# [.NET] 什麼是 Boxing 和 Unboxing ?

###### tags: `Bolger` `.NET` `C#`

## [.NET] 什麼是 Boxing 和 Unboxing ?

### Boxing
Boxing 將儲存 **堆疊(stack)** 的 **實值類型(Value type)** 轉換為 object 類型，此轉換為 **隱含轉換(implicit conversion)** 。

- Boxing 處理時，會在 **堆積(heap)** 產生新物件並將值複製過去。
- 用實值型別實作的任何介面類型也適用。

有點類似實務的例如你有一個東西(錢包)，把它裝箱並放置到某個抽屜，這個抽屜有把鑰匙讓你知道放在哪個抽屜

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</center>

int i = 123;

// Boxing
object o = i;

透過[sharplab][sharplab]反編譯出來的IL ([介紹sharplab][介紹sharplab])

IL_0000: nop
IL_0001: ldc.i4.s 123
IL_0003: stloc.0
IL_0004: ldloc.0
IL_0005: box [System.Private.CoreLib]System.Int32 // Boxing
IL_000a: stloc.1
IL_000b: ret

### Unboxing

**Unboxing** 就是 Boxing 的相反，將 object 類型明確轉換為實值型別。

- 注意 object 是否為 Null 如果是會造成 `NullReferenceException`
- 嘗試不相容的實值類型進行 Unboxing 會造成 `InvalidCastException`

<center>


</center>

object o = 123;

// Unboxing
int i = (int)o;

IL_0000: nop
IL_0001: ldc.i4.s 123
IL_0003: box [System.Private.CoreLib]System.Int32
IL_0008: stloc.0
IL_0009: ldloc.0
IL_000a: unbox.any [System.Private.CoreLib]System.Int32 // Unboxing
IL_000f: stloc.1
IL_0010: ret

### Boxing and Unboxing 效能測試

private static void WithoutBox(object obj)
{
    int i = 123;
}

private static void Boxing(object obj)
{
    object i = 123;
}

private static void UnBoxing(object obj)
{
    int i = (int)obj;
}

# WithoutBox
IL_0000: nop
IL_0001: ldc.i4.s 123
IL_0003: stloc.0
IL_0004: ret

# Boxing
IL_0000: nop
IL_0001: ldc.i4.s 123
IL_0003: box [System.Private.CoreLib]System.Int32
IL_0008: stloc.0
IL_0009: ret

# UnBoxing
IL_0000: nop
IL_0001: ldarg.0
IL_0002: unbox.any [System.Private.CoreLib]System.Int32
IL_0007: stloc.0
IL_0008: ret

測試100,000,00後可以在處理 **Boxing** 的耗時最久幾乎是沒有任何處理快兩倍的時間，**UnBoxing** 也是比較久所以在寫程式上盡量避免 Boxing 或 UnBoxing 產生。

這裡是[範例][gitlab]，如有理解錯誤歡迎大家多多指教，謝謝

WithoutBox Test 耗時 381 毫秒
Boxing Test 耗時 728 毫秒
UnBoxing Test 耗時 455 毫秒

### Boxing 範例

範例 1: 不小心就 Boxing

通常我們在寫 Log 時候，很常在寫 Log 訊息使用字串簡化寫法，但其實不小心就造成 Boxing 產生，此種狀況可能造成效率問題。

var ex1 = 123;

Console.WriteLine($" {ex1}");             // Boxing
Console.WriteLine($" {ex1.ToString()}");

    IL_0000: nop
    IL_0001: ldc.i4.s 123
    IL_0003: stloc.0
    IL_0004: ldstr " {0}"
    IL_0009: ldloc.0
    IL_000a: box [System.Private.CoreLib]System.Int32 // Boxing
    IL_000f: call string [System.Private.CoreLib]System.String::Format(string, object)
    IL_0014: call void [System.Console]System.Console::WriteLine(string)

    IL_0019: nop
    IL_001a: ldstr " "
    IL_001f: ldloca.s 0
    IL_0021: call instance string [System.Private.CoreLib]System.Int32::ToString()
    IL_0026: call string [System.Private.CoreLib]System.String::Concat(string, string)
    IL_002b: call void [System.Console]System.Console::WriteLine(string)
    IL_0030: nop
    IL_0031: ret

範例 2: 容易誤會有 Boxing

重點:Boxing 只有在執行 **實值型別(Value type)** ， string 是 參考型別所以不會產生 Boxing。 ([參考][reference types])

var ex2 = "123";

object obj2 = ex2;

obj2 = "456";

Console.WriteLine($"Example2 string result: {ex2}");
Console.WriteLine($"Example2 object result: {obj2}");

IL_000f: ldstr "Example2 string result: "
IL_0014: ldloc.0
IL_0015: call string [System.Private.CoreLib]System.String::Concat(string, string)
IL_001a: call void [System.Console]System.Console::WriteLine(string)

IL_001f: nop
IL_0020: ldstr "Example2 object result: {0}"
IL_0025: ldloc.1
IL_0026: call string [System.Private.CoreLib]System.String::Format(string, object)
IL_002b: call void [System.Console]System.Console::WriteLine(string)

### 參考資料
[Boxing 和 Unboxing][boxing-and-unboxing]

[boxing 與 unboxing 重點整理與程式範例][bloger-boxing-and-unboxing]

[Why do we need boxing and unboxing][why-need]

[Object type boxing with a reference type variable][boxing-with-a-reference-type-variable]

[Reference types][reference types]

<!-- 路徑 -->

[sharplab]:
https://sharplab.io/

[介紹sharplab]:
https://putaonini.gitlab.io/tool/20211030/3902068567/

[gitlab]:
https://gitlab.com/PuTaoNiNi/csharpsample/-/blob/main/BoxingAndUnboxing/Program.cs

[boxing-and-unboxing]:
https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/types/boxing-and-unboxing

[why-need]:
https://stackoverflow.com/questions/2111857/why-do-we-need-boxing-and-unboxing-in-c

[bloger-boxing-and-unboxing]:
https://nwpie.blogspot.com/2017/04/5-boxing-unboxing.html

[reference types]:
https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/reference-types

<!-- reference types => boxing => object types 是否會再產生一組, 還是指標過去 -->
[boxing-with-a-reference-type-variable]:
https://stackoverflow.com/questions/9092961/object-type-boxing-with-a-reference-type-variable

[Is object a reference type or value type]:
https://stackoverflow.com/questions/17673029/is-object-a-reference-type-or-value-type

<!-- 之前的問題可以補一下 -->
[問題]:
https://www.facebook.com/groups/DotNetUserGroupTaiwan/permalink/2571258316500604/