Double-checked locking in .NET

.NET 4.0 has a new type: Lazy<T> that takes away any concern about getting the pattern wrong. It's part of the new Task Parallel Library.

See the MSDN Parallel Computing Dev Center: http://msdn.microsoft.com/en-us/concurrency/default.aspx

BTW, there's a backport (I believe it is unsupported) for .NET 3.5 SP1 available here.

Implementing the Singleton Pattern in C# talks about this problem in the third version.

It says:

Making the instance variable volatile can make it work, as would explicit memory barrier calls, although in the latter case even experts can't agree exactly which barriers are required. I tend to try to avoid situations where experts don't agree what's right and what's wrong!

The author seems to imply that double locking is less likely to work than other strategies and thus should not be used.

Double-checking locking now works in Java as well as C# (the Java memory model changed and this is one of the effects). However, you have to get it exactly right. If you mess things up even slightly, you may well end up losing the thread safety.

As other answers have stated, if you're implementing the singleton pattern there are much better ways to do it. Personally, if I'm in a situation where I have to choose between implementing double-checked locking myself and "lock every time" code I'd go for locking every time until I'd got real evidence that it was causing a bottleneck. When it comes to threading, a simple and obviously-correct pattern is worth a lot.