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

来自 C# 4 书:

马克还写道:

如果你改变了一个 const 的值，你需要重建所有的客户

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

问题:

1) 为什么会这样?static readonly 和 const 都是 static 吗?

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

2) 这些值实际上保存在哪里?

2) Where actually the values are saved ?

3) 使字段static readonly实际上是如何解决幕后"这个问题的?

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

推荐答案

不，const 是 const，而不是 static - 它是一种特殊情况，具有不同的规则；它仅在编译时(而非运行时)设置，并且处理方式不同

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

这里的关键是下面的意思:

the crux here is what the following means:

var foo = SomeType.StaticValue;

对

var bar = SomeType.ConstValue;

在 first 的情况下，它在运行时从 SomeType 读取值，即通过 ldsfld；但是，在第二种情况下，即编译为值，即如果ConstValue恰好是123，那么第二种是等同于:

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，它来自 SomeType 的事实不存在，因为值 (123) 由 评估编译器，并存储.因此，它需要重建以获取新的价值.

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.

改为static readonly表示保留从SomeType加载值".

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

所以如下:

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;
}

编译为:

.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 Bar 中，ldc 正在加载一个值 直接 (0x1c8 == 456)，使用 Test 完全消失了.

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