列表<int>test = {1, 2, 3} - 这是一个特性还是一个错误? [英] List&lt;int&gt; test = {1, 2, 3} - is it a feature or a bug?

问题描述

如您所知，不允许在列表中使用数组初始化语法.它会给出编译时错误.示例:

As you know, it is not allowed to use the Array-initialisation syntax with Lists. It will give a compile-time error. Example:

List<int> test = { 1, 2, 3} 
// At compilation the following error is shown:
// Can only use array initializer expressions to assign to array types. 

不过今天我做了以下(非常简化):

However today I did the following (very simplified):

class Test
{
     public List<int> Field;
}

List<Test> list = new List<Test>
{
    new Test { Field = { 1, 2, 3 } }
};

上面的代码编译得很好，但运行时会出现对象引用未设置为对象"的运行时错误.

The code above compiles just fine, but when run it will give a "Object references is not set to an object" run-time error.

我希望该代码会出现编译时错误.我要问你的问题是:为什么不呢?这种场景何时可以正确运行，有什么好的理由吗?

I would expect that code to give a compile-time error. My question to you is: Why doesn't it, and are there any good reasons for when such a scenario would run correctly?

这已经使用 .NET 3.5 进行了测试，包括 .Net 和 Mono 编译器.

This has been tested using .NET 3.5, both .Net and Mono compilers.

干杯.

推荐答案

我认为这是一种设计行为.Test = { 1, 2, 3 } 被编译成代码，该代码调用存储在 Test 字段中的列表的 Add 方法.

I think this is a by-design behavior. The Test = { 1, 2, 3 } is compiled into code that calls Add method of the list stored in the Test field.

您得到 NullReferenceException 的原因是 Test 是 null.如果您将 Test 字段初始化为新列表，则代码将起作用:

The reason why you're getting NullReferenceException is that Test is null. If you initialize the Test field to a new list, then the code will work:

class Test {    
  public List<int> Field = new List<int>(); 
}  

// Calls 'Add' method three times to add items to 'Field' list
var t = new Test { Field = { 1, 2, 3 } };

这是很合乎逻辑的——如果你写 new List;{ ... } 然后它创建一个新的列表实例.如果不添加对象构造，它将使用现有实例(或 null).据我所知，C# 规范不包含任何与此场景匹配的显式翻译规则，但它提供了一个示例(请参阅第 7.6.10.3 节):

It is quite logical - if you write new List<int> { ... } then it creates a new instance of list. If you don't add object construction, it will use the existing instance (or null). As far as I can see, the C# spec doesn't contain any explicit translation rule that would match this scenario, but it gives an example (see Section 7.6.10.3):

一个List可以被创建和初始化如下:

A List<Contact> can be created and initialized as follows:

var contacts = new List<Contact> {
    new Contact {
        Name = "Chris Smith",
        PhoneNumbers = { "206-555-0101", "425-882-8080" }
    },
    new Contact {
        Name = "Bob Harris",
        PhoneNumbers = { "650-555-0199" }
    }
};

效果与

var contacts = new List<Contact>();
Contact __c1 = new Contact();
__c1.Name = "Chris Smith";
__c1.PhoneNumbers.Add("206-555-0101");
__c1.PhoneNumbers.Add("425-882-8080");
contacts.Add(__c1);
Contact __c2 = new Contact();
__c2.Name = "Bob Harris";
__c2.PhoneNumbers.Add("650-555-0199");
contacts.Add(__c2);

其中 __c1 和 __c2 是临时变量，否则不可见和无法访问.

where __c1 and __c2 are temporary variables that are otherwise invisible and inaccessible.

这篇关于列表<int>test = {1, 2, 3} - 这是一个特性还是一个错误?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！