使用 null 初始化的 C# 值类型

发布于 2024-10-04 10:04:08 字数 533 浏览 7 评论 0原文

在 C# 3.0 中可以将 null 赋给 int 吗？ type（在CLR中int？是一个结构体）：

int? a = null;

但是当你定义一个自定义结构体时：

struct MyStruct
{

}

在这段代码的编译过程中出现错误：

MyStruct a = null;

错误如下：

Cannot convert null to 'Example.MyStruct' because it is a non-nullable value type

而int？是 CLR 中的一个结构体，我们可以为其分配 null ，这在某种程度上是违反直觉的。我认为 null 被隐式转换或装箱为某个 int ？表示空值的值。是如何精确完成的？如何以可以执行以下行的方式扩展 MyStruct：

MyStruct a = null;

原文

In C# 3.0 you can assign null to int? type (in CLR int? is a struct):

int? a = null;

but when you define a custom struct:

struct MyStruct
{

}

there's an error during the compilation of this code:

MyStruct a = null;

The error is as follows:

Cannot convert null to 'Example.MyStruct' because it is a non-nullable value type

While int? is a struct in CLR it is somehow counterintuitive that we can assign null to it.
I suppose that null is implicitly casted or boxed to a certian int? value that represents the null value. How is it done precisely? How can I extend MyStruct in such a way that it would be possible to execute the line:

MyStruct a = null;

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风轻花落早 2024-10-11 10:04:08

这是因为 int? 实际上是简写 Nullable。您也可以对您的类型执行相同的操作：

MyStruct? a = null;

null 只能隐式转换为 nullable 类型，这意味着任何引用类型或任何可为 null 的值类型 - 后者对于某些 T 意味着 Nullable。

请注意，这种从 null 转换的能力实际上是一种语言功能 - 编译器将其转换为：

int? a = null;

它们

int? a = new int?();

的含义相同 - 基本上是可空值的“null”值值类型是 HasValue 为 false 的值，默认情况下为 false。它与空引用不同 - 尽管像这样装箱空值将导致空引用。

（这是跨越语言、库和 CLR 边界的功能的一个示例。库部分非常简单，CLR 部分仅与装箱和拆箱有关 - 但语言中的提升运算符等方面有很多内容.)

It's because int? is actually shorthand Nullable<int>. You can do the same thing for your type too:

MyStruct? a = null;

null can only be implicitly converted to a nullable type, which means any reference type or any nullable value type - where the latter means Nullable<T> for some T.

Note that this ability to conver from null is really a language feature - the compiler is converting this:

int? a = null;

into

int? a = new int?();

They mean the same thing - basically a "null" value for a nullable value type is a value where HasValue is false, which it will be by default. It's not the same as a null reference - although boxing a null value like that will result in a null reference.

(This is one example of a feature which crosses language, library and CLR boundaries. The library part is quite simple, the CLR part is only to do with boxing and unboxing - but there's quite a lot in terms of lifted operators etc in the language.)

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情域 2024-10-11 10:04:08

结构体是 C# 中的值类型，而不是引用类型，因此它们不能为 null。

MyStruct? = null;

会起作用，但请记住结构是不可变的，例如：

public struct Point 
{
   public int x, y;

   public Point(int p1, int p2) 
   {
       x = p1;
       y = p2;    
   }
}

void Main()
{

    //create a point
    var point1 = new Point(10, 10); 

    //assign point2 to point, point2
    var point2 = point1; 

    //change the value of point2.x
    point2.x = 20; 

    //you will notice that point1.x does not change
    //this is because point2 is set by value, not reference.
    Console.WriteLine(point1.x);
    Console.WriteLine(point2.x);

}

Structs are value types in C#, not reference types, so they can't be null.

MyStruct? = null;

will work, but remember that structs are immutable, as a example:

public struct Point 
{
   public int x, y;

   public Point(int p1, int p2) 
   {
       x = p1;
       y = p2;    
   }
}

void Main()
{

    //create a point
    var point1 = new Point(10, 10); 

    //assign point2 to point, point2
    var point2 = point1; 

    //change the value of point2.x
    point2.x = 20; 

    //you will notice that point1.x does not change
    //this is because point2 is set by value, not reference.
    Console.WriteLine(point1.x);
    Console.WriteLine(point2.x);

}

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