Add-Migration error: Object reference not set to an instance of an object

问题

I'm using code-first concept and just tried adding a new class to the Models folder in hope to create a new table in SQL server database. i've added

Public Overridable Property Diagnosis As DbSet(Of Diagnosis)

in the DBContext.vb.

Then in the package manager console, i entered Add-Migration Diagnosis. But below error comes out. Why is this? Can anyone help me out please?

Object reference not set to an instance of an object.

Full error in Package Manager Console:

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.

I don't know if this is relevant or not, but in my web.config, there is a Default Connection that is already pre-set when i first created my project. When i first did Enable-Migrations i had to specify which connection it was. I've removed that line but it still doesn't work.

回答1:

Previously when i did the add-migration, it didn't work because i simply created a class and added declaration of the respective strings, integer, etc. as variables like this:

Public Property id As String

Now it suddenly popped into my mind that i had to give a more elaborate description on each of the variables such as below:

<Key>
<StringLength(20)>
Public Property id As String

Now it works =')

来源：https://stackoverflow.com/questions/42362172/add-migration-error-object-reference-not-set-to-an-instance-of-an-object