What does [PartCreationPolicy(CreationPolicy.Shared)]

Question

What does [PartCreationPolicy(CreationPolicy.Shared)] mean?

Answer 1

It means that, when requesting an instance of a class decorated with [PartCreationPolicy(CreationPolicy.Shared)], the CompositionContainer will always return the same instance of this class and not create a new one.

[Export]
[PartCreationPolicy(CreationPolicy.Shared)]
class Foo
{

}

The above class will give the following result:

private void Test()
{
  var foo1 = Container.GetExportedValue<Foo>();
  var foo2 = Container.GetExportedValue<Foo>();
  Console.WriteLine(foo1 == foo2); // => True
}

Answer 2

To add to Julien's answer, I think conceptually you can think of it as a Singleton.