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windows C#-对象和集合初始值设定项(中)

article 2024/12/25 18:33:51

带 init 访问器的对象初始值设定项

确保无人更改设计，并且可以使用访问器来限制 init 对象。它有助于限制属性值的设置。

public class Person
{
    public string FirstName { get; set; }
    public string LastName { get; init; }
}

// The `LastName` property can be set only during initialization. It CAN'T be modified afterwards.
// The `FirstName` property can be modified after initialization.
var pet = new Person() { FirstName = "Joe", LastName = "Doe"};

// You can assign the FirstName property to a different value.
pet.FirstName = "Jane";

// Compiler error:
// Error CS8852  Init - only property or indexer 'Person.LastName' can only be assigned in an object initializer,
//               or on 'this' or 'base' in an instance constructor or an 'init' accessor.
// pet.LastName = "Kowalski";

必需的仅限 init 的属性支持不可变结构，同时允许该类型用户使用自然语法。

具有类类型属性的对象初始值设定项

初始化对象时，考虑类类型属性的含义至关重要：

public class HowToClassTypedInitializer
{
    public class EmbeddedClassTypeA
    {
        public int I { get; set; }
        public bool B { get; set; }
        public string S { get; set; }
        public EmbeddedClassTypeB ClassB { get; set; }

        public override string ToString() => $"{I}|{B}|{S}|||{ClassB}";

        public EmbeddedClassTypeA()
        {
            Console.WriteLine($"Entering EmbeddedClassTypeA constructor. Values are: {this}");
            I = 3;
            B = true;
            S = "abc";
            ClassB = new() { BB = true, BI = 43 };
            Console.WriteLine($"Exiting EmbeddedClassTypeA constructor. Values are: {this})");
        }
    }

    public class EmbeddedClassTypeB
    {
        public int BI { get; set; }
        public bool BB { get; set; }
        public string BS { get; set; }

        public override string ToString() => $"{BI}|{BB}|{BS}";

        public EmbeddedClassTypeB()
        {
            Console.WriteLine($"Entering EmbeddedClassTypeB constructor. Values are: {this}");
            BI = 23;
            BB = false;
            BS = "BBBabc";
            Console.WriteLine($"Exiting EmbeddedClassTypeB constructor. Values are: {this})");
        }
    }

    public static void Main()
    {
        var a = new EmbeddedClassTypeA
        {
            I = 103,
            B = false,
            ClassB = { BI = 100003 }
        };
        Console.WriteLine($"After initializing EmbeddedClassTypeA: {a}");

        var a2 = new EmbeddedClassTypeA
        {
            I = 103,
            B = false,
            ClassB = new() { BI = 100003 } //New instance
        };
        Console.WriteLine($"After initializing EmbeddedClassTypeA a2: {a2}");
    }

    // Output:
    //Entering EmbeddedClassTypeA constructor Values are: 0|False||||
    //Entering EmbeddedClassTypeB constructor Values are: 0|False|
    //Exiting EmbeddedClassTypeB constructor Values are: 23|False|BBBabc)
    //Exiting EmbeddedClassTypeA constructor Values are: 3|True|abc|||43|True|BBBabc)
    //After initializing EmbeddedClassTypeA: 103|False|abc|||100003|True|BBBabc
    //Entering EmbeddedClassTypeA constructor Values are: 0|False||||
    //Entering EmbeddedClassTypeB constructor Values are: 0|False|
    //Exiting EmbeddedClassTypeB constructor Values are: 23|False|BBBabc)
    //Exiting EmbeddedClassTypeA constructor Values are: 3|True|abc|||43|True|BBBabc)
    //Entering EmbeddedClassTypeB constructor Values are: 0|False|
    //Exiting EmbeddedClassTypeB constructor Values are: 23|False|BBBabc)
    //After initializing EmbeddedClassTypeA a2: 103|False|abc|||100003|False|BBBabc
}

以下示例演示了对于 ClassB，初始化过程如何涉及到更新特定的值，同时保留原始实例中的其他值。初始值设定项重用当前实例：ClassB 的值为：100003(此处分配的新值)、true(在 EmbeddedClassTypeA 的初始化中保留的值)、BBBabc(EmbeddedClassTypeB 中未更改的默认值)。

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