How to merge a list of lists with same type of items to a single list of items?

Problem

The question seems perplexing, but as described in the following codes, it becomes much clearer:

   List<List<T>> listOfList;
   // add three lists of List<T> to listOfList, for example
   /* listOfList = new {
        { 1, 2, 3}, // list 1 of 1, 3, and 3
        { 4, 5, 6}, // list 2
        { 7, 8, 9}  // list 3
        };
   */
   List<T> list = null;
   // how to merger all the items in listOfList to list?
   // { 1, 2, 3, 4, 5, 6, 7, 8, 9 } // one list
   // list = ???

Not sure if you can do it with C# LINQ or Lambda?

In other words, how can I concatenate or “flatten” a collection of lists?

Asked by David.Chu.ca

Solution #1

Use the SelectMany extension method to get a list of all the people in your group.

list = listOfList.SelectMany(x => x).ToList();

Answered by JaredPar

Solution #2

The C# integrated syntax version is as follows:

var items =
    from list in listOfList
    from item in list
    select item;

Answered by Joe Chung

Solution #3

Is this what you mean?

var listOfList = new List<List<int>>() {
    new List<int>() { 1, 2 },
    new List<int>() { 3, 4 },
    new List<int>() { 5, 6 }
};
var list = new List<int> { 9, 9, 9 };
var result = list.Concat(listOfList.SelectMany(x => x));

foreach (var x in result) Console.WriteLine(x);

9 9 9 1 2 3 4 5 6 9 9 9 1 2 3 4 5 6 9 9 9 1 2 3 4 5 6 9 9 9 1

Answered by IRBMe

Solution #4

For List>> etc., make use of

list.SelectMany(x => x.SelectMany(y => y)).ToList();

This was mentioned in a comment, but it merits its own response, in my opinion.

Answered by Arman

Post is based on https://stackoverflow.com/questions/1191054/how-to-merge-a-list-of-lists-with-same-type-of-items-to-a-single-list-of-items