static readonly field initializer vs static constructor initialization

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    
}