c# テキストボックスに全角文字バイト数制限を制御したい

using System.ComponentModel;

namespace Jeanne.Windows.Forms {

    public class ExTextBox : System.Windows.Forms.TextBox {

      #region　コンストラクタ　

        private System.ComponentModel.Container components = null;

        public ExTextBox() {
            this.components = new System.ComponentModel.Container();
            this.MaxByteLength = int.MaxValue;
        }

      #endregion

      #region　MaxByteLength プロパティ　

        private int _MaxByteLength;

        [Category("動作")]
        [DefaultValue(int.MaxValue)]
        [Description("エディット コントロールに入力できる最大文字バイト数を指定します。")]
        [RefreshProperties(RefreshProperties.Repaint)]
        public int MaxByteLength {
            get {
                return this._MaxByteLength;
            } set {
                this._MaxByteLength = value;

                int iMaxLength = (int)System.Math.Ceiling(value / 2);

                if (iMaxLength > this.MaxLength) {
                    this.MaxLength = iMaxLength;
                }
            }
        }

      #endregion

      #region　MaxLength プロパティ (override)　

        [RefreshProperties(RefreshProperties.Repaint)]
        public override int MaxLength {
            get {
                return base.MaxLength;
            } set {
                base.MaxLength = value;

                int iMaxByteLength = value * 2;

                if (iMaxByteLength > this.MaxByteLength) {
                    this.MaxByteLength = iMaxByteLength;
                }
            }
        }

      #endregion

      #region　Dispose メソッド (override)　

        protected override void Dispose(bool disposing) {
            if (disposing) {
                if (this.components != null) {
                    components.Dispose();
                }
            }

            base.Dispose(disposing);
        }

      #endregion

      #region　IsInputChar メソッド (override)　

        protected override bool IsInputChar(char charCode) {
            base.IsInputChar(charCode);

            return false;
        }

      #endregion

      #region　ProcessDialogChar メソッド　

        protected override bool ProcessDialogChar(char charCode) {
            base.ProcessDialogChar(charCode);

            System.Text.Encoding sjisEncoding = System.Text.Encoding.GetEncoding("Shift_JIS");
            int inputByteCount = sjisEncoding.GetByteCount(charCode.ToString());
            int textByteCount  = sjisEncoding.GetByteCount(this.Text);

            if (char.IsControl(charCode)) {
                return false;
            }

            if ((textByteCount + inputByteCount) > this.MaxByteLength) {
                return true;
            }

            return false;
        }

      #endregion

    }

}