using System;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.ComponentModel;
using System.Text;

namespace AspItalia.WebControls
{
  /// <summary>
  /// Descrizione di riepilogo per WebCustomControl2.
  /// </summary>
  [DefaultProperty("Text"), 
    ToolboxData("<{0}:WebCustomControl2 runat=server></{0}:WebCustomControl2>")]
  public class WebCustomControl2 : System.Web.UI.WebControls.TextBox
  {
    public WebCustomControl2()
    {
      
    }
    protected override void OnLoad(EventArgs e)
    {
      base.OnLoad (e);
      StringBuilder mySB = new StringBuilder(180);
      mySB.Append("<script type=\"text/javascript\">");
      mySB.Append("function ValutaLunghezza(elmnt,content)");
      mySB.Append("{if (content.length==elmnt.maxLength){");
      mySB.Append(this.Page.GetPostBackClientEvent(this,string.Empty));
      mySB.Append("}}</script>");
  
      this.Page.RegisterClientScriptBlock("ValutaLunghezza",mySB.ToString());
    }

    protected override void AddAttributesToRender(HtmlTextWriter writer)
    {
      base.AddAttributesToRender (writer);
      writer.AddAttribute("onkeyup","ValutaLunghezza(this,this.value)");
    }

  }
}

if (Request.Form.Get(myControl.ClientID) != null ).....