學習C#

C#空值條件運算符

[{academy-video-youtube({"vid": "eOpmjGF4qxo", "start_time": "0", "title": "C# Null-Conditional Operator (? 運算子) 解釋！

在他的影片之一中，Gerald Versluis 遇到了關於他範例程式碼中問號 (?) 的問題。這促使他建立了一段詳細的影片來解釋C# 中的空條件運算子。如果您曾經想知道?[ ]在 C# 中的作用，Gerald 的影片是一個很好的資源，可讓您清楚理解這一點。讓我們一步步地跟進他的解釋。

通過檔案頁面的快速解釋

Gerald 首先參考了微軟官方檔案中的成員存取運算符和表達式。他突出介紹了在 C# 6 引入的空條件運算符部分。在錄製時，最新版本為 C# 9，而 C# 10 即將到來。

Gerald 說明當您使用a為空，整個表達式會計算為空而不是丟擲空引用異常。否則，將正常檢索值。

雖然文件很有幫助，但 Gerald 更喜歡實作範例，所以他轉向互動的編碼環境。

通過 try.dot.net 的範例

為了演示，Gerald 切換到 try.dot.net，一個互動的 C#環境。他提到這個工具運行在 Blazor 上，允許使用者直接在瀏覽器中執行 C#程式碼。

他簡要展示了 try.dot.net 中預載的預設斐波那契數列範例，然後清除它以開始集中於空條件運算子的新的範例。

空條件運算子解釋

Gerald 建立了一個簡單的 Person 類別，具有以下屬性：

public class Person
{
    // The Name property of the Person
    public string Name { get; set; }
    
    // A reference to another Person object representing the partner
    public Person Partner { get; set; }
    
    // An array representing the Person's hobbies
    public string[] Hobbies { get; set; }
}

public class Person
{
    // The Name property of the Person
    public string Name { get; set; }
    
    // A reference to another Person object representing the partner
    public Person Partner { get; set; }
    
    // An array representing the Person's hobbies
    public string[] Hobbies { get; set; }
}

Public Class Person
    ' The Name property of the Person
    Public Property Name As String

    ' A reference to another Person object representing the partner
    Public Property Partner As Person

    ' An array representing the Person's hobbies
    Public Property Hobbies As String()
End Class

Public Class Person
    ' The Name property of the Person
    Public Property Name As String

    ' A reference to another Person object representing the partner
    Public Property Partner As Person

    ' An array representing the Person's hobbies
    Public Property Hobbies As String()
End Class

他將Person物件初始化為空：

// Initialize a Person object as null
Person person = null;

// Attempting to access a property of a null object will throw a NullReferenceException
Console.WriteLine(person.Name);

// Initialize a Person object as null
Person person = null;

// Attempting to access a property of a null object will throw a NullReferenceException
Console.WriteLine(person.Name);

' Initialize a Person object as Nothing
Dim person As Person = Nothing

' Attempting to access a property of a Nothing object will throw a NullReferenceException
Console.WriteLine(person.Name)

' Initialize a Person object as Nothing
Dim person As Person = Nothing

' Attempting to access a property of a Nothing object will throw a NullReferenceException
Console.WriteLine(person.Name)

運行此程式碼會引發空引用例外，因為person為空。為了防止這種情況，開發者通常在存取屬性之前新增一個空檢查：

// Check if the person object is not null before accessing its properties
if (person != null)
{
    Console.WriteLine(person.Name);
}

// Check if the person object is not null before accessing its properties
if (person != null)
{
    Console.WriteLine(person.Name);
}

' Check if the person object is not null before accessing its properties
If person IsNot Nothing Then
    Console.WriteLine(person.Name)
End If

' Check if the person object is not null before accessing its properties
If person IsNot Nothing Then
    Console.WriteLine(person.Name)
End If

雖然這有效，但會增加多餘的程式碼行。 Gerald 解釋了空條件運算子如何簡化這一過程：

// Using the null-conditional operator to safely access the Name property
Console.WriteLine(person?.Name);

// Using the null-conditional operator to safely access the Name property
Console.WriteLine(person?.Name);

' Using the null-conditional operator to safely access the Name property
Console.WriteLine(person?.Name)

' Using the null-conditional operator to safely access the Name property
Console.WriteLine(person?.Name)

現在，如果person為空，表達式只會返回空值而不會當機。

屬性上的空條件運算子

Gerald 通過設定一個Person物件擴展了範例：

// Create a new Person object
Person person = new Person { Name = "Gerald" };

// Access the Name property safely
Console.WriteLine(person?.Name);

// Create a new Person object
Person person = new Person { Name = "Gerald" };

// Access the Name property safely
Console.WriteLine(person?.Name);

' Create a new Person object
Dim person As New Person With {.Name = "Gerald"}

' Access the Name property safely
Console.WriteLine(person?.Name)

' Create a new Person object
Dim person As New Person With {.Name = "Gerald"}

' Access the Name property safely
Console.WriteLine(person?.Name)

由於person不是空值，正確列印出**"Gerald"**。

接下來，他展示了一個巢狀的範例：

// Safely access the Name property of the Partner object
Console.WriteLine(person?.Partner?.Name);

// Safely access the Name property of the Partner object
Console.WriteLine(person?.Partner?.Name);

' Safely access the Name property of the Partner object
Console.WriteLine(person?.Partner?.Name)

' Safely access the Name property of the Partner object
Console.WriteLine(person?.Partner?.Name)

如果person.Partner為空，則整個表達式會計算為空而不是引發異常。這樣可以避免不必要的 if 條件。

鏈式空條件運算子

Gerald 解釋了空條件運算子可以被連結使用，當處理深層巢狀的物件時，使程式碼更加清晰。您可以使用以下方式代替編寫多個空檢查：

// Chain null-conditional operators to safely access nested objects
Console.WriteLine(person?.Partner?.Name);

// Chain null-conditional operators to safely access nested objects
Console.WriteLine(person?.Partner?.Name);

' Chain null-conditional operators to safely access nested objects
Console.WriteLine(person?.Partner?.Name)

' Chain null-conditional operators to safely access nested objects
Console.WriteLine(person?.Partner?.Name)

如果person.Partner為空，結果將為空，從而防止運行時錯誤。

陣列上的空條件運算子

Gerald 將討論延伸至陣列和集合。他在 Person 類別中新增了一個 Hobbies 陣列，並展示了如何安全地存取元素：

// Safely access the first element of the Hobbies array
Console.WriteLine(person?.Hobbies?[0]);

// Safely access the first element of the Hobbies array
Console.WriteLine(person?.Hobbies?[0]);

' Safely access the first element of the Hobbies array
Console.WriteLine(If(person?.Hobbies?.ElementAtOrDefault(0), Nothing))

' Safely access the first element of the Hobbies array
Console.WriteLine(If(person?.Hobbies?.ElementAtOrDefault(0), Nothing))

如果Hobbies為空，則表達式會評估為空而不是引發例外。

結論

Gerald 的影片深入探討了 C# 中的空條件運算子。通過使用?[ ]，開發者可以編寫更簡潔、更安全的程式碼，避免空引用異常。不論是處理屬性、巢狀物件或陣列，空條件運算子使程式碼更簡潔且更易讀。

欲進一步了解完整演示，請務必查看Gerald Versluis 的影片，在其中他以實作範例解釋一切！

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學習C#

C#空值條件運算符

通過檔案頁面的快速解釋

通過 try.dot.net 的範例

空條件運算子解釋

屬性上的空條件運算子

鏈式空條件運算子

陣列上的空條件運算子

結論

Earn More by Sharing What You Love

Let's Stay in Touch!

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