If a static readonly member calls a static method to get a value, is it done synchronously?

Question

Given:

public class MyClass
{
    private static readonly Dictionary<string,int> mydict = CreateDictionary();

    private static Dictionary<string,int> CreateDictionary() { ... }
}

Is this done synchronously? (i.e. can two quick instantiations of MyClass cause CreateDictionary() to be called twice?

Answer 1

Yes, it's thread safe. Is the C# static constructor thread safe?

Static constructors are guaranteed to be run only once per application domain, before any instances of a class are created or any static members are accessed. http://msdn.microsoft.com/en-us/library/aa645612.aspx

Static field initialization is part of the static constructor. The fact that the field is readonly doesn't change anything

Some IL code as requested (taken from Try Roslyn http://goo.gl/ayIMG0)

.method private hidebysig specialname rtspecialname static 
    void .cctor () cil managed 
{
    // Method begins at RVA 0x205f
    // Code size 11 (0xb)
    .maxstack 8

    IL_0000: call class [mscorlib]System.Collections.Generic.Dictionary`2<string, int32> MyClass::CreateDictionary()
    IL_0005: stsfld class [mscorlib]System.Collections.Generic.Dictionary`2<string, int32> MyClass::mydict
    IL_000a: ret
} // end of method MyClass::.cctor

where .cctor is the special name of the static constructors. The call to CreateDictionary and the assignment to mydict are quite evident.

Answer 2

The accepted answer is correct; the initialization will happen either zero or one times but never twice. But I would add a caveat. In your example the CLR and the C# language reserve the right to initialize the field earlier than you might expect. If you write:

public class MyClass
{
    private static readonly Dictionary<string,int> mydict = CreateDictionary();
    static MyClass() {}

then the CLR and C# guarantee that the field will be initialized when the first static method is called on MyClass or when the first instance of MyClass is created. If you omit the static constructor then the CLR and C# are permitted, but not required, to initialize the field at any time prior to those events. In particular, suppose you have a method M which calls a static method of MyClass. The CLR might decide to run the static initializer of MyClass.mydict when M is jitted, and not when M actually calls the static method. This can in some rare situations lead to surprising results.

Do a web search on the beforefieldinit optimization for more details. Jon Skeet has a good article on this.