从程序员的角度来看，还有一些其他的极端情况可能导致不可预测的行为。

不可变值类型和只读字段

    // Simple mutable structure. 
    // Method IncrementI mutates current state.
    struct Mutable
    {
        public Mutable(int i) : this() 
        {
            I = i;
        }

        public void IncrementI() { I++; }

        public int I { get; private set; }
    }

    // Simple class that contains Mutable structure
    // as readonly field
    class SomeClass 
    {
        public readonly Mutable mutable = new Mutable(5);
    }

    // Simple class that contains Mutable structure
    // as ordinary (non-readonly) field
    class AnotherClass 
    {
        public Mutable mutable = new Mutable(5);
    }

    class Program
    {
        void Main()
        {
            // Case 1. Mutable readonly field
            var someClass = new SomeClass();
            someClass.mutable.IncrementI();
            // still 5, not 6, because SomeClass.mutable field is readonly
            // and compiler creates temporary copy every time when you trying to
            // access this field
            Console.WriteLine(someClass.mutable.I);

            // Case 2. Mutable ordinary field
            var anotherClass = new AnotherClass();
            anotherClass.mutable.IncrementI();

            // Prints 6, because AnotherClass.mutable field is not readonly
            Console.WriteLine(anotherClass.mutable.I);
        }
    }

可变值类型和数组

假设我们有一个可变结构的数组，我们为该数组的第一个元素调用IncrementI方法。你希望从这个电话中得到什么行为?它应该改变数组的值还是只改变一个副本?

    Mutable[] arrayOfMutables = new Mutable[1];
    arrayOfMutables[0] = new Mutable(5);

    // Now we actually accessing reference to the first element
    // without making any additional copy
    arrayOfMutables[0].IncrementI();

    // Prints 6!!
    Console.WriteLine(arrayOfMutables[0].I);

    // Every array implements IList<T> interface
    IList<Mutable> listOfMutables = arrayOfMutables;

    // But accessing values through this interface lead
    // to different behavior: IList indexer returns a copy
    // instead of an managed reference
    listOfMutables[0].IncrementI(); // Should change I to 7

    // Nope! we still have 6, because previous line of code
    // mutate a copy instead of a list value
    Console.WriteLine(listOfMutables[0].I);

因此，只要您和团队的其他成员清楚地了解您在做什么，可变结构就不是邪恶的。但是有太多的极端情况，当程序行为与预期不同时，这可能会导致微妙的难以产生和难以理解的错误。

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)。

就我个人而言，当我看代码时，下面的代码看起来相当笨拙:

data.value.set ( data.value.get () + 1 ) ;

而不是简单地

数据.值++ ;或数据值 = 数据值 + 1 ;

数据封装在传递类时非常有用，并且您希望确保以受控的方式修改值。然而，当你拥有公共的set和get函数，它们所做的仅仅是将值设置为传递进来的值时，这比简单地传递公共数据结构有什么改进呢?

当我在类中创建私有结构时，我创建了该结构来将一组变量组织到一个组中。我希望能够在类范围内修改该结构，而不是获得该结构的副本并创建新实例。

对我来说，这阻止了有效使用用于组织公共变量的结构，如果我想要访问控制，我会使用类。

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。

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”。

当某种东西可以变异时，它就获得了一种认同感。

struct Person {
    public string name; // mutable
    public Point position = new Point(0, 0); // mutable

    public Person(string name, Point position) { ... }
}

Person eric = new Person("Eric Lippert", new Point(4, 2));

Because Person is mutable, it's more natural to think about changing Eric's position than cloning Eric, moving the clone, and destroying the original. Both operations would succeed in changing the contents of eric.position, but one is more intuitive than the other. Likewise, it's more intuitive to pass Eric around (as a reference) for methods to modify him. Giving a method a clone of Eric is almost always going to be surprising. Anyone wanting to mutate Person must remember to ask for a reference to Person or they'll be doing the wrong thing.

如果你让类型是不可变的，这个问题就消失了;如果我不能修改eric，无论我收到eric还是eric的克隆对我来说都没有区别。更一般地说，如果类型的所有可观察状态都保存在以下成员中，则按值传递是安全的:

不可变的 引用类型 安全通过价值

如果满足这些条件，那么可变值类型的行为就像引用类型一样，因为浅拷贝仍然允许接收方修改原始数据。

The intuitiveness of an immutable Person depends on what you're trying to do though. If Person just represents a set of data about a person, there's nothing unintuitive about it; Person variables truly represent abstract values, not objects. (In that case, it'd probably be more appropriate to rename it to PersonData.) If Person is actually modeling a person itself, the idea of constantly creating and moving clones is silly even if you've avoided the pitfall of thinking you're modifying the original. In that case it'd probably be more natural to simply make Person a reference type (that is, a class.)

诚然，函数式编程已经告诉我们，使所有东西都不可变是有好处的(没有人可以秘密地保留对eric的引用并改变他)，但由于这在OOP中不是惯用的，因此对于使用您的代码的其他人来说仍然是不直观的。