WCF clent/Serveur : Aidez moi a trouver l'erreur svp

Private void btn_EnregChamps_Click(object sender, EventArgs e)
        {// champsSelect  est une listebox
           
            
            foreach (object item in ChampsSelect.Items)
            {
              
                string Champ_id = Client.Champ_Recherche((ChampsSelect.Tag as List<Champ>), item.ToString());
               
            }
           
        }

using System;
using System.ServiceModel;
using System.ServiceModel.Channels;
using System.IO;
using System.Configuration;
using System.Diagnostics;
using System.Windows.Forms;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Data.Odbc;
using System.Runtime.Serialization;
using Model;
using Infragistics.Win.UltraWinTree;


namespace eDipWCFLibrary
{
    
    [ServiceContract()]
    public interface IService 
    {
        
        [OperationContract]
        string Champ_Recherche(List<Champ> chps, string Valeur);
        
       
    }
    
 
    public partial class Service : IService 
    {
         public string Champ_Recherche(List<Champ> chps, string Valeur)
        {
            foreach (Champ chp in chps)
            {
                string chpval = string.Format("{0} - [{1}]", chp.NOM, chp.LIBELLE);
                if (chpval == Valeur)
                {
                    return chp.CHAMP_ID;
                }
            }
            return "";
        }
      }
}

(ChampsSelect.Tag)List<Champ>

foreach (object item in ChampsSelect.Items)
            {
              
                
                Champ  toto = ((ChampsSelect.Tag)List<Champ>).Find(delegate (Champ titi) {return item.ToString() == string.Format("{0} - [{1}]", titi.NOM, titi.LIBELLE);});
               string Champ_id = toto.CHAMP_ID;;
            }