____  _____  ___  ___  ____  ____   ____   ____ ___  ____
          |    \(____ |/___)/ _ \|  _ \|    \ / ___) / ___) _ \|    \
          | | | / ___ |___ | |_| | | | | | | ( (___ ( (__| |_| | | | |
          |_|_|_\_____(___/ \___/|_| |_|_|_|_|\____|_)____)___/|_|_|_|

So, when pointing the XmlSerializer at a document that contains a list of abstract base objects, I came across a fairly standard exception:

System.InvalidOperationException: There was an error generating the XML document. System.InvalidOperationException: The type was not expected. Use the XmlInclude or SoapInclude attribute to specify types that are not known statically.

But, even after I put in the XmlIncludes, OR when using the serializer constructor overload that takes a list of types, I was still getting the error.

Turns out my problem, strangely, was this line:

[XmlRootAttribute(
  "Document", 
  Namespace = "http://www.cuttlefishindustries.com/mycoolnamespace", 
  IsNullable = false)]

I didn't investigate it that much but it appears that if you specify a Namespace, the serializer ignores the additional types you supply (either via XmlInclude or the constructor overload). Once I took out my document's XML root attribute, my serializer worked. Huh. So, there ya go, hopefully this'll save someone some time, sometime.