Casting an Item Collection from a listbox to a generic list

Try this (using System.Linq):

OfType() is an extension method, so you need to use System.Linq

List<string> selectedFields = new List<string>();
selectedFields.AddRange(chkDFMFieldList.CheckedItems.OfType<string>());

Or just do it in one line:

List<string> selectedFields = chkDFMFieldList.CheckedItems.OfType<string>().ToList();