selected = from b in new ResourceEntryTable().ResourceEntries
where (string.IsNullOrEmpty(txtCategory.Value) || b.PartitionKey.Equals(txtCategory.Value))
where (string.IsNullOrEmpty(txtSubcategory.Value) || b.subCategory.Equals(txtSubcategory.Value)) where (string.IsNullOrEmpty(txtCity.Value) || b.city.Equals(txtCity.Value))
select b;

Tough to squeeze into the comment box. But point is to use a || on the where statement to negate the statement if there is no value. Also nice with Azure, no need to pull back a full data set with data services.