HowTo: Thread safe singleton implementation of a class
Normally, if you would like a singleton implementation of a class, you would go about it a little bit like this:
public class BloggingAbout
{
private static BloggingAbout bloggingAbout;
private BloggingAbout()
{
}
public static BloggingAbout BloggingAboutInstance
{
get
{
if (bloggingAbout == null)
{
bloggingAbout = new BloggingAbout();
}
return bloggingAbout;
}
}
}
The trouble here is that this implementation is not thread safe. The reason I post this is I recently ran into some code of an old colleague of mine (from my former employee) and some tests showed his singleton class was instantiated several times when the application was stressed with requests from the start, which caused unwanted lag in the application. Apparently him not implementing the Singleton pattern thread safe caused this, so I fixed it.
It's not thread safe because two different threads could both evaluate if (bloggingAbout == null) and find it to be true and create an instance, which violates the singleton pattern. To make sure this doesn't happen, you could use locking which would result in this code:
public class BloggingAbout
{
private static BloggingAbout bloggingAbout;
private static object lockObject = new object();
private BloggingAbout()
{
}
public static BloggingAbout BloggingAboutInstance
{
get
{
lock(lockObject)
{
if (bloggingAbout == null)
{
bloggingAbout = new BloggingAbout();
}
return bloggingAbout;
}
}
}
}
The lock statement ensures that one thread does not enter a critical section of code while another thread is in that critical section. If another thread attempts to enter a locked section of code, it will wait (block) until the object is released. This is the reason the lockObject has to exist even before calling the static property BloggingAboutInstance.