AGSFeatureSet DisplayFieldName property is always null?

[MattCooper]

Am I assuming incorrectly that the AGSFeatureSet property "displayFieldName" should return the primary display field as set in the map service's source MXD?  I have it set in the MXD, can see it in the rest services directory but this property is always null.  Am I doing something wrong or attempting to use this field incorrectly?

Thanks,
Matt

[DiveshGoyal]

How are you getting the feature set?
Is it in response to a query on the server?

[MattCooper]

Yes, the featureset is returned from a query task on a feature service.

Thanks,
Matt