具有公共可变字段或属性的结构并不邪恶。

Struct methods (as distinct from property setters) which mutate "this" are somewhat evil, only because .net doesn't provide a means of distinguishing them from methods which do not. Struct methods that do not mutate "this" should be invokable even on read-only structs without any need for defensive copying. Methods which do mutate "this" should not be invokable at all on read-only structs. Since .net doesn't want to forbid struct methods that don't modify "this" from being invoked on read-only structs, but doesn't want to allow read-only structs to be mutated, it defensively copies structs in read-only contexts, arguably getting the worst of both worlds.

尽管在只读上下文中处理自突变方法存在问题，但是，可变结构通常提供的语义要比可变类类型优越得多。考虑以下三个方法签名:

struct PointyStruct {public int x,y,z;};
class PointyClass {public int x,y,z;};

void Method1(PointyStruct foo);
void Method2(ref PointyStruct foo);
void Method3(PointyClass foo);

对于每种方法，请回答以下问题:

假设该方法没有使用任何“不安全”代码，它会修改foo吗? 如果在调用方法之前没有对'foo'的外部引用，那么在调用方法之后是否可以存在外部引用?

答案:

问题1: Method1(): no(意图明确) Method2(): yes(明确的意图) Method3(): yes(不确定意图) 问题2: Method1():没有 Method2(): no(除非不安全) Method3():是的

Method1 can't modify foo, and never gets a reference. Method2 gets a short-lived reference to foo, which it can use modify the fields of foo any number of times, in any order, until it returns, but it can't persist that reference. Before Method2 returns, unless it uses unsafe code, any and all copies that might have been made of its 'foo' reference will have disappeared. Method3, unlike Method2, gets a promiscuously-sharable reference to foo, and there's no telling what it might do with it. It might not change foo at all, it might change foo and then return, or it might give a reference to foo to another thread which might mutate it in some arbitrary way at some arbitrary future time. The only way to limit what Method3 might do to a mutable class object passed into it would be to encapsulate the mutable object into a read-only wrapper, which is ugly and cumbersome.

结构数组提供了美妙的语义。给定矩形类型的RectArray[500]，如何将元素123复制到元素456，然后在不影响元素456的情况下，将元素123的宽度设置为555是显而易见的。"RectArray[432] = RectArray[321];…;RectArray[123]。宽度= 555;"。知道Rectangle是一个具有名为Width的整数字段的结构体，就可以知道关于上述语句的所有信息。

Now suppose RectClass was a class with the same fields as Rectangle and one wanted to do the same operations on a RectClassArray[500] of type RectClass. Perhaps the array is supposed to hold 500 pre-initialized immutable references to mutable RectClass objects. in that case, the proper code would be something like "RectClassArray[321].SetBounds(RectClassArray[456]); ...; RectClassArray[321].X = 555;". Perhaps the array is assumed to hold instances that aren't going to change, so the proper code would be more like "RectClassArray[321] = RectClassArray[456]; ...; RectClassArray[321] = New RectClass(RectClassArray[321]); RectClassArray[321].X = 555;" To know what one is supposed to do, one would have to know a lot more both about RectClass (e.g. does it support a copy constructor, a copy-from method, etc.) and the intended usage of the array. Nowhere near as clean as using a struct.

To be sure, there is unfortunately no nice way for any container class other than an array to offer the clean semantics of a struct array. The best one could do, if one wanted a collection to be indexed with e.g. a string, would probably be to offer a generic "ActOnItem" method which would accept a string for the index, a generic parameter, and a delegate which would be passed by reference both the generic parameter and the collection item. That would allow nearly the same semantics as struct arrays, but unless the vb.net and C# people can be pursuaded to offer a nice syntax, the code is going to be clunky-looking even if it is reasonably performance (passing a generic parameter would allow for use of a static delegate and would avoid any need to create any temporary class instances).

就我个人而言，我对Eric Lippert等人对可变值类型的憎恨感到恼火。它们提供了比到处使用的混杂引用类型清晰得多的语义。尽管.net对值类型的支持有一些限制，但在许多情况下，可变值类型比任何其他类型的实体都更适合。

具有公共可变字段或属性的结构并不邪恶。

Struct methods (as distinct from property setters) which mutate "this" are somewhat evil, only because .net doesn't provide a means of distinguishing them from methods which do not. Struct methods that do not mutate "this" should be invokable even on read-only structs without any need for defensive copying. Methods which do mutate "this" should not be invokable at all on read-only structs. Since .net doesn't want to forbid struct methods that don't modify "this" from being invoked on read-only structs, but doesn't want to allow read-only structs to be mutated, it defensively copies structs in read-only contexts, arguably getting the worst of both worlds.

尽管在只读上下文中处理自突变方法存在问题，但是，可变结构通常提供的语义要比可变类类型优越得多。考虑以下三个方法签名:

struct PointyStruct {public int x,y,z;};
class PointyClass {public int x,y,z;};

void Method1(PointyStruct foo);
void Method2(ref PointyStruct foo);
void Method3(PointyClass foo);

对于每种方法，请回答以下问题:

假设该方法没有使用任何“不安全”代码，它会修改foo吗? 如果在调用方法之前没有对'foo'的外部引用，那么在调用方法之后是否可以存在外部引用?

