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

The solution for me was that I had made a mistake in my implementation of GetResourceType in my IDataServiceQueryProvider.

When querying a Resource Property that was a ResourceSetReference I was returning the ResourceType of the parent resource, and not the type of the related resource.