Question

当我尝试跑步时

dotnet ef migration add Init

我遇到错误

无法创建类型为“ IdentityContext”的对象。

我知道是什么引起了问题。我想学习使用消息总线并将其添加到我的项目中，因此我运行了一些课程并实现了它。 Ofc巴士运作正常。但是问题是我无法再通过EF迁移。

以前是我的Startup.cs。

public class Program
{
    public static void Main(string[] args)
    {
        CreateWebHostBuilder(args).Build().Run();
    }

    public static IWebHostBuilder CreateWebHostBuilder(string[] args) =>
        WebHost.CreateDefaultBuilder(args)
            .UseStartup<Startup>();
}

现在看起来像这样。

public static void Main(string[] args)
{
  ServiceHost.Create<Startup>(args)
    .UseRabbitMq()
    .Build()
    .Run();
}

和ServiceHost类

public void Run() => _webHost.Run();

public static HostBuilder Create<TStartup>(string[] args) where TStartup : class
{
  Console.Title = typeof(TStartup).Namespace;
  var config = new ConfigurationBuilder()
    .AddEnvironmentVariables()
    .AddCommandLine(args)
    .Build();
  var webHostBuilder = WebHost.CreateDefaultBuilder()
    .UseConfiguration(config)
    .UseStartup<TStartup>();

  return new HostBuilder(webHostBuilder.Build());
}

因此，任何人都会提出建议，为什么迁移停止工作？对我来说，它看起来应该可以工作，但事实并非如此。

我在Startup.cs中有办公室

services.AddEntityFrameworkNpgsql().AddDbContext<IdentityContext>(options =>
    options.UseNpgsql(Configuration.GetConnectionString("IdentityConnection")));

编辑 IdentityContext类：

  public class IdentityContext : DbContext
  {
    public IdentityContext(DbContextOptions<IdentityContext> options) : base(options)
    {
      while (!Debugger.IsAttached)
      {
        Thread.Sleep(100);
      }
    }
    public DbSet<User> Users { get; set; }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
      modelBuilder.Entity<User>().HasIndex(user => user.Email).IsUnique();
      modelBuilder.Entity<User>().HasIndex(user => user.Username).IsUnique();
    }
  }

编辑2。详细迁移

Using project '/home/msek/Projects/inz/CrossX/src/CrossX.Identity/CrossX.Identity.csproj'.
Using startup project '/home/msek/Projects/inz/CrossX/src/CrossX.Identity/CrossX.Identity.csproj'.
Writing '/home/msek/Projects/inz/CrossX/src/CrossX.Identity/obj/CrossX.Identity.csproj.EntityFrameworkCore.targets'...
dotnet msbuild /target:GetEFProjectMetadata /property:EFProjectMetadataFile=/tmp/tmpCq3PQa.tmp /verbosity:quiet /nologo /home/msek/Projects/inz/CrossX/src/CrossX.Identity/CrossX.Identity.csproj
Writing '/home/msek/Projects/inz/CrossX/src/CrossX.Identity/obj/CrossX.Identity.csproj.EntityFrameworkCore.targets'...
dotnet msbuild /target:GetEFProjectMetadata /property:EFProjectMetadataFile=/tmp/tmpR48yu8.tmp /verbosity:quiet /nologo /home/msek/Projects/inz/CrossX/src/CrossX.Identity/CrossX.Identity.csproj
dotnet build /home/msek/Projects/inz/CrossX/src/CrossX.Identity/CrossX.Identity.csproj /verbosity:quiet /nologo

