I have list, i want to convert it to autoCompleteStringCollection.. And I don't want use foreach. _textbox.AutoCompleteMode = AutoCompleteMode.Append; _textbox.AutoCompleteSource = AutoCompleteSource.CustomSource; _textbox.AutoCompleteCustomSource = user.GetNameUsers() as AutoCompleteStringCollection; Note user.GetNameUsers() is list. Code doesn't work, it become null. Thank you