Entity Framework Core 添加迁移期间的 NullReferenceException

Question

我正在尝试在我的控制台应用程序中测试 EF Core。 我的项目结构就像

Project_name
 - Data Folder
   - AppDbContext.cs
 - Models Folder
   - Person.cs
   - Student.cs
   - Lecturer.cs
   .... etc
 
 - Program.cs  

在我的AppDbContext.cs我配置如下

    public class AppDbContext: DbContext
    {
    /*  public AppDbContext(DbContextOptions<AppDbContext> options): base(options)
        {}*/

        public DbSet<Person> People { get; set; }
        public DbSet<Student> Students { get; set; }
        
        public DbSet<Lecturer> Lecturers { get; set; }
        public DbSet<Course> Coures { get; set; }
        public DbSet<Department> Departments { get; set; }
        public DbSet<LecturerCourse> lecturerCourses{ get; set; }
        public DbSet<Enrollment> Enrollments { get; set; }
        public DbSet<CourseNotice> courseNotices { get; set; }


        protected override void OnConfiguring(DbContextOptionsBuilder builder)
        {
            builder.UseSqlServer(@"Server = (localdb)\\mssqllocaldb; Database = efcore_consoleApp; Trusted_Connection = True; MultipleActiveResultSets = true;");
        }

        protected override void OnModelCreating(ModelBuilder builder)
        {
            base.OnModelCreating(builder);

            // Customize Course and AppUser relationship using FluentApi
            builder.Entity<LecturerCourse>()
                .HasKey(ca => new { ca.CourseId, ca.LecturerId });

            builder.Entity<LecturerCourse>()
                .HasOne(cu => cu.Course)
                .WithMany(c => c.LecturerCourses)
                .HasForeignKey(cu => cu.CourseId);

            builder.Entity<LecturerCourse>()
                .HasOne(cu => cu.Lecturer)
                .WithMany(u => u.LecturerCourses)
                .HasForeignKey(cu => cu.LecturerId);

            builder.Entity<Enrollment>()
                .HasKey(e => new { e.CourseId, e.StudentId });

            builder.Entity<Enrollment>()
                .HasOne(c => c.Course)
                .WithMany(e => e.Enrollments)
                .HasForeignKey(fk => fk.CourseId);

            builder.Entity<Enrollment>()
                .HasOne(s => s.Student)
                .WithMany(e => e.Enrollments)
                .HasForeignKey(fk => fk.StudentId);
        }

    }

我已经安装了一些需要的软件包，例如

- EntityFrameworkCore(5.0.8)
- EntityFrameworkCoreDesign(5.0.8)
- EntityFrameworkCoreSqlServer(5.0.8)
- EntityFrameworkCoreTools(5.0.8)

当我尝试添加迁移Add-Migration ... ，错误此类Object reference not set to an instance of an object. 发生堆栈跟踪，如

