public partial class TextBoxEx : TextBox
    {
        public TextBoxEx()
        {
            InitializeComponent();
        }

        属性#region 属性

        private int m_MaxByteLength = 0;
        [Description("获取或设置用户可在文本框控件中键入或粘贴的最大字节数。0 为允许无限长度。")]
        /**//// <summary>
        /// 获取或设置用户可在文本框控件中键入或粘贴的最大字节数。0 为允许无限长度。
        /// </summary>
        public int MaxByteLength
        {
            get { return m_MaxByteLength; }
            set { m_MaxByteLength = value; }
        }
}

        protected override void WndProc(ref Message m)
        {
            //如果该属性没有设置，则允许输入
            if (m_MaxByteLength == 0)
            {
                base.WndProc(ref m);
                return;
            }

switch (m.Msg)
            {
                case WM_CHAR:
                    int i = (int)m.WParam;
                    bool isBack = (i == (int)Keys.Back);
                    bool isCtr = (i == 24) || (i == 22) || (i == 26) || (i == 3);

if (isBack || isCtr)
                    {
                        //控制键不作处理
                    }
                    else
                    {
                        char c = (char)i;
                        if (CheckByteLengthFlow(c.ToString()))
                        {
                            break;
                        }
                    }
                    base.WndProc(ref m);
                    break;
                case WM_PASTE:
                    IDataObject iData = Clipboard.GetDataObject();//取剪贴板对象
                    if (iData.GetDataPresent(DataFormats.Text)) //判断是否是Text
                    {
                        string text = (string)iData.GetData(DataFormats.Text);//取数据
                        if (CheckByteLengthFlow(text))
                        {
                            m.Result = (IntPtr)0;//不可以粘贴
                            break;
                        }
                    }
                    base.WndProc(ref m);
                    break;
                default:
                    base.WndProc(ref m);
                    break;
            }
        }

/// <summary>
        /// 判断即将输入的文本长度是否溢出
        /// </summary>
        /// <param name="text">文本</param>
        /// <returns>是否溢出</returns>
        private bool CheckByteLengthFlow(string text)
        {
            int len = GetByteLength(text);    //输入的字符的长度
            int tlen = GetByteLength(this.Text);  //文本框原有文本的长度
            int slen = GetByteLength(this.SelectedText);    //文本框选中文本的长度
            return (m_MaxByteLength < (tlen - slen + len));
        }

/// <summary>
        /// 计算文本字节长度，区分多字节字符
        /// </summary>
        /// <param name="text">文本</param>
        /// <returns>文本字节长度</returns>
        private int GetByteLength(string text)
        {
            return System.Text.Encoding.Default.GetBytes(text).Length;
        }

public string RealText
        {
            get
            {
                if (m_MaxByteLength == 0)
                {
                    return this.Text;
                }

                if (m_MaxByteLength >= GetByteLength(this.Text))
                {
                    return this.Text;
                }

                string text = this.Text;

                if (string.IsNullOrEmpty(text))
                {
                    return text;
                }

                char[] c = text.ToCharArray();
                StringBuilder sb = new StringBuilder();
                int count = 0;

                for (int i = 0; i < c.Length; i++)
                {
                    count += GetByteLength(c[i].ToString());
                    if (m_MaxByteLength >= count)
                    {
                        sb.Append(c[i]);
                    }
                }

                return sb.ToString();
            }
        }