静态只读字段初始化程序与静态构造函数初始化

Question

以下是初始化静态只读字段的两种不同方法.两种方法之间有区别吗？如果是的话,什么时候应该优先于另一个呢？

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;
    }
}

Answer 1

这两者之间有一个细微的区别,可以在IL代码中看到 - 将显式静态构造函数告知C#编译器不要将类型标记为beforefieldinit.beforefieldinit影响何时运行类型初始化程序,例如,在C#中编写惰性单例时,了解这一点很有用.

简而言之,区别在于:

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

在所有其他方面,它们是相同的.反射器的输出:

A类:

.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
} 

B级:

.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    
}

Answer 2

该beforefieldinit属性表示初始化是如何发生的.

在显式静态构造函数初始化的情况下,静态成员的初始化在访问类型时发生.在类A的情况下给出的示例中,初始化将仅在首次引用connectionString时发生,而在类B的情况下,初始化将在第一次引用类型类B时发生,而不一定访问connectionString.

只有C#(.NET 4.0)使我们能够控制如何初始化静态成员.使用VB.NET只能使用非beforefieldinit方法,而使用C++/CLI时,只能使用beforefieldinit机制.

Answer 3

他们基本上是相同的,但如果你碰巧有两个只读分配一个静态字段和静态类型构造,只读分配第一次出现.