Custom wcf data provider and debugging a relationship error

Question

I'm implementing a custom data provider, I have gotten it to the point that it returns data and can be filtered, but am having some trouble getting relationships to work.

When querying the metadata the relationships look correct, and when querying a table the related property links appear, but when attempting to access a ResourceReference property I get the following exception:

    Object reference not set to an instance of an object.
        System.NullReferenceException
        stacktrace at System.Data.Services.Providers.DataServiceProviderWrapper.GetResourceAssociationSet(ResourceSetWrapper resourceSet, ResourceType resourceType, ResourceProperty resourceProperty)
   at System.Data.Services.Providers.DataServiceProviderWrapper.GetContainer(ResourceSetWrapper sourceContainer, ResourceType sourceResourceType, ResourceProperty navigationProperty)
   at System.Data.Services.Providers.DataServiceProviderWrapper.GetResourceProperties(ResourceSetWrapper resourceSet, ResourceType resourceType)
   at System.Data.Services.Serializers.SyndicationSerializer.WriteObjectProperties(IExpandedResult expanded, Object customObject, ResourceType resourceType, Uri absoluteUri, String relativeUri, SyndicationItem item, DictionaryContent content, EpmSourcePathSegment currentSourceRoot)
   at System.Data.Services.Serializers.SyndicationSerializer.WriteEntryElement(IExpandedResult expanded, Object element, ResourceType expectedType, Uri absoluteUri, String relativeUri, SyndicationItem target)
   at System.Data.Services.Serializers.SyndicationSerializer.WriteTopLevelElement(IExpandedResult expanded, Object element)
   at System.Data.Services.Serializers.Serializer.WriteRequest(IEnumerator queryResults, Boolean hasMoved)
   at System.Data.Services.ResponseBodyWriter.Write(Stream stream)

Here's a sample of how I create the relationships:

var sourceReference = new ResourceProperty(
                relatedType.ResourceTypeName,
                ResourcePropertyKind.ResourceReference,
                relatedType.ResourceType);
            sourceReference.CanReflectOnInstanceTypeProperty = false;
            compoundType.ResourceType.AddProperty(sourceReference);

var destinationReference = new ResourceProperty(
                compoundType.ResourceSetName,
                ResourcePropertyKind.ResourceSetReference,
                compoundType.ResourceType);
            destinationReference.CanReflectOnInstanceTypeProperty = false;
            source.ResourceType.AddProperty(destinationReference);

var sourceAssociation = new ResourceAssociationSet(
                        "source",
                        new ResourceAssociationSetEnd(compoundType.ResourceSet, compoundType.ResourceType, sourceReference),
                        new ResourceAssociationSetEnd(relatedType.ResourceSet, relatedType.ResourceType, null));

var destinationAssociation = new ResourceAssociationSet(
                             "destination",
                             new ResourceAssociationSetEnd(relatedType.ResourceSet, relatedType.ResourceType, destinationReference),
                             new ResourceAssociationSetEnd(compoundType.ResourceSet, compoundType.ResourceType, null));

From looking at the sample code on the OData website I thought I'd done it all correctly, and cannot determine my error. Any ideas? or tips on debugging a custom WCF Data service?

Update: Here's what happens just before the null exception.

Have a resource set Collars with a relationship to Projects so I do this query: blah.svc/Collars(1)/Project

My override of GetResourceAssociationSet in my IDataServiceMetadataProvider gets called with the parameters ResourceSet = Collars, ResourceType = Collar, Property = Project and I return the association set specified above. GetResourceAssociationSet is then called again with ResourceSet = Projects, ResourceType = Collar, Property = Project and I return the same association set.

Then in System.Data.Services.Providers.GetResourceAssociationSetEnd the variables passed in are resourceSet = Projects, resourceType = Collar, resourceProperty = Project, this function returns null.

Which makes thisEnd in System.Data.Services.Providers.DataServiceProviderWrapper.GetResourceAssociationSet equal to null. Then GetRelatedResourceAssociationSetEnd is called with the same variables and also returns null.

So it then crashes with the call

ResourceSetWrapper relatedSet = this.ValidateResourceSet(relatedEnd.ResourceSet); 

because relatedEnd is null.

Answer 1

Well, in my debugging I noticed the last time GetResourceAssociationSet was called before the error occurred was for a case where the resourceSet and resourceType parameters had different values (in the case of a derived type).

So, I looked and found this

WCF data services (OData), query with inheritance limitation?

...and lo and behold this uninformative null reference exception (at least in my case) is caused by that same issue. I removed the offending property (and then moved it to the base resource set, even though it doesn't exist there in practice), and the issue was resolved.

Interesting side note (in case this helps the original poster): ResourceType.Properties includes properties from Base Types. Had to change some code to use PropertiesDeclaredOnThisType instead...