how to use a static Object without mofidifying my Object

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C#3.0

C#

Hi Guy, i Need your help please.
I receive a Data of Type List<classdata> from a WebService. and insert it in a DataTable this List is Static. see the Code:

        private static List<ClassData> DataList;  
       public static void DataListUpdate()
        {
                DataList = webservice.GetCommissioningClientView().ToList();
        }



        public static DataTable GetDataListTable()
        {
            try
            {

                DataTable DTable = new DataTable();
                DTable.Columns.Add("DataDesc");
                DTable.Columns.Add("DataID");

var SortDataByPriority = from table in DataList                                         
                         orderby table.highestPriority descending
                         select table;

              
                foreach (var DataObject in SortDataByPriority )
                {
                    ClassData orderData = DataObject as ClassData;
                    
                    foreach (var element in DataObject.GetDetails)
                    {
                        row = DTable.NewRow();
                        row[0] = element.MatDesc;
                        row[1] = element.EntryMaterialID;


                           
   if (DTable.Columns.Contains(DataObject.DataID)) 
 {

   if((DTable.Rows[foundRowIndex][DataObject.DataID] as DataElement) != null);
 {
    //### Hieer is my Problem ####  
      
    (DTable.Rows[foundRowIndex][DataObject.DataID] as DataElement).ElementQuantity += Element.EntryQuantity ; 
  //###

  }

  else 
   {
      DTable.Rows[foundRowIndex][DataObject.DataID] = ObjectElement;
   }
}

after verification i observe that the ObjectClass Propertie value has been changed too.
for example
DataElement.Quantity = 23 // Initial Value
after Addition
DataElement.Quantity = 46; not my Purpose.

my purpose is just to Change a value of a a Object insert a new Value in my DataTable and the OjectClass stay without Change.
like this;

##before Addition
DataElement.Quantity = 23 // Initial
##after Addition
DataElement.Quantity = 23 // Object stay intact

##DataTable
Object with a value: DataElement.Quantity = 46

but when i made a Change the Value of the Propertie it Change a value of my ObjectClass.
Please where i'm doing wrong??