private void txtSearch_TextChanged(object sender, EventArgs e)
        {
           AutoCompleteStringCollection result = new AutoCompleteStringCollection();

           //do my logic too add the list of items to result
           txtSearch.AutoCompleteCustomSource = result;
        }

private void txtSearch_keypress(object sender, KeyPressEventArgs e)
        {
            if (e.KeyChar ==(char)( Keys.Enter))
            {
                cmdSearch.PerformClick();
                e.Handled = true;

            }

        }

this.txtSearch.KeyPress += new KeyPressEventHandler(this.txtSearch_keypress);