如何在第二层的linq中分组

Question

public class ShipmentItem
{
  public int? ShipmentId { get; set; }
  public int? TotalCountShipmentItems { get; set; }          
  public List<PurchaseOrderItem> ShipmentItemList { get; set; }
}
public class PurchaseOrderItem
{
  public int itemId{get;set}
  public string ItemNumber { get; set; }
  public string Description { get; set; }
  public int? TotQuantity { get; set; }                
}

我想要结果格式

 data: [
    {
      ShipmentId : 15,
      TotalCountShipmentItems : 4,
      ShipmentItemList : [
        {
         itemId: 10,
         ItemNumber: 1012,
         Description : desss,
        TotQuantity : 20,/*sum of all items of id 10*/
        },
       {
         itemId: 11,
         ItemNumber: 1013,
         Description : d333s,
        TotQuantity : 50,/*sum of all items of id 11*/
        }
]
}]

我希望结果按shipmentId分组，然后按Group by ItemId分组 试图分组，但运气不佳，但没有奏效 会导致序列问题

我在哪里犯错？我们不能将它分组到获取列表中吗？