ToolTip tt;
private void Form1_Load(object sender, EventArgs e)
{
      tt=new ToolTip();
}
private void comboBox1_DrawItem(object sender, DrawItemEventArgs e)
 {
       e.DrawBackground();
       e.DrawFocusRectangle();
       Rectangle r0 = e.Bounds;
       e.Graphics.DrawString(dt.Rows[e.Index]["address"].ToString(), e.Font, new Pen(e.ForeColor).Brush, r0);
       Size sz = TextRenderer.MeasureText(dt.Rows[e.Index]["address"].ToString(), e.Font);
       if (sz.Width > 80)
          tt.Show(dt.Rows[e.Index]["address"].ToString(), comboBox1, r0.X + r0.Width, r0.Y + r0.Height);
       else
           tt.Hide(comboBox1);
}

private void comboBox1_DropDownClosed(object sender, EventArgs e)
{
       tt.Hide(comboBox1);
}