清单< INT>测试= {1，2，3} - 它是一个功能或一个错误？ [英] List<int> test = {1, 2, 3} - is it a feature or a bug?

问题描述

如你所知，这是不允许使用阵列的初始化语法列表。它会给一个编译时错误。例如：

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（很简单）：

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，但在运行时会给出一个对象引用未设置为对象运行时错误。

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

我希望这些代码给一个编译时错误。我对你的问题是：为什么没有它，有什么好的理由时，这种情况会正常运行。

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测试？ ，同时提供.NET和Mono的编译器。

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

干杯。

推荐答案

我认为这是一个由设计行为。在测试= {1，2，3} 被编译到调用代码添加存储在列表中的方法测试字段。

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.

为什么你要原因的NullReferenceException 是测试是空。如果初始化测试字段到一个新的列表，然后代码将工作：

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 - 如果你写新的List< INT> {...} 然后创建列表的新实例。如果不添加对象构造，它将使用现有的实例（或空）。据我所看到的，C＃规范不包含任何明确的转换规则，将匹配这个场景，但它提供了一个例子（见栏目7.6.10.3 ）：

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):

的 A 列表<联系与GT; 可以创建和初始化如下：的

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" }
    }
};

的具有为的

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);

的，其中 __ C1 和 __ C2 是临时变量是不可见的，交通不便。的

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

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