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

Time Elapsed 00:00:01.17
dotnet exec --depsfile /home/msek/Projects/inz/CrossX/src/CrossX.Identity/bin/Debug/netcoreapp2.2/CrossX.Identity.deps.json --additionalprobingpath /home/msek/.nuget/packages --runtimeconfig /home/msek/Projects/inz/CrossX/src/CrossX.Identity/bin/Debug/netcoreapp2.2/CrossX.Identity.runtimeconfig.json /home/msek/.dotnet/tools/.store/dotnet-ef/2.2.2/dotnet-ef/2.2.2/tools/netcoreapp2.2/any/tools/netcoreapp2.0/any/ef.dll migrations add Init --assembly /home/msek/Projects/inz/CrossX/src/CrossX.Identity/bin/Debug/netcoreapp2.2/CrossX.Identity.dll --startup-assembly /home/msek/Projects/inz/CrossX/src/CrossX.Identity/bin/Debug/netcoreapp2.2/CrossX.Identity.dll --project-dir /home/msek/Projects/inz/CrossX/src/CrossX.Identity/ --language C# --working-dir /home/msek/Projects/inz/CrossX/src/CrossX.Identity --verbose --root-namespace CrossX.Identity
Using assembly 'CrossX.Identity'.
Using startup assembly 'CrossX.Identity'.
Using application base '/home/msek/Projects/inz/CrossX/src/CrossX.Identity/bin/Debug/netcoreapp2.2'.
Using working directory '/home/msek/Projects/inz/CrossX/src/CrossX.Identity'.
Using root namespace 'CrossX.Identity'.
Using project directory '/home/msek/Projects/inz/CrossX/src/CrossX.Identity/'.
Finding DbContext classes...
Finding IDesignTimeDbContextFactory implementations...
Finding application service provider...
Finding IWebHost accessor...
No CreateWebHostBuilder(string[]) method was found on type 'CrossX.Identity.Program'.
No application service provider was found.
Finding DbContext classes in the project...
Found DbContext 'IdentityContext'.
Microsoft.EntityFrameworkCore.Design.OperationException: Unable to create an object of type 'IdentityContext'. For the different patterns supported at design time, see https://go.microsoft.com/fwlink/?linkid=851728 ---> System.MissingMethodException: No parameterless constructor defined for this object.
   at System.RuntimeTypeHandle.CreateInstance(RuntimeType type, Boolean publicOnly, Boolean wrapExceptions, Boolean& canBeCached, RuntimeMethodHandleInternal& ctor)
   at System.RuntimeType.CreateInstanceSlow(Boolean publicOnly, Boolean wrapExceptions, Boolean skipCheckThis, Boolean fillCache)
   at Microsoft.EntityFrameworkCore.Design.Internal.DbContextOperations.<>c__DisplayClass12_3.<FindContextTypes>b__13()
   --- End of inner exception stack trace ---
   at Microsoft.EntityFrameworkCore.Design.Internal.DbContextOperations.<>c__DisplayClass12_3.<FindContextTypes>b__13()
   at Microsoft.EntityFrameworkCore.Design.Internal.DbContextOperations.CreateContext(Func`1 factory)
   at Microsoft.EntityFrameworkCore.Design.Internal.DbContextOperations.CreateContext(String contextType)
   at Microsoft.EntityFrameworkCore.Design.Internal.MigrationsOperations.AddMigration(String name, String outputDir, String contextType)
   at Microsoft.EntityFrameworkCore.Design.OperationExecutor.AddMigrationImpl(String name, String outputDir, String contextType)
   at Microsoft.EntityFrameworkCore.Design.OperationExecutor.OperationBase.<>c__DisplayClass3_0`1.<Execute>b__0()
   at Microsoft.EntityFrameworkCore.Design.OperationExecutor.OperationBase.Execute(Action action)
Unable to create an object of type 'IdentityContext'. For the different patterns supported at design time, see https://go.microsoft.com/fwlink/?linkid=851728

Answer 1

确保您有没有参数的 IdentityContext 的默认构造函数。它应该可以工作。

Answer 2

如果您正在使用新的 .NET 5 顶级代码功能，那么这绝对是迁移中断的原因。在 Program 类中使用顶级语句会使 EF 工具无法访问它，因为它既不包含命名空间也不包含类名。

顶级语句：

Console.WriteLine("Hello, World!");

经典Program类：

namespace MyApp
{
    public class Program
    {
        public static void Main(string[] args)
        {
            Console.WriteLine("Hello, World!");
        }
    }
}

如果是这样，那就回到经典模式。

Answer 3

我的问题是我没有从解决方案中将我的api项目设置为Startup Project，因此它找不到任何Startup.cs或类似的东西。

Answer 4

EF 需要能够获取 DbContext 的实例。

来自an answer here：

<块引用>

确保您将默认启动项目设置为 WebApplication。
然后在包管理器控制台 将默认项目设置为数据项目。这将使用 WebApplication 配置并添加迁移到数据项目。

如果您正在运行 update-database 命令，那么您需要将默认项目设置为 Startup.cs 文件所在的 WebAPI 项目。

Answer 5

好像您实现了IdentityContext，但是在您的应用程序中的某个地方，它仍然尝试引用DbContext。确保Identitycontext正在扩展DbContext。

https://docs.microsoft.com/en-us/ef/core/miscellaneous/configuring-dbcontext

编辑：如https://docs.microsoft.com/en-gb/ef/core/miscellaneous/cli/dbcontext-creation

中所述实现了IDesignTimeDbContextFactory。

Answer 6

在名为 CreateWebHostBuilder 的Program类中创建一个方法。将您的主要方法主体移至该类，然后从Main方法调用该类。新的实现将是：

public static void Main(string[] args)
{
    CreateWebHostBuilder(args).Build().Run();
}

public static IWebHostBuilder CreateWebHostBuilder(string[] args) =>
    ServiceHost.Create<Startup>(args)
                .UseRabbitMq()
                .Build()
                .Run();

ef工具环顾CreateWebHostBuilder方法并在找不到任何方法时返回错误。

Answer 7

就我而言，问题在于没有按照程序中设置的方式检测到启动：

public static IWebHostBuilder CreateWebHostBuilder(string[] args, Type startup)
{
   return WebHost.CreateDefaultBuilder(args).UseStartup(startup);
}

传递类型直接解决问题：

.UseStartup<Startup>()

无法创建“ DbContext”类型的对象

7 个答案: