Cross-Process Drag and Drop of custom object type in WinForms C#

This question is close to what I'm interested in, but not quite.

I have a .NET WinForms application written in C#. I have a ListView control which displays an array of C# objects. I've hooked it up so that you can drag/drop these listview items to a different form in the same application, and it properly passes the array of objects (type Session) to the drop handler for that other form.

However, I now want to support cross-process drag/drop where I run multiple instances of my application. This appears that it's going to work (e.g. GetDataPresent succeeds), but ultimately throws an exception when I actually try to retrieve the data-- cannot cast object[] to Session[].

if (e.Data.GetDataPresent("Fiddler.Session[]"))
{
   Session[] oDroppedSessions;
   try
   {
      oDroppedSessions = (Session[])e.Data.GetData("Fiddler.Session[]");
   }
   catch (Exception eX)
   {  // reaches here 
   }
}

Anyone know if I must implement ISerializable for my objects in order to make this work? Ordinarily, I'd simply try it, but implementing ISerializable for this class would be quite non-trivial, and I'm worried that there may be weird side-effects of doing so.

UPDATE: Implementing ISerializable doesn't help-- the method is never called. Similarly, adding the Serializable attribute to the class has no impact at all. Any other ideas?