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我使用的是代码优先的概念，并试图向Models文件夹中添加新类，以期在SQL Server数据库中创建新表。我添加了添加迁移错误：未将对象引用设置为对象的实例
Public Overridable Property Diagnosis As DbSet(Of Diagnosis)
在DBContext.vb中。
然后在包管理器控制台中，我输入了Add-Migration Diagnosis。但是，下面的错误出来了。为什么是这样？任何人都可以帮我吗？
Object reference not set to an instance of an object.
完全错误的软件包管理器控制台：
System.NullReferenceException: Object reference not set to an instance of an object.
at System.Web.UI.ParseChildrenAttribute.GetHashCode()
at System.Collections.Generic.ObjectEqualityComparer`1.GetHashCode(T obj)
at System.Linq.Set`1.InternalGetHashCode(TElement value)
at System.Linq.Set`1.Find(TElement value, Boolean add)
at System.Linq.Enumerable.<ExceptIterator>d__72`1.MoveNext()
at System.Collections.Generic.List`1..ctor(IEnumerable`1 collection)
at System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)
at System.Data.Entity.ModelConfiguration.Utilities.AttributeProvider.<GetAttributes>b__3(PropertyInfo pi)
at System.Collections.Concurrent.ConcurrentDictionary`2.GetOrAdd(TKey key, Func`2 valueFactory)
at System.Data.Entity.ModelConfiguration.Utilities.AttributeProvider.GetAttributes(PropertyInfo propertyInfo)
at System.Data.Entity.ModelConfiguration.Conventions.PropertyAttributeConfigurationConvention`1.<.ctor>b__0(ConventionTypeConfiguration ec)
at System.Data.Entity.ModelConfiguration.Conventions.TypeConvention.ApplyCore(Type memberInfo, ModelConfiguration modelConfiguration)
at System.Data.Entity.ModelConfiguration.Conventions.TypeConventionBase.Apply(Type memberInfo, ModelConfiguration modelConfiguration)
at System.Data.Entity.ModelConfiguration.Configuration.ConventionsConfiguration.ApplyModelConfiguration(Type type, ModelConfiguration modelConfiguration)
at System.Data.Entity.ModelConfiguration.Conventions.Convention.ApplyModelConfiguration(Type type, ModelConfiguration modelConfiguration)
at System.Data.Entity.ModelConfiguration.Configuration.ConventionsConfiguration.ApplyModelConfiguration(Type type, ModelConfiguration modelConfiguration)
at System.Data.Entity.ModelConfiguration.Mappers.TypeMapper.MapComplexType(Type type, Boolean discoverNested)
at System.Data.Entity.ModelConfiguration.Mappers.PropertyMapper.MapPrimitiveOrComplexOrEnumProperty(PropertyInfo propertyInfo, Func`1 structuralTypeConfiguration, Boolean discoverComplexTypes)
at System.Data.Entity.ModelConfiguration.Mappers.PropertyMapper.MapIfNotNavigationProperty(PropertyInfo propertyInfo, EntityType entityType, Func`1 entityTypeConfiguration)
at System.Data.Entity.ModelConfiguration.Mappers.TypeMapper.<>c__DisplayClass14.<MapEntityType>b__e(PropertyMapper m, PropertyInfo p)
at System.Data.Entity.ModelConfiguration.Mappers.TypeMapper.MapStructuralElements[TStructuralTypeConfiguration](Type type, ICollection`1 annotations, Action`2 propertyMappingAction, Func`1 structuralTypeConfiguration)
at System.Data.Entity.ModelConfiguration.Mappers.TypeMapper.MapEntityType(Type type)
at System.Data.Entity.ModelConfiguration.Mappers.NavigationPropertyMapper.Map(PropertyInfo propertyInfo, EntityType entityType, Func`1 entityTypeConfiguration)
at System.Data.Entity.ModelConfiguration.Mappers.TypeMapper.MapEntityType(Type type)
at System.Data.Entity.DbModelBuilder.MapTypes(EdmModel model)
at System.Data.Entity.DbModelBuilder.Build(DbProviderManifest providerManifest, DbProviderInfo providerInfo)
at System.Data.Entity.DbModelBuilder.Build(DbConnection providerConnection)
at System.Data.Entity.Internal.LazyInternalContext.CreateModel(LazyInternalContext internalContext)
at System.Data.Entity.Internal.RetryLazy`2.GetValue(TInput input)
at System.Data.Entity.Internal.LazyInternalContext.InitializeContext()
at System.Data.Entity.Internal.LazyInternalContext.get_ModelBeingInitialized()
at System.Data.Entity.Infrastructure.EdmxWriter.WriteEdmx(DbContext context, XmlWriter writer)
at System.Data.Entity.Utilities.DbContextExtensions.<>c__DisplayClass1.<GetModel>b__0(XmlWriter w)
at System.Data.Entity.Utilities.DbContextExtensions.GetModel(Action`1 writeXml)
at System.Data.Entity.Utilities.DbContextExtensions.GetModel(DbContext context)
at System.Data.Entity.Migrations.DbMigrator..ctor(DbMigrationsConfiguration configuration, DbContext usersContext, DatabaseExistenceState existenceState, Boolean calledByCreateDatabase)
at System.Data.Entity.Migrations.DbMigrator..ctor(DbMigrationsConfiguration configuration)
at System.Data.Entity.Migrations.Design.MigrationScaffolder..ctor(DbMigrationsConfiguration migrationsConfiguration)
at System.Data.Entity.Migrations.Design.ToolingFacade.ScaffoldRunner.Run()
at System.AppDomain.DoCallBack(CrossAppDomainDelegate callBackDelegate)
at System.AppDomain.DoCallBack(CrossAppDomainDelegate callBackDelegate)
at System.Data.Entity.Migrations.Design.ToolingFacade.Run(BaseRunner runner)
at System.Data.Entity.Migrations.Design.ToolingFacade.Scaffold(String migrationName, String language, String rootNamespace, Boolean ignoreChanges)
at System.Data.Entity.Migrations.AddMigrationCommand.Execute(String name, Boolean force, Boolean ignoreChanges)
at System.Data.Entity.Migrations.AddMigrationCommand.<>c__DisplayClass2.<.ctor>b__0()
at System.Data.Entity.Migrations.MigrationsDomainCommand.Execute(Action command)
Object reference not set to an instance of an object.
我不知道这是否是相关与否，但在我的web.config中，有一个默认的连接是已经预当我第一次创建我的项目时设置。当我第一次做Enable-Migrations时，我不得不指定它是哪个连接。我已删除该行，但仍然无效。
你已经能够成功地给你建在尝试命令之前解决方案？另外..请确保您的包管理器控制台中的命令针对安装了实体Gramework的正确Project。 – Derek
@Derek 0成功，0失败，1最新，0跳过。在构建解决方案后，我已经尝试过在包管理器控制台中进行添加迁移，但仍然存在相同的错误通过“违反正确的项目”，在默认项目下拉列表中选择我的项目名称？ – Nurul
我不知道你的解决方案是如何分解的，但如果你有多个项目，你需要确保包管理器控制台的目标是安装了实体框架的项目。 – Derek