I’m used to post about specific scenarios and their resolution. Here I’m posting a sort of note to myself because I always forgot where is located this list of SharePoint built in fields ID and Content Types ID.
The list of Fields ID : Microsoft.SharePoint.SPBuiltInFieldId
The list of Content Types ID : Microsoft.SharePoint.SPBuiltInContentTypeId
So if you want to get the user who created an item you can use the following code :
string creator = (string)spItem[SPBuiltInFieldId.Author];