C# ComboBoxToolTip 控制的问题 在DropDownStyle 为 DropDown时没问题 设置为DropDownList就运行不了
附上源码：

C# code

public class ComboBoxToolTip : ComboBox    {        private ToolTip itemToolTip;        /// <summary>        /// ComboBox 控件选择项带ToolTip提示的        /// 当 选择项 长度大于本身 ComboBox 控件时 生成ToolTip提示        /// </summary>        public ComboBoxToolTip()        {            this.DrawMode = DrawMode.OwnerDrawFixed;            this.DrawItem += new DrawItemEventHandler(DrawItemInList);            this.DropDownClosed += new EventHandler(DropDownListIsClosed);            this.itemToolTip = new ToolTip();        }        void DropDownListIsClosed(object sender, EventArgs e)        {            this.itemToolTip.Hide(this);        }        private bool IsTruncated(string text, Graphics graphics, Font font, Rectangle destinationBounds)        {            SizeF size = graphics.MeasureString(text, font);            Rectangle offsetToZeroRect = new Rectangle(0, 0, destinationBounds.Width, destinationBounds.Height);            return !offsetToZeroRect.Contains((int)size.Width, (int)size.Height);        }        void DrawItemInList(object sender, DrawItemEventArgs e)        {            e.DrawBackground();           string text = this.Items[e.Index].ToString();            bool truncated;            using (Graphics graphics = e.Graphics)            {                graphics.DrawString(text, e.Font, Brushes.Black, e.Bounds, StringFormat.GenericDefault);                truncated = IsTruncated(text, graphics, e.Font, e.Bounds);            }            if ((e.State == DrawItemState.Selected) && (truncated))            {                this.itemToolTip.Show(text, this, e.Bounds.X + e.Bounds.Width, e.Bounds.Y + e.Bounds.Height);            }            else            {                this.itemToolTip.Hide(this);            }            e.DrawFocusRectangle();        }    }