C＃类的构造函数默认值的问题

Question

我有下面的类：

public class Topic 
    { 
     public string Topic { get; set; } 
     public string Description { get; set; } 
     public int Count { get; set; } 
    } 

我想有当与下面是创建类伯爵总是被设置为零：

var abc = new Topic { 
    Topic = "test1", 
    Description = "description1" 
} 

我与构造函数有点混淆。当我创建abc时，这是可能的还是需要指定主题，描述和计数？

Answer 1

您有几个不同的选项。

1）int默认为零，所以如果您不初始化它将为零。

2）你可以使用一个构造函数

public Topic(){ Count = 0;} 

3）您可以使用支持字段，而不是自动财产和初始化为零

private int _count = 0; 
public int Count { 
    get {return _count} 
    set {_count = value; } 
} 

Answer 2

一个int的缺省值是0。

所有值类型具有默认值，因为它们不能null。

请参阅此MSDN页面上的Initializing Value Types。

Answer 3

Count将在初始化时默认为0，因为它是值类型，不能是null。

Answer 4

编辑

正如我从这个答案的评论中学到的，在初始化器调用中忽略()是完全有效的。

正确的语法是 我希望的语法仍然是：

var abc = new Topic() { 
    Topic = "test1", 
    Description = "description1" 
} 

（注意()）。

这会将Count初始化为0，因为0是int的默认值。如果你想始终指定主题和描述，添加一个显式的构造函数：

public Topic(string topic, string description) 
{ 
    Topic = topic; 
    Description = description; 
    // You may also set Count explicitly here, but if you want "0" you don't need to 
} 

Answer 5

这下面的语句不仅是一个构造函数：

var abc = new Topic { 
    Topic = "test1", 
    Description = "description1" 
} 

这是一个构造函数和一个对象初始化。

真正发生的是new Topic()被首先调用，因此将所有值初始化为它们的默认值（属性Topic为null，说明为空，Count为0）。之后，将值“test1”分配给Topic，并将值“description1”分配给Description。

所有值类型都具有不同于null的默认值（因为它们不能为空），并且引用类型默认为null。

Answer 6

公共类节目 { 公共静态无效的主要（） {

// Declare a StudentName by using the constructor that has two parameters. 
    StudentName student1 = new StudentName("Craig", "Playstead"); 

    // Make the same declaration by using a collection initializer and sending 
    // arguments for the first and last names. The default constructor is 
    // invoked in processing this declaration, not the constructor that has 
    // two parameters. 
    StudentName student2 = new StudentName 
    { 
     FirstName = "Craig", 
     LastName = "Playstead", 
    }; 

    // Declare a StudentName by using a collection initializer and sending 
    // an argument for only the ID property. No corresponding constructor is 
    // necessary. Only the default constructor is used to process object 
    // initializers. 
    StudentName student3 = new StudentName 
    { 
     ID = 183 
    }; 

    // Declare a StudentName by using a collection initializer and sending 
    // arguments for all three properties. No corresponding constructor is 
    // defined in the class. 
    StudentName student4 = new StudentName 
    { 
     FirstName = "Craig", 
     LastName = "Playstead", 
     ID = 116 
    }; 

    System.Console.WriteLine(student1.ToString()); 
    System.Console.WriteLine(student2.ToString()); 
    System.Console.WriteLine(student3.ToString()); 
    System.Console.WriteLine(student4.ToString()); 
} 

// Output: 
// Craig 0 
// Craig 0 
// 183 
// Craig 116 

}

公共类StudentName {

// The default constructor has no parameters. The default constructor 
// is invoked in the processing of object initializers. 
// You can test this by changing the access modifier from public to 
// private. The declarations in Main that use object initializers will 
// fail. 
public StudentName() { } 

// The following constructor has parameters for two of the three 
// properties. 
public StudentName(string first, string last) 
{ 
    FirstName = first; 
    LastName = last; 
} 

// Properties. 
public string FirstName { get; set; } 
public string LastName { get; set; } 
public int ID { get; set; } 

public override string ToString() 
{ 
    return FirstName + " " + ID; 
} 

}