MICROLANDDataSet is a class, not an object. You have declared a variable named 'dataset' and assigned an object of that type to it, so you should be using 'dataset.Orders' and 'dataset.Inventory'. What you're doing is like taking dog for a walk rather than taking A dog for a walk. One is a type of thing and the other is a thing. You might know what a dog is but you can't take that knowledge for a walk; you have to actually have a dog.