EFCore 连接MySQL数据库查询数据提示This MySqlConnection is already in use

using (MyDBContext db =new MyDBContext())
{
        Order order= db.Orders.Single(o => o.Id==2);
        printChildren(1, db, order);
}

static void printChildren(int depLevel,MyDBContext ctx,Order ou)
{
    var children = ctx.Orders.Where(o => o.Parent == ou);
    foreach (var child in children)
    {
        Console.WriteLine(new String('\t', depLevel) + child.Name);
        //递归
        printChildren(depLevel + 1, ctx, child);
    }
}