Private constructor

As the name suggests a private constructor is a constructor which is declared as 'private' and so is  only accessible from within the class itself.

It is used in the following scenarios:

1. If you want to prevent a class from being instantiated (because it only contains shared members) you can introduce a private constructor which does nothing. You need to include such a constructor, otherwise the system will by default provide a default public constructor which you don't want.

However, rather than using a class with a private constructor in this scenario, you can use a module to achieve much the same result.

2. If you want to control the number of instances being created, you can only allow instantiation via a shared 'Create' method which calls a private constructor internally. This approach is used for creating singletons.

3. If you're using the factory method pattern (see  http://en.wikipedia.org/wiki/Factory_method_pattern), you can force clients to use the shared factory methods by only having a private constructor.