这段代码对我有用
ToolTip toolTip1 = new ToolTip();
void initMethod()
{
    lstList.MouseMove += new MouseEventHandler(lstList_MouseMove);//mousemove handler
    this.lstList.ShowItemToolTips = true;            
    toolTip1.SetToolTip(lstList,"");// init the tooltip
    ...
    ListViewItem value = lstList.Items.Add(name, name, 0);
    ...
    if (lstList.Columns.Contains(lstColErrorCode))
    {                                
        ListViewItem.ListViewSubItem lvs =  value.SubItems.Add(new ListViewItem.ListViewSubItem(value, errorCode.ToString()));
        lvs.Tag = "mydecimal"; // only the decimal subitem will be tooltiped
    }
}
listview 中的 mousemove 事件:
void lstList_MouseMove(object sender, MouseEventArgs e)
{
    ListViewItem item = lstList.GetItemAt(e.X, e.Y);
    ListViewHitTestInfo info = lstList.HitTest(e.X, e.Y);
    if ((item != null) && (info.SubItem != null) && (info.SubItem.Tag!=null) && (info.SubItem.Tag.ToString() == "mydecimal"))
    {
        toolTip1.SetToolTip(lstList,((decimal)info.SubItem.Text).ToString("X"));
    }
    else
    {
        toolTip1.SetToolTip(lstList, "");
    }
}