Jan 12, 2016 01:52 PM|Rion Williams|LINK
You could perform the conversion within your lambda statements if I understand you correctly :
DBDocuments.Where(x => DBDocumentAdditional.Find(a => a.DocumentId == Convert.ToInt64(x.DocumentId)))
.ForEach(t => t.Additional = DBDocumentAdditional );
You may have throw an AsEnumerable() or a ToList() call prior to your Find method to ensure that LINQ knows how to resolve that.