SharePoint Lessons Learned

The Workflow History list content type, which can be found in the hidden Workflow History list, is unusual in that it has no parent content type. Normally, in a loop such as

foreach(SPContentType listType in list.ContentTypes)

listType.Parent is a valid content type which is a site content type, but it is null for the Workflow History list content type on the Workflow History list.

Note: The Workflow History list being hidden, one cannot navigate to it via View All Site Content. But if one knows its GUID, one can navigate to it via ~/_layouts/listedit.aspx?List=%7B123456%7D, where 123456 stands for the actual GUID.