AutoCompleteStringCollection AutoComp = null;

txtFName.AutoCompleteMode = AutoCompleteMode.Suggest;
txtFName.AutoCompleteSource = AutoCompleteSource.CustomSource;

AutoComp.Add("AAAA");   
AutoComp.Add("BBBB");   
AutoComp.Add("GGGG");   

txtFName.AutoCompleteCustomSource = AutoComp;