CategoryDropdownlist.Items.Insert(0,new ListItem("NessunFiltro","-1"));

using System;
using System.Web;
using System.Text;
using System.Web.UI;
using System.Web.Caching;
using System.Web.UI.WebControls;
using System.ComponentModel;
[assembly: TagPrefixAttribute("DropList","Drop")]
namespace DropList
{
 /// <summary>
 /// Descrizione di riepilogo per Drop.
 /// </summary>
 /// 
 [ToolboxData("<{0}:Drop runat=server></{0}:Drop>")]
 public class Drop: System.Web.UI.WebControls.DropDownList
 {
  public  Drop()
  {
   this.PreRender += new EventHandler(Drop_PreRender);
  }
  protected  void Drop_PreRender (object sender, EventArgs e)
  {
   if(NeutralText.Length > 0 & NeutralValue.Length > 0)
   {
    this.Items.Insert(0,new ListItem(NeutralText,NeutralValue));
   }
  }
  [Browsable(true)]
  public string NeutralText
  {
   get
   {
    object obj1 = this.ViewState["NeutralText"];
    if (obj1 != null)
    {
     return ((string) obj1);
    }
    return string.Empty;
   }
   set {ViewState["NeutralText"] = value;}
  }
  [Browsable(true)]
  public string NeutralValue
  {
   get 
   {
    object obj1 = this.ViewState["NeutralValue"];
    if (obj1 != null)
    {
     return ((string) obj1);
    }
    return string.Empty;
   }
   set {ViewState["NeutralValue"] = value;}
  }
 
 }
}