當麻許不寫程式

這套件主要是在用於 "扁平化" 你的資料模型，當你的模型是巢狀的時候他會進行把巢狀的物件拉到第一層來

直接看案例，這邊是我原本的模型

  
  
public class Order
{
    public int Id { get; set; }
    public Customer Customer { get; set; } = default!;
    public List Lines { get; set; } = new();
    public DateTime CreatedAt { get; set; }
}

public class Customer
{
    public int Id { get; set; }
    public string Name { get; set; } = "";
    public Address Address { get; set; } = default!;
}

public class Address
{
    public string City { get; set; } = "";
    public string Country { get; set; } = "";
}

public class OrderLine
{
    public string ProductName { get; set; }
    public decimal Price { get; set; }
    public int Quantity { get; set; }
}

    
  
  

這邊我先給出，將這資料序列畫成 JSON 之後的長相

     
[
  {
    "Id": 1,
    "Customer": {
      "Id": 1,
      "Name": "許當麻",
      "Address": {
        "City": "台北市",
        "Country": "台灣"
      }
    },
    "Lines": [
      {
        "ProductName": "筆記型電腦",
        "Price": 28000,
        "Quantity": 1
      },
      {
        "ProductName": "無線滑鼠",
        "Price": 1200,
        "Quantity": 1
      }
    ],
    "CreatedAt": "2026-01-14T11:59:31.505265+08:00"
  },
  {
    "Id": 2,
    "Customer": {
      "Id": 2,
      "Name": "林怡君",
      "Address": {
        "City": "新北市",
        "Country": "台灣"
      }
    },
    "Lines": [
      {
        "ProductName": "27吋螢幕",
        "Price": 9500,
        "Quantity": 1
      }
    ],
    "CreatedAt": "2026-01-13T11:59:31.5133105+08:00"
  }
]   
     
     
  

之後我們只需要建立一個物件，上面加上 Attribute 就可以，之後在 LINQ 下進行 .Select 輸出

   
    
    [Flatten(typeof(Order))]
    public partial class OrderFlatten
    {
    }

    
  

測試轉換結果

         
    	 var flattenData = FakeOrderData.FakeData1
                                  .AsQueryable()
                                  .Where(x => x.Id <= 2)
                                  .Select(OrderFlatten.Projection)
                                  .ToList();

         Console.WriteLine(JsonConvert.SerializeObject(flattenData));
  
  

輸出結果

         
[
  {
    "Id": 1,
    "CustomerId": 1,
    "CustomerName": "許當麻",
    "CustomerAddressCity": "台北市",
    "CustomerAddressCountry": "台灣",
    "CreatedAt": "2026-01-14T11:59:31.505265+08:00"
  },
  {
    "Id": 2,
    "CustomerId": 2,
    "CustomerName": "林怡君",
    "CustomerAddressCity": "新北市",
    "CustomerAddressCountry": "台灣",
    "CreatedAt": "2026-01-13T11:59:31.5133105+08:00"
  }
]
  
  

這樣是不是就非常簡單的將 Customer=> Name , AdddressCity ..等都往上拉了一個層級，甚至你不用寫轉換的程式碼

再來他還有一些其他常見的用法，如果你在進行關聯你可以透過調整 Arrtibute 把子項目的 Id 不要顯示出來

    
    [Flatten(typeof(Order), IgnoreNestedIds = true)]
    public partial class OrderFlattenIgonreIds
    {


    }
    
    //使用方法
	var flattenIgonreNestedIdsData = FakeOrderData.FakeData1
                                   .AsQueryable()
                                   .Where(x => x.Id <= 2)
                                   .Select(OrderFlattenIgonreIds.Projection)
                                   .ToList();

	Console.WriteLine(JsonConvert.SerializeObject(flattenIgonreNestedIdsData));
    
  

輸出結果

         
[
  {
    "Id": 1,
    "CustomerName": "許當麻",
    "CustomerAddressCity": "台北市",
    "CustomerAddressCountry": "台灣",
    "CreatedAt": "2026-01-14T11:59:31.505265+08:00"
  },
  {
    "Id": 2,
    "CustomerName": "林怡君",
    "CustomerAddressCity": "新北市",
    "CustomerAddressCountry": "台灣",
    "CreatedAt": "2026-01-13T11:59:31.5133105+08:00"
  }
]
  
  

不過，如果如果你是要比較複雜要多一個 多出來的屬性，還是得要自己作 mapping 自己寫一個 Projection

  
    [Flatten(typeof(Order))]
    public partial class OrderFlattenWithTotalAmount
    {
        public decimal TotalAmount { get; set; }

        public static Expression> WithTotalAmount =>
       o => new OrderFlattenWithTotalAmount
       {
           // Flatten 對應的基本欄位（你要「照名填」）
           Id = o.Id,
           CustomerId = o.Customer.Id,
           CustomerName = o.Customer.Name,
           CustomerAddressCity = o.Customer.Address.City,
           CustomerAddressCountry = o.Customer.Address.Country,
           CreatedAt = o.CreatedAt,
           TotalAmount = o.Lines.Sum(l => l.Price * l.Quantity)
       };
    }
    
    //使用方法
	var customerData = FakeOrderData.FakeData1
                                   .AsQueryable()
                                   .Where(x => x.Id <= 2)
                                   .Select(OrderFlattenWithTotalAmount.WithTotalAmount)
                                   .ToList();

    Console.WriteLine(JsonConvert.SerializeObject(customerData));

  
  

輸出結果

         
[
  {
    "TotalAmount": 29200,
    "Id": 1,
    "CustomerId": 1,
    "CustomerName": "許當麻",
    "CustomerAddressCity": "台北市",
    "CustomerAddressCountry": "台灣",
    "CreatedAt": "2026-01-14T11:59:31.505265+08:00"
  },
  {
    "TotalAmount": 9500,
    "Id": 2,
    "CustomerId": 2,
    "CustomerName": "林怡君",
    "CustomerAddressCity": "新北市",
    "CustomerAddressCountry": "台灣",
    "CreatedAt": "2026-01-13T11:59:31.5133105+08:00"
  }
]
  
  

做個結論，在快速處理一些 API 輸出時，你有扁平化的需求，他是一個好東西，但是他遇到是 List<object> ，或是陣列型的屬性時

他會略過不處理，所以在一些 "圖方便" 的狀況他是一個不錯的選擇，但是如果要需要客制化還是得寫 expreesion ，你說他雞肋嗎

倒也不至於，只是在一些狀況下的確可以快速產生一個需要的物件去輸出，但是現實雖然常常都是比較複雜的..

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本文原文首發於個人部落格：[C#] 別再手寫 Select - Facet.Net 扁平化 Model 的優點與實務限制

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The bug existed in all possible states.
Until I ran the code.