How Select and SelectMany Works in C#

Select

The Select method in LINQ projects each element of a sequence into a new form. It is used to transform elements in a collection.

Example

public void SelectMethod()
{
    var numbers = new List<int> { 1, 2, 3, 4, 5 };
    var squares = numbers.Select(x => x * x);

    foreach (var square in squares)
    {
        Console.WriteLine(square);
    }
}

In this example, Select takes each number in the list, squares it, and returns a new collection of squared numbers.

SelectMany

The SelectMany method projects each element of a sequence to an IEnumerable<T> and flattens the resulting sequences into one sequence. It is useful when working with collections of collections

public void SelectManyMethod()
{
    var lists = new List<List<int>>
    {
       new List<int>{1,2,3},
       new List<int>{4,5,6},
       new List<int>{7,8,9}
    };
    var flatList = lists.SelectMany(x => x);
    foreach (var list in flatList)
    {
        Console.WriteLine(list);
    }
}

Output