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

Question

Use the SelectMany extension method

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

Here's the C# integrated syntax version:

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

Do you mean this?

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);

Results in: 9 9 9 1 2 3 4 5 6

For List<List<List<x>>> and so on, use

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

This has been posted in a comment, but it does deserves a separate reply in my opinion.