void Main()
{
     var query=GetClassID(3);
     query.ToList().ForEach(q=>Console.WriteLine("{0}\t{1}\t{2}",q.Id,q.Name,q.Parent));
     /*
        Id       Name       Parent
        5        B2         3
        8        D1         5
        9        D2         5
       10        D3         5
     */
}
public IEnumerable<Tim_LinqTable> GetClassID(int p_id)
{
            var query = from c in this.Tim_LinqTables
                        where c.Parent  == p_id
                        select c;
         
           return  query.ToList().Concat(query.ToList().SelectMany(t => GetClassID(t.Id)));              
}