Static constructor can run after the non-static constructor. Is this a compiler bug?

问题

The output from the following program is:

Non-Static
Static
Non-Static

Is this a compiler bug? I expected:

Static
Non-Static
Non-Static

because I thought the static constructor was ALWAYS called before the non-static constructor.

I tested this with Visual Studio 2010 using both .net 3.5 and .net 4.0.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace StaticConstructorBug
{
    class Program
    {
        static void Main(string[] args)
        {
            var mc = new MyClass();

            Console.ReadKey();
        }
    }

    public class MyClass
    {
        public MyClass()
        {
            Console.WriteLine("Non-static");
        }

        static MyClass()
        {
            Console.WriteLine("Static");
        }

        public static MyClass aVar = new MyClass();
    }
}

回答1:

See ECMA 334 §17.4.5.1:

17.4.5.1 Static field initialization

The static field variable initializers of a class declaration correspond to a sequence of assignments that are executed in the textual order in which they appear in the class declaration. If a static constructor (§17.11) exists in the class, execution of the static field initializers occurs immediately prior to executing that static constructor. Otherwise, the static field initializers are executed at an implementation-dependent time prior to the first use of a static field of that class

Specifically: "execution of the static field initializers occurs immediately prior to executing that static constructor".

Your static MyClass aVar must be initialized before your static constructor executes (or, at least, it must appear that way). Without that static member, the static constructor should be called before any non-static constructors.

If you still want a MyClass singleton, you can put it in a container class and refer to it using that, e.g.:

public static class MyClassSingleton
{
    public static MyClass aVar = new MyClass();
}

回答2:

It is caused by line public static MyClass aVar = new MyClass();.

In fact the aVar = new MyClass(); is prepend to the static contrstructor. So your static constructor:

static MyClass() {
    Console.WriteLine("Static");
}

is changed to:

static MyClass() {
    aVar = new MyClass(); // this will run instance contstructor and prints "Non-Static"
    Console.WriteLine("Static");
}

回答3:

This public static MyClass aVar = new MyClass(); is part of your static constructor. If you look at it with reflector you will see the following:

static MyClass()
{
    aVar = new Program.MyClass();
    Console.WriteLine("Static");
}

So your result should be obvious now.

回答4:

From MSDN Link:

A static constructor is called automatically to initialize the class before the first instance is created or any static members are referenced.

My Guess this is because of the static instantiation of the instance on the last line, but according to MSDN the static constructor should happen before the first instance is called.

来源：https://stackoverflow.com/questions/2925611/static-constructor-can-run-after-the-non-static-constructor-is-this-a-compiler

标签

static-constructor