Monostate vs. Singleton

Question

What are the scenarios when one would use a Monostate pattern instead of singleton inorder to maintain a global object?

Edit: I know what Singleton and Monostate patterns are. Have also implemented Singleton in quite a few scenarios. Just want to know the scenarios (case examples) where MonoState pattern needs to be implemented.

For eg. I need to maintain list of columns per screen in my windows forms app. I could use a Singleton Dictionary in this case. However, I am storing a List in the static global var and I wanted to provide indexers (since I need to dynamically add new entry to the list if key is not present) where I could specify ScreenDetails.ScreenName as a key & get the ScreenDetails.ColumnsTable. Since indexers can't operate on a static class I changed the pattern to Monostate.

So I would like to know which other scenarios may compel a user to use Monostate instead of Singletons.

Answer 1

monostate and singleton are two faces of the same medal (global state):

• monostate forces a behaviour (only one value along all class instances)
• singleton forces a structural constraint (only one instance)

singleton usage is not transparent

i.e.:

Singleton singleton = Singleton.getInstance(); 

monostate usage is transparent

i.e.: MonoState m1 = new MonoState(); MonoState m2 = new MonoState(); // same internal state of m1 (e.g. static)