如何實作一個含自動實作屬性的輕量型類別 (C# 程式設計手冊)

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這個範例顯示如何建立不可變的輕量型類別，只用來封裝一組自動實作屬性。 當您必須使用參考類型語意時，請使用這種建構，而不是結構。

您可以使用下列方式來建立不可變的屬性：

  • 只宣告 get 存取子，讓屬性在該類型的建構函式外的任何位置皆為不可變的。
  • 宣告 init 存取子，而不是 set 存取子，讓屬性只能在建構函式中設定，或使用物件初始設定式。
  • 將 set 存取子宣告為 private。 屬性可在類型內設定，但是它對於取用者是不可變的。

您可以將 required 修飾詞加到屬性宣告，以強制呼叫者將屬性設為初始化新物件的一部分。

下列範例示範只有 get 存取子的屬性與具有 get 和 private set 的屬性有何不同。

class Contact
{
    public string Name { get; }
    public string Address { get; private set; }

    public Contact(string contactName, string contactAddress)
    {
        // Both properties are accessible in the constructor.
        Name = contactName;
        Address = contactAddress;
    }

    // Name isn't assignable here. This will generate a compile error.
    //public void ChangeName(string newName) => Name = newName;

    // Address is assignable here.
    public void ChangeAddress(string newAddress) => Address = newAddress;
}

範例

下列範例顯示兩個方式來實作不可變的類別，該類別具有自動實作屬性。 每一種方法會宣告具有私用 set 的其中一個屬性，以及僅具有 get 的其中一個屬性。 第一個類別僅使用建構函式來初始化屬性，第二個類別使用會呼叫建構函式的靜態 Factory 方法。

// This class is immutable. After an object is created,
// it cannot be modified from outside the class. It uses a
// constructor to initialize its properties.
class Contact
{
    // Read-only property.
    public string Name { get; }

    // Read-write property with a private set accessor.
    public string Address { get; private set; }

    // Public constructor.
    public Contact(string contactName, string contactAddress)
    {
        Name = contactName;
        Address = contactAddress;
    }
}

// This class is immutable. After an object is created,
// it cannot be modified from outside the class. It uses a
// static method and private constructor to initialize its properties.
public class Contact2
{
    // Read-write property with a private set accessor.
    public string Name { get; private set; }

    // Read-only property.
    public string Address { get; }

    // Private constructor.
    private Contact2(string contactName, string contactAddress)
    {
        Name = contactName;
        Address = contactAddress;
    }

    // Public factory method.
    public static Contact2 CreateContact(string name, string address)
    {
        return new Contact2(name, address);
    }
}

public class Program
{
    static void Main()
    {
        // Some simple data sources.
        string[] names = ["Terry Adams","Fadi Fakhouri", "Hanying Feng",
                            "Cesar Garcia", "Debra Garcia"];
        string[] addresses = ["123 Main St.", "345 Cypress Ave.", "678 1st Ave",
                                "12 108th St.", "89 E. 42nd St."];

        // Simple query to demonstrate object creation in select clause.
        // Create Contact objects by using a constructor.
        var query1 = from i in Enumerable.Range(0, 5)
                    select new Contact(names[i], addresses[i]);

        // List elements cannot be modified by client code.
        var list = query1.ToList();
        foreach (var contact in list)
        {
            Console.WriteLine("{0}, {1}", contact.Name, contact.Address);
        }

        // Create Contact2 objects by using a static factory method.
        var query2 = from i in Enumerable.Range(0, 5)
                        select Contact2.CreateContact(names[i], addresses[i]);

        // Console output is identical to query1.
        var list2 = query2.ToList();

        // List elements cannot be modified by client code.
        // CS0272:
        // list2[0].Name = "Eugene Zabokritski";
    }
}

/* Output:
    Terry Adams, 123 Main St.
    Fadi Fakhouri, 345 Cypress Ave.
    Hanying Feng, 678 1st Ave
    Cesar Garcia, 12 108th St.
    Debra Garcia, 89 E. 42nd St.
*/

編譯器會針對每個自動實作屬性建立支援欄位。 欄位無法直接從原始程式碼存取。

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