A little note on SPBuiltinContentTypeId usage

Recently, I got a question from a developer who was wondering why the following statement caused a System.NullReferenceException:

SPContentType itemContentType = myCustomList.ContentTypes[SPBuiltInContentTypeId.Item];

The SPBuiltinContentTypeId Class contains static readonly fields representing all out-of-the-box ContentTypes in SharePoint Foundation (or WSS). The thing here is that when a Content Type is assigned to a List by using “Add from existing site content types” there actually is a new ContentType being generated under the hood. This newly generated ContentType actually inherits from the ContentType you selected to assign so it (initially) behaves exactly the same as it’s parent but it will have it’s own unique ID.

The SPBuiltInContentTypeId Class’ fields represent only the SPContentTypeIds of the ContentTypes typically instantiated at SiteCollection scope. Referencing a List-assigned version of the “same” ContentType won’t work using the overload of SPContentTypeCollection’s (SPList.ContentTypes) indexer taking a SPBuiltInContentTypeId because the ContentType has it’s own unique ID. In order to reference the List-scoped ContentType you should use one of the other two overloads of this class’ indexer instead. The first is taking an Int32 representing the index of a ContentType whithin the collection. However, for a more predictable result I’d recommend using the overload accepting a string representing a ContentType’s name:

SPContentType itemContentType = myCustomList.ContentTypes["Item"];