System.NullReferenceException: Object reference not set to an instance of an object.
   at Microsoft.EntityFrameworkCore.Metadata.Internal.InternalEntityTypeBuilder.SetOrAddForeignKey(ForeignKey foreignKey, InternalEntityTypeBuilder principalEntityTypeBuilder, IReadOnlyList`1 dependentProperties, Key principalKey, String propertyBaseName, Nullable`1 isRequired, Nullable`1 configurationSource)
   at Microsoft.EntityFrameworkCore.Metadata.Internal.InternalEntityTypeBuilder.CreateForeignKey(InternalEntityTypeBuilder principalEntityTypeBuilder, IReadOnlyList`1 dependentProperties, Key principalKey, String propertyBaseName, Nullable`1 required, ConfigurationSource configurationSource)
   at Microsoft.EntityFrameworkCore.Metadata.Internal.InternalEntityTypeBuilder.HasRelationship(EntityType targetEntityType, Nullable`1 navigationToTarget, Nullable`1 inverseNavigation, Nullable`1 setTargetAsPrincipal, ConfigurationSource configurationSource, Nullable`1 required)
   at Microsoft.EntityFrameworkCore.Metadata.Internal.InternalEntityTypeBuilder.HasRelationship(EntityType targetEntityType, MemberInfo navigationMember, ConfigurationSource configurationSource, Nullable`1 targetIsPrincipal)
   at Microsoft.EntityFrameworkCore.Metadata.Builders.EntityTypeBuilder.HasOneBuilder(MemberIdentity navigationId, EntityType relatedEntityType)
   at Microsoft.EntityFrameworkCore.Metadata.Builders.EntityTypeBuilder`1.HasOne[TRelatedEntity](Expression`1 navigationExpression)
   at efcore_consoleApp.Data.AppDbContext.OnModelCreating(ModelBuilder builder) in C:\Users\PC_1\source\repos\efcore_consoleApp\efcore_consoleApp\AppDbContext.cs:line 44
   at Microsoft.EntityFrameworkCore.Infrastructure.ModelCustomizer.Customize(ModelBuilder modelBuilder, DbContext context)
   at Microsoft.EntityFrameworkCore.Infrastructure.ModelSource.CreateModel(DbContext context, IConventionSetBuilder conventionSetBuilder, ModelDependencies modelDependencies)
   at Microsoft.EntityFrameworkCore.Infrastructure.ModelSource.GetModel(DbContext context, IConventionSetBuilder conventionSetBuilder, ModelDependencies modelDependencies)
   at Microsoft.EntityFrameworkCore.Internal.DbContextServices.CreateModel()
   at Microsoft.EntityFrameworkCore.Internal.DbContextServices.get_Model()
   at Microsoft.EntityFrameworkCore.Infrastructure.EntityFrameworkServicesBuilder.<>c.<TryAddCoreServices>b__7_3(IServiceProvider p)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitFactory(FactoryCallSite factoryCallSite, RuntimeResolverContext context)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor`2.VisitCallSiteMain(ServiceCallSite callSite, TArgument argument)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitCache(ServiceCallSite callSite, RuntimeResolverContext context, ServiceProviderEngineScope serviceProviderEngine, RuntimeResolverLock lockType)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitScopeCache(ServiceCallSite singletonCallSite, RuntimeResolverContext context)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor`2.VisitCallSite(ServiceCallSite callSite, TArgument argument)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitConstructor(ConstructorCallSite constructorCallSite, RuntimeResolverContext context)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor`2.VisitCallSiteMain(ServiceCallSite callSite, TArgument argument)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitCache(ServiceCallSite callSite, RuntimeResolverContext context, ServiceProviderEngineScope serviceProviderEngine, RuntimeResolverLock lockType)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitScopeCache(ServiceCallSite singletonCallSite, RuntimeResolverContext context)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor`2.VisitCallSite(ServiceCallSite callSite, TArgument argument)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.Resolve(ServiceCallSite callSite, ServiceProviderEngineScope scope)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.DynamicServiceProviderEngine.<>c__DisplayClass1_0.<RealizeService>b__0(ServiceProviderEngineScope scope)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.ServiceProviderEngine.GetService(Type serviceType, ServiceProviderEngineScope serviceProviderEngineScope)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.ServiceProviderEngineScope.GetService(Type serviceType)
   at Microsoft.Extensions.DependencyInjection.ServiceProviderServiceExtensions.GetRequiredService(IServiceProvider provider, Type serviceType)
   at Microsoft.Extensions.DependencyInjection.ServiceProviderServiceExtensions.GetRequiredService[T](IServiceProvider provider)
   at Microsoft.EntityFrameworkCore.DbContext.get_DbContextDependencies()
   at Microsoft.EntityFrameworkCore.DbContext.get_InternalServiceProvider()
   at Microsoft.EntityFrameworkCore.DbContext.Microsoft.EntityFrameworkCore.Infrastructure.IInfrastructure<System.IServiceProvider>.get_Instance()
   at Microsoft.EntityFrameworkCore.Infrastructure.Internal.InfrastructureExtensions.GetService[TService](IInfrastructure`1 accessor)
   at Microsoft.EntityFrameworkCore.Infrastructure.AccessorExtensions.GetService[TService](IInfrastructure`1 accessor)
   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, String namespace)
   at Microsoft.EntityFrameworkCore.Design.OperationExecutor.AddMigrationImpl(String name, String outputDir, String contextType, String namespace)
   at Microsoft.EntityFrameworkCore.Design.OperationExecutor.AddMigration.<>c__DisplayClass0_0.<.ctor>b__0()
   at Microsoft.EntityFrameworkCore.Design.OperationExecutor.OperationBase.<>c__DisplayClass3_0`1.<Execute>b__0()
   at Microsoft.EntityFrameworkCore.Design.OperationExecutor.OperationBase.Execute(Action action)

我是 EF Core 和 .NET 开发的新手，所以希望能得到帮助！

Answer 1

在通过查看堆栈跟踪提到问题可能在第line 44 。 我再次检查了原始代码，就像在我拥有的Models文件夹中一样

[Keyless]
public class Person {
  // some general properties of person 
}

public class Lecturer: Person {
  public string Id { get; set; }

  // some properties that derived from `Person` 
}

public class Student: Person {
  public string Id { get; set; }

  // some properties that derived from `Person` 
}

尝试删除Person类后

public class Lecturer {
  public string Id { get; set; }

  // some properties
}

public class Student {
  public string Id { get; set; }

  // some properties 
}

现在它起作用了。 老实说，我仍然不知道究竟是什么导致了这个问题。 喜欢看到有经验的人可以给出一些解释。