静态只读字段初始VS静态构造函数初始化 [英] static readonly field initializer vs static constructor initialization

问题描述

下面是两种不同的方法来初始化静态只读域。有两种方法之间的差异？如果是的话，应该何时之一pferred比其他$ P $？

  A级
{
    私人静态只读字符串的connectionString = WebConfigurationManager.ConnectionStrings [SomeConnection]的ConnectionString。
}B类
{
    私人静态只读字符串的connectionString;    静态B（）
    {
        的connectionString = WebConfigurationManager.ConnectionStrings [SomeConnection]的ConnectionString。
    }
}
 

解决方案

有这两个，它可以在IL code可以看出之间有一个微妙的不同 - 把一个明确的静态构造函数告诉C＃编译器不标记其类型为 beforefieldinit 。当类型初始化运行并了解这是有用的，当写懒单身的beforefieldinit影响一>，例如

在短暂的区别是这样的：

 的.class民营汽车ANSI beforefieldinit一个
的.class私人汽车ANSI B
 

在所有其他方面它们是相同的。从反射输出：

A类：

 的.class民营汽车ANSI beforefieldinit一个
    扩展[mscorlib程序] System.Object的
{
    。方法私人hidebysig specialname rtspecialname静态无效.cctor（）CIL管理
    {
        .maxstack 8
        L_0000：ldsfld类[mscorlib程序] System.Collections.Generic.Dictionary`2＆LT;字符串，类连接＆GT; WebConfigurationManager ::的ConnectionStrings
        L_0005：ldstrSomeConnection
        ！L_000a：callvirt实例1 [mscorlib程序] System.Collections.Generic.Dictionary`2＆LT;字符串，类连接＆GT; :: get_Item（！0）
        L_000f：ldfld字符串连接::的ConnectionString
        L_0014：stsfld字符串A ::的connectionString
        L_0019：RET
    }    市民。方法hidebysig specialname rtspecialname实例无效.ctor（）CIL管理
    {
        .maxstack 8
        L_0000：ldarg.0
        L_0001：呼叫实例无效[mscorlib程序] System.Object的构造函数::（）。
        L_0006：RET
    }    技术领域私有静态initonly字符串的connectionString
}
 

B类：

 的.class民营汽车ANSI B
    扩展[mscorlib程序] System.Object的
{
    。方法私人hidebysig specialname rtspecialname静态无效.cctor（）CIL管理
    {
        .maxstack 8
        L_0000：NOP
        L_0001：ldsfld类[mscorlib程序] System.Collections.Generic.Dictionary`2＆LT;字符串，类连接＆GT; WebConfigurationManager ::的ConnectionStrings
        L_0006：ldstrSomeConnection
        ！L_000b：callvirt实例1 [mscorlib程序] System.Collections.Generic.Dictionary`2＆LT;字符串，类连接＆GT; :: get_Item（！0）
        L_0010：ldfld字符串连接::的ConnectionString
        L_0015：stsfld字符串B ::的connectionString
        L_001a：RET
}    市民。方法hidebysig specialname rtspecialname实例无效.ctor（）CIL管理
    {
        .maxstack 8
        L_0000：ldarg.0
        L_0001：呼叫实例无效[mscorlib程序] System.Object的构造函数::（）。
        L_0006：RET
    }
    技术领域私有静态initonly字符串的connectionString
}
 

Below are two different ways to initialize static readonly fields. Is there a difference between the two approaches? If yes, when should one be preferred over the other?

class A
{
    private static readonly string connectionString = WebConfigurationManager.ConnectionStrings["SomeConnection"].ConnectionString;
}

class B
{
    private static readonly string connectionString;

    static B()
    {
        connectionString = WebConfigurationManager.ConnectionStrings["SomeConnection"].ConnectionString;
    }
}

解决方案

There is one subtle difference between these two, which can be seen in the IL code - putting an explicit static constructor tells the C# compiler not to mark the type as beforefieldinit. The beforefieldinit affects when the type initializer is run and knowing about this is useful when writing lazy singletons in C#, for example.

In brief the difference is this:

.class private auto ansi beforefieldinit A
.class private auto ansi B

In all other aspects they are the same. Output from Reflector:

Class A:

.class private auto ansi beforefieldinit A
    extends [mscorlib]System.Object
{
    .method private hidebysig specialname rtspecialname static void .cctor() cil managed
    {
        .maxstack 8
        L_0000: ldsfld class [mscorlib]System.Collections.Generic.Dictionary`2<string, class Connection> WebConfigurationManager::ConnectionStrings
        L_0005: ldstr "SomeConnection"
        L_000a: callvirt instance !1 [mscorlib]System.Collections.Generic.Dictionary`2<string, class Connection>::get_Item(!0)
        L_000f: ldfld string Connection::ConnectionString
        L_0014: stsfld string A::connectionString
        L_0019: ret 
    }

    .method public hidebysig specialname rtspecialname instance void .ctor() cil managed
    {
        .maxstack 8
        L_0000: ldarg.0 
        L_0001: call instance void [mscorlib]System.Object::.ctor()
        L_0006: ret 
    }

    .field private static initonly string connectionString
} 

Class B:

.class private auto ansi B
    extends [mscorlib]System.Object
{
    .method private hidebysig specialname rtspecialname static void .cctor() cil managed
    {
        .maxstack 8
        L_0000: nop 
        L_0001: ldsfld class [mscorlib]System.Collections.Generic.Dictionary`2<string, class Connection> WebConfigurationManager::ConnectionStrings
        L_0006: ldstr "SomeConnection"
        L_000b: callvirt instance !1 [mscorlib]System.Collections.Generic.Dictionary`2<string, class Connection>::get_Item(!0)
        L_0010: ldfld string Connection::ConnectionString
        L_0015: stsfld string B::connectionString
        L_001a: ret 
}

    .method public hidebysig specialname rtspecialname instance void .ctor() cil managed
    {
        .maxstack 8
        L_0000: ldarg.0 
        L_0001: call instance void [mscorlib]System.Object::.ctor()
        L_0006: ret 
    }


    .field private static initonly string connectionString    
}

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