静态只读VS常量 - 不同的组件POV？ [英] Static readonly vs const — different assemblies POV?

问题描述

有关于这个主题的许多问题，但没有（除了之一，但仍然是一个缺一< 。/ A>）正在处理以下情形

There are many questions about this subject , but none (except one but still a short one) are dealing with the following scenario.

这是C＃4本书：

马克还写道：

如果你改变一个常量的值，则需要重建所有的
客户

if you change the value of a const, you need to rebuild all the clients

问：

1），这是为什么？都是静态只读和常量 - 静态

1) Why is that? Are both static readonly and const— static?

2）的其中，的实际值保存？

2) Where actually the values are saved ?

3）如何制作字段静态只读实际上解决这个问题？

3) How does making a field static readonly actually solve this problem "behind the scene" ?

推荐答案

没有，一个const是一个常量，不是一个静态的 - 这是一个特殊的情况下，不同的规则;它的仅设置在编译时（而不是运行时），它的处理方式不同。

no, a const is a const, not a static - it is a special-case, with different rules; it is only set at compile-time (not runtime), and it is handled differently

这里的关键是以下方法：

the crux here is what the following means:

var foo = SomeType.StaticValue;

VS

vs

var bar = SomeType.ConstValue;

在第一个的情况下，从<$读取在运行时的值C $ C> SOMETYPE ，即通过一个 ldsfld ;然而，在第二种情况下，被编译的的值，也就是说，如果 ConstValue 恰好是 123 ，那么第二个就是相同为：

in the first case, it reads the value at runtime from SomeType, i.e. via a ldsfld; however, in the second case, that is compiled to the value, i.e. if ConstValue happens to be 123, then the second is identical to:

var bar = 123;

在运行时，它是从哪里来的事实 SOMETYPE 不存在，作为值（ 123 ）进行了评估由编译器，然后保存。因此，它需要一个重新拾起新值。

at runtime, the fact that it came from SomeType does not exist, as the value (123) was evaluated by the compiler, and stored. Hence it needs a rebuild to pick up new values.

更改为静态只读表示加载值从 SOMETYPE 被保留

Changing to static readonly means that the "load the value from SomeType" is preserved.

所以以下内容：

static int Foo()
{
    return Test.Foo;
}
static int Bar()
{
    return Test.Bar;
}
...
static class Test
{
    public static readonly int Foo = 123;
    public const int Bar = 456;
}

编译如下：

compiles as:

.method private hidebysig static int32 Bar() cil managed
{
    .maxstack 8
    L_0000: ldc.i4 0x1c8
    L_0005: ret 
}

.method private hidebysig static int32 Foo() cil managed
{
    .maxstack 8
    L_0000: ldsfld int32 ConsoleApplication2.Test::Foo
    L_0005: ret 
}

请注意，在酒吧的 LDC 加载值直接（0x1c8 == 456），与<$ C $ 。C>测试完全消失了。

Note that in the Bar, the ldc is loading a value directly (0x1c8 == 456), with Test completely gone.

有关完整的常量的是的使用静态字段，但是实现 - 它是的文字的领域，这意味着：在编译器进行评估，而不是在运行时

For completeness, the const is implemented with a static field, but - it is a literal field, meaning: evaluated at the compiler, not at runtime.

.field public static literal int32 Bar = int32(0x1c8)
.field public static initonly int32 Foo

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