EndUpdate is called automatically by any property or method that changes the list of strings. Call BeginUpdate before directly modifying the strings in the list, and EndUpdate after. When implementing properties or methods that change the list in descendants of TWideStrings, call BeginUpdate before the changes are made, and EndUpdate when the changes are complete. 

TWideStrings simply keeps track of when the list of strings is being changed. Some descendants of TWideStrings use this information to perform certain actions, such as telling a control to repaint, when updates are complete. 

BeginUpdate 

UpdateCount