An interesting C# feature that I just stumbled across by accident: You can provide a custom collection initializer for any IEnumerable<T> using an extension method. It allows you to turn something like this:

into this:

by simply providing the Add method that is required for collection initializers as an extension method:

Definitely not something for everyday use, but I can imagine it could clean up some data-heavy bits of code a lot. It appears that this was introduced in C# 6.0, along with indexer initializers.