Question

I have a function that returns IEnumerable<IEnumerable<string>>. I want to use this function, building on the basic IEnumerable, assigning it to a List<List<string>>. This would be easy if I just had one dimension (i.e. one collection), and would be solved by using the ToList() method – but how do i do this for an IEnumerable<IEnumberable<string>> without iterating through each of the items?

Answer 1

You could use Enumerable.SelectMany

For example(from MSDN)

PetOwner[] petOwners = 
    { new PetOwner { Name="Higa, Sidney", 
          Pets = new List<string>{ "Scruffy", "Sam" } },
      new PetOwner { Name="Ashkenazi, Ronen", 
          Pets = new List<string>{ "Walker", "Sugar" } },
      new PetOwner { Name="Price, Vernette", 
          Pets = new List<string>{ "Scratches", "Diesel" } } };

// Query using SelectMany().
IEnumerable<string> query1 = petOwners.SelectMany(petOwner => petOwner.Pets);

Probably the best resource on this topic(J.Skeet):

Reimplementing LINQ to Objects: Part 9 – SelectMany

Here’s another example with a List<List<string>> like in your question:

var lists = new List<List<string>>();
lists.Add(new List<String>(new[] { "A", "B", "C" }));
lists.Add(new List<String>(new[] { "D", "E", "F", "G" }));
lists.Add(new List<String>(new[] { "H", "I", "J", "K", "L" }));
var all = lists.SelectMany(i => i);
String allLetters= String.Format("All letters: {0}",String.Join(",",all));