Okay, so, at this point I'm sure you're just trolling.
But, I'll make a last attempt anyway.
It's the way you allow classes to use the collection initializer. It's a C# thing. I've linked the documentation.
I'm not gonna preach theory, but, I'm certain you already know it's not the decorator pattern being applied here, just look at the constructor...
Sure, there are elements similar to the decorator, but, you can't really decorate anything with this class.