答案:

问题1: Method1(): no(意图明确) Method2(): yes(明确的意图) Method3(): yes(不确定意图) 问题2: Method1():没有 Method2(): no(除非不安全) Method3():是的

Method1 can't modify foo, and never gets a reference. Method2 gets a short-lived reference to foo, which it can use modify the fields of foo any number of times, in any order, until it returns, but it can't persist that reference. Before Method2 returns, unless it uses unsafe code, any and all copies that might have been made of its 'foo' reference will have disappeared. Method3, unlike Method2, gets a promiscuously-sharable reference to foo, and there's no telling what it might do with it. It might not change foo at all, it might change foo and then return, or it might give a reference to foo to another thread which might mutate it in some arbitrary way at some arbitrary future time. The only way to limit what Method3 might do to a mutable class object passed into it would be to encapsulate the mutable object into a read-only wrapper, which is ugly and cumbersome.

结构数组提供了美妙的语义。给定矩形类型的RectArray[500]，如何将元素123复制到元素456，然后在不影响元素456的情况下，将元素123的宽度设置为555是显而易见的。"RectArray[432] = RectArray[321];…;RectArray[123]。宽度= 555;"。知道Rectangle是一个具有名为Width的整数字段的结构体，就可以知道关于上述语句的所有信息。

Now suppose RectClass was a class with the same fields as Rectangle and one wanted to do the same operations on a RectClassArray[500] of type RectClass. Perhaps the array is supposed to hold 500 pre-initialized immutable references to mutable RectClass objects. in that case, the proper code would be something like "RectClassArray[321].SetBounds(RectClassArray[456]); ...; RectClassArray[321].X = 555;". Perhaps the array is assumed to hold instances that aren't going to change, so the proper code would be more like "RectClassArray[321] = RectClassArray[456]; ...; RectClassArray[321] = New RectClass(RectClassArray[321]); RectClassArray[321].X = 555;" To know what one is supposed to do, one would have to know a lot more both about RectClass (e.g. does it support a copy constructor, a copy-from method, etc.) and the intended usage of the array. Nowhere near as clean as using a struct.

To be sure, there is unfortunately no nice way for any container class other than an array to offer the clean semantics of a struct array. The best one could do, if one wanted a collection to be indexed with e.g. a string, would probably be to offer a generic "ActOnItem" method which would accept a string for the index, a generic parameter, and a delegate which would be passed by reference both the generic parameter and the collection item. That would allow nearly the same semantics as struct arrays, but unless the vb.net and C# people can be pursuaded to offer a nice syntax, the code is going to be clunky-looking even if it is reasonably performance (passing a generic parameter would allow for use of a static delegate and would avoid any need to create any temporary class instances).

就我个人而言，我对Eric Lippert等人对可变值类型的憎恨感到恼火。它们提供了比到处使用的混杂引用类型清晰得多的语义。尽管.net对值类型的支持有一些限制，但在许多情况下，可变值类型比任何其他类型的实体都更适合。

如果使用得当，我不相信它们是邪恶的。我不会把它放在我的生产代码中，但我会把它放在像结构化单元测试模拟这样的东西中，其中结构的生命周期相对较小。

使用Eric示例，也许您想要创建该Eric的第二个实例，但是要进行调整，因为这是您的测试的性质(即复制，然后修改)。如果我们只是在测试脚本的剩余部分中使用Eric2，那么Eric的第一个实例发生什么并不重要，除非您计划使用Eric2作为测试比较。

这对于测试或修改那些浅层定义特定对象(结构体的重点)的遗留代码非常有用，但是通过使用不可变的结构体，可以避免令人讨厌的使用。

从哪里开始;-p

埃里克·利珀特的博客总是很适合引用:

这是可变的另一个原因 值类型是邪恶的。试着总是 使值类型不可变。

首先，您很容易丢失更改……例如，从列表中获取内容:

Foo foo = list[0];
foo.Name = "abc";

这改变了什么?没有什么有用的…

属性也是一样:

myObj.SomeProperty.Size = 22; // the compiler spots this one

强迫你做:

Bar bar = myObj.SomeProperty;
bar.Size = 22;
myObj.SomeProperty = bar;

不那么关键的是规模问题;可变对象往往有多个属性;然而，如果你有一个包含两个int型，一个string型，一个DateTime型和一个bool型的结构体，你会很快消耗大量内存。使用类，多个调用方可以共享对同一个实例的引用(引用很小)。

我不会说可变性是邪恶的，但可变性通常是程序员急于提供最大功能的标志。在现实中，这通常是不需要的，反过来，使界面更小，更容易使用，更难使用错误(=更健壮)。

其中一个例子就是竞态条件中的读/写和写/写冲突。这些在不可变结构中根本不可能发生，因为写操作不是有效的操作。

另外，我认为可变性几乎从来都不是真正需要的，程序员只是认为它在未来可能会出现。例如，改变日期是没有意义的。相反，在旧日期的基础上创建一个新的日期。这是一个廉价的操作，所以性能不是一个考虑因素。

可变结构体并不邪恶。

在高绩效环境下，它们是绝对必要的。例如，当缓存线和垃圾收集成为瓶颈时。

我不认为在这些完全有效的用例中使用不可变结构体是“邪恶的”。

我可以看到c#的语法没有帮助区分值类型或引用类型的成员的访问，所以我完全赞成使用强制不变性的不可变结构，而不是可变结构。

然而，与其简单地给不可变结构贴上“邪恶”的标签，我建议接受这种语言，提倡更有帮助和建设性的经验法则。

例如:"struct是默认复制的值类型。如果你不想复制他们，你需要一份推荐信。 “首先尝试使用只读结构体”。