Check Datatable one column value with other fields Using LINQ

Suppose i have Users_Name in DataTable and in ohter hand,I have Reciepient_Name is other table,and i want to check whether Reciepient_Name is there in a DataTable,if yes assign Reciepient_Name with User_name otherwise assign Reciepient_Name to empty.

Using System.LINQ; //for working with FirstOrDefault

if (!string.IsNullOrEmpty(recipient_name)) //string field coming from other table assume it will have some value
{
     if (dt_user_reciepent.Rows.Count > 0) //assume that dt_user_reciepent is already filled with user_name
     {                            
            var res = (from obj in dt_user_reciepent.AsEnumerable()
                       where  obj.Field<string>("User_name").ToLower().Contains(recpient_name)
                       select new
                       {
                            User_name =  obj.Field<string>("User_name")
                       }).FirstOrDefault();

             if (res != null)
             {
                 recipient_name = res.User_ID;       
             }
             else
             {
                 recipient_name = string.Empty;       
             }
     }
}