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();
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…