How to join together all the elements in an IEnumerable of IEnumerables?

Question

Just in case you're wondering how this came up, I'm working with some resultsets from Entity Framework.

I have an object that is an IEnumerable<IEnumerable<string>>; basically, a list of lists of strings.

I want to merge all the lists of strings into one big list of strings.

What is the best way to do this in C#.net?

Answer 1

Use the LINQ SelectMany method:

IEnumerable<IEnumerable<string>> myOuterList = // some IEnumerable<IEnumerable<string>>...
IEnumerable<String> allMyStrings = myOuterList.SelectMany(sl => sl);

To be very clear about what's going on here (since I hate the thought of people thinking this is some kind of sorcery, and I feel bad that some other folks deleted the same answer):

SelectMany is an extension method ( a static method that through syntactic sugar looks like an instance method on a specific type) on IEnumerable<T>. It takes your original enumeration of enumerations and a function for converting each item of that into a enumeration.

Because the items are already enumerations, the conversion function is simple- just return the input (sl => sl means "take a paremeter named sl and return it"). SelectMany then provides an enumeration over each of these in turn, resulting in your "flattened" list..