlogContext : DbContext
    {
        public DbSet<Post> Posts { get; set; }

        public DbSet<Comment> Comments { get; set; }

        public BlogContext(DbContextOptions<BlogContext> options) : base(options)
        {

        }
    }
}

// Startup.cs

using Microsoft.EntityFrameworkCore;
using Microsoft.Extensions.DependencyInjection;

public class Startup
{
    public Startup(IConfiguration configuration)
    {
         Configuration = configuration;
    }

    public IConfiguration Configuration { get; }

    public string ConnectionString => Configuration.GetConnectionString("Blog");

    public void ConfigureServices(IServiceCollection services)
    {
        services.AddDbContext<BlogContext>(opt => { opt.UseSqlite(ConnectionString); });
    }
}

无论是Startup和BlogContext住在同一个项目。

回答

public class Program
{
    public static void Main(string[] args)
        => CreateHostBuilder(args).Build().Run();

    // EF Core uses this method at design time to access the DbContext
    public static IHostBuilder CreateHostBuilder(string[] args)
        => Host.CreateDefaultBuilder(args)
            .ConfigureWebHostDefaults(
                webBuilder => webBuilder.UseStartup<Startup>());
}