“GetOrCreate” - does that idiom have an established name?

Question

Ok, consider this common idiom that most of us have used many times (I assume):

class FooBarDictionary
{
    private Dictionary fooBars;

          


        

Answer 1

In C#...

... I've got a DefaultingDictionary<> which does about this. As a bonus

• you can specify a default value or factory function to create values for missing keys:
• it comes with implicit conversion from IDictionary<> (wrapping the dictionary)
• it comes with extension methods to morph any dictionary into a DefaultingDictionary<> on the fly

Full Code:

• https://gist.github.com/2789882#file_defaulting_dictionary.cs

The extensions .AsDefaulting can be used to transparently use any IDictionary<> as a defaulting one, so you can opt to use any dictionary (even e.g. obtained from a thirdparty API) as a defaulting one, and the underlying container will be updated with any 'auto-vivified' items.

Use it like

IDictionary dict = LoadFromDatabase();

// using a fixed value
SomeFunc(dict.AsDefaulting(defaultItem));

// using an independent generator function
var defaulting = dict.AsDefaulting(() => new MyItem { Id = System.Guid.NewGuid() });

// using a keydepedent generator function
var defaulting = dict.AsDefaulting(key => LazyLoadFromDatabase(key));

Some test cases

are included:

• https://gist.github.com/2789882#file_defaulting_dictionary_tests.cs