自动适应输入内容高度的TextBox控件

#region 附 AutoTextBox 控件源码
using System;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.ComponentModel;

namespace WebExcel.UI.WebControls
{
     ///   <summary>
     ///  Summary description for AutoLengthTextBox.
     ///   </summary>
    [DefaultProperty("Text"), 
        ToolboxData("<{0}:AutoTextArea runat=server></{0}:AutoTextArea>")]
     public  class AutoTextArea : System.Web.UI.WebControls.TextBox
    {
        [DefaultValue(200)]
         public  int MaxHeight
        {
             get
            {
                 object obj = ViewState["MaxHeight"];
                 return obj ==  null ? 200 : ( int)obj;
            }
             set
            {
                ViewState["MaxHeight"] = value;
            }
        }

        [DefaultValue(60)]
         public  int MinHeight
        {
             get
            {
                 object obj = ViewState["MinHeight"];
                 return obj ==  null ? 60 : ( int)obj;
            }
             set
            {
                ViewState["MinHeight"] = value;
            }
        }

         protected  override  void OnPreRender(EventArgs e)
        {
             this.Attributes["minHeight"] =  this.MinHeight.ToString();
             if (  this.Height == Unit.Empty )
            {
                 this.Height =  this.MinHeight;
            }
             else
            {
                 this.Height = ( int)Math.Max( this.MinHeight,  this.Height.Value);
            }
             base.OnPreRender (e);
        }

         ///   <summary>  
         ///  Render this control to the output parameter specified.
         ///   </summary>
         ///   <param name="output">  The HTML writer to write out to  </param>
         protected  override  void Render(HtmlTextWriter output)
        {
             string strCode;
             if (  this.MaxHeight <= 0 )
            {
                strCode = "this.style.height=Math.max(this.minHeight,this.scrollHeight)+(this.offsetHeight-this.clientHeight)";
            }
             else
            {
                strCode = "this.style.height=(this.scrollHeight>200)?200:Math.max(this.minHeight,this.scrollHeight)+(this.offsetHeight-this.clientHeight)";
            }
             base.Attributes["onpropertychange"] = strCode;
             //  base.Attributes["onfocus"] = "this.height=this.height";
             if (  base.Rows == 0 )
            {
                 base.Rows = 1;
            }
             base.TextMode = TextBoxMode.MultiLine;
             base.Render(output);
        }
    }
}
#endregion