if win form try this
public void AutoComplete()
{
AutoCompleteStringCollection AutoItem = new AutoCompleteStringCollection();
foreach (DataRow rw in Dt.rows)
{
AutoItem.Add(Rw["columnname"].ToString());
}
texbox.AutoCompleteMode = AutoCompleteMode.Suggest;
texbox.AutoCompleteSource = AutoCompleteSource.CustomSource;
texbox.AutoCompleteCustomSource = AutoItem;
}