Static instances

[John Jenkins]

Hi,
I have been doing some work on an app, that has a particular requirement for an object that can only have a single instance active. A previous developer had simply created single static class.

private static MemoryStreamCreator memThing=new MemoryStreamCreator();

I thought that perhaps a Singleton pattern may have been more appropriate.

1. Is this sensible or is the code above ok? The only concern I had was that another developer in another class, within the same project could use the same line.

If it is, I was reading a blog

http://blogs.msdn.com/cjohnson/archive/2004/06/10/152179.aspx

Which said creating an instance from within a Singleton was not thread safe. Can someone provide me with some code which will guarantee thread safety? I had tried to use the code from the above link, in this example but there appears to be an issue. I think it is related to the fact that the _synclock object is not static. Can someone explain the code supplied in the link.



public class Yada
{
private static MyFunkyObject _foo;
private Object _synclock = new Object();

public static MyFunkyObject Foo
{

get
{

lock(_synclock)
{
if(_foo == null)
_foo = new MyFunkyObject();
}

return _foo;
}
}
}





I have never had to to try to lock an object from a static method. I normally just use

lock(this)......

Also if creating the singleton is not threadsafe then am I better going with the original implementation?



Thanks

 

[Jimbo]

I thought that perhaps a Singleton pattern may have been more appropriate.

I have never had to to try to lock an object from a static method. I
normally just use

lock(this)......

Also if creating the singleton is not threadsafe then am I better going
with the original implementation?

Try this:

using System;
using System.Runtime.CompilerServices;

namespace Test
{
public class Singleton
{

private static Singleton s;

protected Singleton()
{
}

[MethodImpl(MethodImplOptions.Synchronized)]
public static UddiCache getInstance()
{
if (singleton == null)
{
s = new Singleton();
}
return s;
}
}
}

Jimbo

 

[Jimbo]

I thought that perhaps a Singleton pattern may have been more
appropriate.

I have never had to to try to lock an object from a static method. I
normally just use

lock(this)......

Also if creating the singleton is not threadsafe then am I better
going with the original implementation?

Try this:

using System;
using System.Runtime.CompilerServices;

namespace Test
{
public class Singleton
{

private static Singleton s;

protected Singleton()
{
}

[MethodImpl(MethodImplOptions.Synchronized)]
public static UddiCache getInstance()
{
if (singleton == null)
{
s = new Singleton();
}
return s;
}
}
}

Jimbo

D'oh! In making the code generic I ****ed it up. The getInstance()
method should be:

public static Singleton getInstance()
{
if (s == null)
{
s = new Singleton();
}
return s;
}

 

[Jeff Louie]

This may help:

http://www.geocities.com/Jeff_Louie/OOP/oop4.htm
http://www.geocities.com/Jeff_Louie/IL_Net/singleton_pattern.htm

At the IL level there is not a lot of difference between a singleton or
static call. If
you need the added level of indirection, subclassing or use of an
interface
contract, or access control then by all means use the singleton.

Regards,
Jeff

 

[Jon Skeet [C# MVP]]

Hi, I have been doing some work on an app, that has a particular
requirement for an object that can only have a single instance
active. A previous developer had simply created single static class.

private static MemoryStreamCreator memThing=new MemoryStreamCreator();

I thought that perhaps a Singleton pattern may have been more appropriate.

That's a normal implementation for a Singleton though - it's not clear
to me what the previous developer had done...

1. Is this sensible or is the code above ok? The only concern I had
was that another developer in another class, within the same project
could use the same line.

If it is, I was reading a blog

http://blogs.msdn.com/cjohnson/archive/2004/06/10/152179.aspx

Which said creating an instance from within a Singleton was not
thread safe.

No, it says that creating an instance without any locking isn't thread-
safe.

Can someone provide me with some code which will
guarantee thread safety?

See http://www.pobox.com/~skeet/csharp/singleton.html

I have never had to to try to lock an object from a static method. I
normally just use

lock(this)......

That's a bad idea too. See
http://www.pobox.com/~skeet/csharp/threads/lockchoice.shtml