Question

I have a List<State> of State entity objects, and each State object has collections of other objects, such as Licenses, Taxes, Bonds, etc

There’s also a collection for ExpiredObjects, which is a list of any object which is expired and needs to be renewed. For some reason, this property is giving me a NullReferenceException when I try and access it for one specific state (Nevada), but I can’t for the life of me figure out what is actually null since I don’t see any null values anywhere.

Here’s my code that throws the exception. It loops through all the states, and adds all the ExpiredObjects to a view-specific collection which gets displayed. My test code is still included

    private List<ExpiredObject> LoadAllExpiredObjects()
    {
        var list = new List<ExpiredObject>();
        foreach (var state in States)
        {
            // This tells me the state is not null, and neither is state.ExpiredObjects
            Debug.WriteLine(string.Format("{0}|{1}|{2}", 
                state.Name, state == null, state.ExpiredObjects == null));

            try
            {
                var x = state.ExpiredObjects;
                Debug.WriteLine(x == null);

                // Exception occurs on the "for" line on the Nevada state only
                // Contents in for loop do not execute
                foreach (var item in x)
                {
                    Debug.WriteLine(string.Format("{0}", item));
                    list.Add(item);
                }

                // This used to be the only line of code before I started testing
                // It fails on Nevada
                //list.AddRange(state.ExpiredObjects);
            }
            catch (Exception ex)
            {
                Debug.WriteLine(ex.Message);
                Debug.WriteLine(ex.StackTrace);
            }
        }
        return list;
    }

The stack trace of the error is this:

A first chance exception of type 'System.NullReferenceException' occurred in System.Data.Entity.dll
Object reference not set to an instance of an object.
   at System.Data.EntityKey.AddHashValue(Int32 hashCode, Object keyValue)
   at System.Data.EntityKey.GetHashCode()
   at System.Collections.Generic.GenericEqualityComparer`1.GetHashCode(T obj)
   at System.Collections.Generic.Dictionary`2.FindEntry(TKey key)
   at System.Collections.Generic.Dictionary`2.TryGetValue(TKey key, TValue& value)
   at System.Data.Objects.ObjectStateManager.TryGetEntityEntry(EntityKey key, EntityEntry& entry)
   at System.Data.Common.Internal.Materialization.Shaper.HandleEntityAppendOnly[TEntity](Func`2 constructEntityDelegate, EntityKey entityKey, EntitySet entitySet)
   at lambda_method(Closure , Shaper )
   at System.Data.Common.Internal.Materialization.Coordinator`1.ReadNextElement(Shaper shaper)
   at System.Data.Common.Internal.Materialization.Shaper`1.SimpleEnumerator.MoveNext()
   at System.Data.Objects.DataClasses.RelatedEnd.Merge[TEntity](IEnumerable`1 collection, MergeOption mergeOption, Boolean setIsLoaded)
   at System.Data.Objects.DataClasses.EntityCollection`1.Load(List`1 collection, MergeOption mergeOption)
   at System.Data.Objects.DataClasses.EntityCollection`1.Load(MergeOption mergeOption)
   at System.Data.Objects.DataClasses.RelatedEnd.Load()
   at System.Data.Objects.DataClasses.RelatedEnd.DeferredLoad()
   at System.Data.Objects.DataClasses.EntityCollection`1.GetEnumerator()
   at MyNamespace.StatesViewModel.LoadAllExpiredObjects() in C:\Users\me\Desktop\MySolution\StatesViewModel.cs:line 217

I also get the exact same error when I select Nevada and it tries to bind a DataGrid to the ExpiredObjects collection (if I comment out that binding, the code works fine)

Does anyone know what could be causing this error?

Answer 1

If it’s only for Nevada then it must be a data problem, do a thorough check in the db.

And to summarize, the core problem was:

• It was db-first.
• … was returning a null value for one of the fields that was marked in EF as not null