如何使用自动实现的属性实现轻量类（C# 编程指南）

  • 项目

本示例演示如何创建一个仅用于封装一组自动实现的属性的不可变轻型类。 当你必须使用引用类型语义时，请使用此种构造而不是结构。

可通过以下方法来实现不可变的属性：

  • 仅声明 get 访问器，使属性除了能在该类型的构造函数中可变，在其他任何位置都不可变。
  • 声明 init 访问器而不是 set 访问器，这使属性只能在构造函数中进行设置，或者通过使用对象初始值设定项设置。
  • 将 set 访问器声明为专用。 属性可在该类型中设置，但它对于使用者是不可变的。

可以将 required 修饰符添加到属性声明中，以强制调用方将属性设置为初始化新对象的一部分。

下面的示例显示了只有 get 访问器的属性与具有 get 和 private set 的属性的区别。

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;
}

示例

下面的示例演示了实现具有自动实现属性的不可变类的两种方法。 这两种方法均使用 private set 声明其中一个属性，使用单独的 get 声明另一个属性。 第一个类仅使用构造函数来初始化属性，第二个类则使用可调用构造函数的静态工厂方法。

// 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.
*/

编译器为每个自动实现的属性创建了支持字段。 这些字段无法直接从源代码进行访问。

请参阅