Difference between FieldLinks and Field in Sharepoint

Question

Hi all:

I'm in the middle of trying to copy a custom content type from one web to another. I've googled around and found some examples that use FieldLinks and Fields. I'm kind of lost as to which one to use, since when I get the FieldLinks from my source web, I get 3 fields; while retrieving from Fields only returned me 2 fields... the custom field is missing. I'm pretty darn sure that I've added the fields at the proper level since I did it via the interface. But when retrieving it using code... the numbers just don't add up.

So besides from that strange problem, I want to know what is the difference between FieldLinks and Fields, and when dealing with them in Content Types (programmatically) which one should I use?

Thanks.

Answer 1

SPFields are fields themselves, while SPFieldLinks are references to the fields. This is a good read that will explain things in detail. In general practice, it is safer to use SPFieldLinks when you are working on the actual content type definition. However, I'll give a quick summary here.

Lists and Webs contain the actual fields with field data. A content type, on the other hand, only holds Field Reference, which simply points at the corresponding field in the list or web. This gets a bit confusing, because content types have both an SPFieldLinkCollection and an SPFieldCollection.

The SPFieldLinkCollection is used in the actual definition of the content type, and is what you would want to use in your situation of copying a content type from one web to another. SPFieldLinks correspond to the actual elements in the XML Schema for a content type.

Comparatively, when you call on a content type's SPFieldCollection and retrieve a Field from it, what is actually happening is that the content type is checking the corresponding field reference, and then looking up in the list/web to get the actual field. Basically, think of the SPFieldCollection in the same way a lookup works: it is worthless without both the lookup value and the lookup source.