//-----------------------------------------------------------
        /// <summary>
        /// DropDown時発生します。
        /// </summary>
        //-----------------------------------------------------------
        protected override void OnDropDown(EventArgs e)
        {
            this.BackColor = backColor;
            AdjustDropDownWidth();
            base.OnDropDown(e);
        }

        //-----------------------------------------------------------
        /// <summary>
        /// AdjustDropDownWidth
        /// </summary>
        //-----------------------------------------------------------
        private void AdjustDropDownWidth()
        {
            Graphics g = this.CreateGraphics();
            Font font = this.Font;
            string str = "";
            int width = this.Width;
            int vScrollWidth = 0;
            int newWidth = 0;

            try
            {
                vScrollWidth = (this.Items.Count > this.MaxDropDownItems) ?
                    SystemInformation.VerticalScrollBarWidth : 0;

                foreach (object s in this.Items)
                {
                    if (s != null)
                    {
                        if (this.DataSource == null)
                            str = s.ToString();
                        else if (s is DataRow)
                            str = ((DataRow)s)[this.DisplayMember].ToString();
                        else if (s is DataRowView)
                            str = ((DataRowView)s)[this.DisplayMember].ToString();
                        else
                            str = " ";

                        newWidth = (int)g.MeasureString(str.ToString().Trim(), font).Width + vScrollWidth;
                        if (width < newWidth)
                            width = newWidth;
                    }
                }
                this.DropDownWidth = width;
            }
            catch (Exception e)
            {
                throw e;
            }
            finally
            {
                if (g != null)
                    g.Dispose();
            }
        }