实体框架SaveChanges（）NullReferenceException，DisplayNameAttribute

Question

I'm stuck and looking for any ideas.

I have one entity that fails on save changes (all other entities work). I have stripped away my code and attributes to reproduce with nothing extra. I have checked that I can save changes accessing SQL Server directly. I have refreshed the EF model, by dropping the entity and re-adding it. This stripped down code involves no DisplayNames.

EF 6.1

Here is the failing test code:

    [TestMethod()]
    public void SaveEntity_ChangeData_ExpectNoErrors()
    {
        CaseEntities caseEntities = new CaseEntities();
        // Get an existing Entity (this line works)
        var entity = caseEntities.RecordsDistributionPreferences.Find("0001");
        // Change a field
        entity.ModifiedBy = "duncan";

        // get row version before save
        byte[] rowVersionBefore = entity.RowVersion;

        // Save Changes (this next line throws null reference exeption)
        caseEntities.SaveChanges();

        // get row version after save
        byte[] rowVersionAfter = entity.RowVersion;

        // Check that row version are different
        Assert.AreNotEqual(BitConverter.ToInt64(rowVersionBefore.Reverse().ToArray(), 0), BitConverter.ToInt64(rowVersionAfter.Reverse().ToArray(), 0));
    }

Exeception and StackTrace:

NullReferenceException {"Object reference not set to an instance of an object."}
at System.ComponentModel.DisplayNameAttribute.GetHashCode()
at System.Collections.Generic.ObjectEqualityComparer`1.GetHashCode(T obj)
at System.Collections.Generic.HashSet`1.InternalGetHashCode(T item)
at System.Collections.Generic.HashSet`1.AddIfNotPresent(T value)
at System.Collections.Generic.HashSet`1.UnionWith(IEnumerable`1 other)
at System.Collections.Generic.HashSet`1..ctor(IEnumerable`1 collection, IEqualityComparer`1 comparer)
at System.Collections.Generic.HashSet`1..ctor(IEnumerable`1 collection)
at System.Data.Entity.ModelConfiguration.Utilities.AttributeProvider.GetAttributes(Type type)
at System.Data.Entity.ModelConfiguration.Utilities.AttributeProvider.<>c__DisplayClass4.<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.Internal.Validation.EntityValidatorBuilder.BuildPropertyValidator(PropertyInfo clrProperty)
at System.Data.Entity.Internal.Validation.EntityValidatorBuilder.BuildValidatorsForProperties(IEnumerable`1 clrProperties, IEnumerable`1 edmProperties, IEnumerable`1 navigationProperties)
at System.Data.Entity.Internal.Validation.EntityValidatorBuilder.BuildTypeValidator[T](Type clrType, IEnumerable`1 edmProperties, IEnumerable`1 navigationProperties, Func`3 validatorFactoryFunc)
at System.Data.Entity.Internal.Validation.EntityValidatorBuilder.BuildEntityValidator(InternalEntityEntry entityEntry)
at System.Data.Entity.Internal.Validation.ValidationProvider.GetEntityValidator(InternalEntityEntry entityEntry)
at System.Data.Entity.Internal.InternalEntityEntry.GetValidationResult(IDictionary`2 items)
at System.Data.Entity.DbContext.ValidateEntity(DbEntityEntry entityEntry, IDictionary`2 items)
at System.Data.Entity.DbContext.GetValidationErrors()
at System.Data.Entity.Internal.InternalContext.SaveChanges()
at System.Data.Entity.Internal.LazyInternalContext.SaveChanges()
at System.Data.Entity.DbContext.SaveChanges()
at SCC.Case.Entities.Test.RecordsDistributionPreferenceTest.SaveEntity_ChangeData_ExpectNoErrors() in c:\Dev\SCC\Case\Dev\src\SCC.Case.Entities.Test\RecordsDistributionPreferenceTest.cs:line 40

Answer 1

Found it!

The problem was with a different entity that the saving entity had a foreign key to. This other entity was simply missing a localized resource item for the DisplayName.

I found the issue my stripping all the foreign keys for the offending DB table, then refreshing the DataModel (from DB). Error was gone.  So I added each foreign key back, refresh, tested until I got the error. Then I took a look at the DisplayNames for the foreign key entity and found one missing. 

Answer 2

i suggest you to put Try Catch and watch inner message, there you will see what's happen and what variable gets null.

try{
        CaseEntities caseEntities = new CaseEntities();
        // Get an existing Entity (this line works)
        var entity = caseEntities.RecordsDistributionPreferences.Find("0001");
        // Change a field
        entity.ModifiedBy = "duncan";

        // get row version before save
        byte[] rowVersionBefore = entity.RowVersion;

        // Save Changes (this next line throws null reference exeption)
        caseEntities.SaveChanges();

        // get row version after save
        byte[] rowVersionAfter = entity.RowVersion;

        // Check that row version are different
        Assert.AreNotEqual(BitConverter.ToInt64(rowVersionBefore.Reverse().ToArray(), 0), BitConverter.ToInt64(rowVersionAfter.Reverse().ToArray(), 0));
    }
catch (NullReferenceException ex){

}

put a breakPoint in the Catch line, and watch inner Message