Singleton Thread-Safety Question

[Cool Guy]

In the following implementation of Singleton, is the instance constructor
guaranteed to execute only once regardless of how many threads try to
access it at one time?

class Test {
public static readonly Test Instance = new Test();

Test() {
// set up state. this mustn't execute more than once
}
}

[Jon Skeet [C# MVP]]

Re: Singleton Thread-Safety Question

Cool Guy <coolguy@abc.xy z> wrote:[color=blue]
> In the following implementation of Singleton, is the instance constructor
> guaranteed to execute only once regardless of how many threads try to
> access it at one time?
>
> class Test {
> public static readonly Test Instance = new Test();
>
> Test() {
> // set up state. this mustn't execute more than once
> }
> }[/color]

Yes, that's guaranteed to only execute once. There are some caveats
about ordering when there's potential for a deadlock, but in simple
situations like the above (which 99% of static initializers fall into)
it's fine.

--
Jon Skeet - <skeet@pobox.co m>

http://www.pobox.com/~skeet

If replying to the group, please do not mail me too

[Cool Guy]

Re: Singleton Thread-Safety Question

"Jon Skeet [C# MVP]" <skeet@pobox.co m> wrote:
[color=blue][color=green]
>> class Test {
>> public static readonly Test Instance = new Test();
>>
>> Test() {
>> // set up state. this mustn't execute more than once
>> }
>> }[/color]
>
> Yes, that's guaranteed to only execute once. There are some caveats
> about ordering when there's potential for a deadlock, but in simple
> situations like the above (which 99% of static initializers fall into)
> it's fine.[/color]

Could you give an example of one such caveat?

[Jon Skeet [C# MVP]]

Re: Singleton Thread-Safety Question

Cool Guy <coolguy@abc.xy z> wrote:[color=blue][color=green]
> > Yes, that's guaranteed to only execute once. There are some caveats
> > about ordering when there's potential for a deadlock, but in simple
> > situations like the above (which 99% of static initializers fall into)
> > it's fine.[/color]
>
> Could you give an example of one such caveat?[/color]

Sure. It's not actually a matter of initializing twice, but of the
incompletely initialized state being visible temporarily. For instance:

using System;

public class A
{
static string Something = B.Bar;
public static string Foo = "Hello";
}

public class B
{
public static string Bar = "There";

static B()
{
Console.WriteLi ne ("In B, A.Foo='{0}'", A.Foo);
}
}

public class Test
{
static void Main()
{
Console.WriteLi ne (A.Foo);
}
}

The results are:
In B, A.Foo=''
Hello

There are further rules for multithreaded system - I suggest you look
in the CLI spec, section 9.5.3.3 of partition 1 for further details.

--
Jon Skeet - <skeet@pobox.co m>

http://www.pobox.com/~skeet

If replying to the group, please do not mail me too