让TextArea支持MaxLength的自定义文本框控件

众所周知，TextArea并不支持MaxLength属性，所以我们就用JS来实现MaxLength的功能。
代码如下：

 1  public class TextMultiLine : TextBox
 2    {
 3        private const string SCP_SELECTPRODUCT_ONLY_SCRIPT_ID = "{A19D31C2-A21B-4acd-92DA-19A4237C54A8}";
 4        /**//// <summary>
 5        /// 
 6        /// </summary>
 7        public override TextBoxMode TextMode
 8        {
 9            get
            {
                return TextBoxMode.MultiLine;
            }
        }

        /**//// <summary>
        /// 
        /// </summary>
        /// <param name="writer"></param>
        protected override void AddAttributesToRender(HtmlTextWriter writer)
        {
            base.AddAttributesToRender(writer);
            writer.AddAttribute("onpaste", "return false;");
            writer.AddAttribute("onpcopy", "return false;");
            if (base.MaxLength > 0)
            {
                writer.AddAttribute("onkeydown", "textCounter(this,"+base.MaxLength+");");
                writer.AddAttribute("onkeyup", "textCounter(this,"+base.MaxLength+");");
            }
        }

        /**//// <summary>
        /// 
        /// </summary>
        /// <param name="e"></param>
        protected override void OnPreRender(EventArgs e)
        {
            base.OnPreRender(e);
            RenderJavaScript();
        }
        /**//// <summary>
        /// 
        /// </summary>
        private void RenderJavaScript()
        {
            string SCP_SELECTPRODUCT_ONLY_SCRIPT =
                    "function textCounter(field, maxlimit)\n" +
                    "{\n" +
                        "if (field.value.length > maxlimit) \n" +
                        "{\n" +
                //如果元素区字符数大于最大字符数，按照最大字符数截断； 
                            "field.value = field.value.substring(0, maxlimit); \n" +
                        "}\n" +
                    "}";                    

            if (!Page.ClientScript.IsClientScriptBlockRegistered(SCP_SELECTPRODUCT_ONLY_SCRIPT_ID))
            {
                Page.ClientScript.RegisterClientScriptBlock(Page.GetType(), SCP_SELECTPRODUCT_ONLY_SCRIPT_ID, SCP_SELECTPRODUCT_ONLY_SCRIPT,true);
            }
        }
    }