Overview

A projection operation transforms an object into a new form that often consists only of those properties that will be subsequently used.

Methods

Select vs SelectMany

Select

Select produces one output value for every input value, like so:

To project the first letter in each string in a list of strings:

List<string> words = ["an", "apple", "a", "day"];

In query syntax:

var query = from word in words
            select word.Substring(0, 1);

In method syntax:

IEnumerable<string> result = words.Select(selector => selector.ToUpper());

SelectMany

SelectMany produces a single output that contains the concatenated sub-collections from each input value. The transform function that is passed must return an enumerable sequence of values for each source value. These enumerable sequences are then concatenated to create one large sequence, like so:

To project each word from each string in a list of strings:

List<string> phrases = ["an apple a day", "the quick brown fox"];

In query syntax:

var query = from phrase in phrases
            from word in phrase.Split(' ')
            select word;

In method syntax:

var query = phrases.SelectMany(phrases => phrases.Split(' '));

Zip

Zip methods work on sequences of two or more possibly heterogeneous types.

Assume these collections:

// An int array with 7 elements.
IEnumerable<int> numbers = [1, 2, 3, 4, 5, 6, 7];
// A char array with 6 elements.
IEnumerable<char> letters = ['A', 'B', 'C', 'D', 'E', 'F'];
// A string array with 8 elements.
IEnumerable<string> emoji = [ "🤓", "🔥", "🎉", "👀", "⭐", "💜", "✔", "💯"];

Overload 1 — Using Zip to return tuples

The first Zip overload returns tuples. To project the first two sequences together:

foreach ((int number, char letter) in numbers.Zip(letters))
    Console.WriteLine($"Number: {number} zipped with letter: '{letter}'");

Output:

Number: 1 zipped with letter: 'A'
Number: 2 zipped with letter: 'B'
Number: 3 zipped with letter: 'C'
Number: 4 zipped with letter: 'D'
Number: 5 zipped with letter: 'E'
Number: 6 zipped with letter: 'F'

ℹ️ Important

The output sequence of a zip operation is never longer in length than the shortest input sequence.
Each element needs a corresponding element in the other sequence to “zip” with.

Overload 2 — Adding a third sequence

The second Zip overload also returns a tuple. To project all three sequences together:

foreach ((int number, char letter, string em) in numbers.Zip(letters, emoji))
    Console.WriteLine($"Number: {number} is zipped with letter: '{letter}' and emoji: {em}");

Output:

Number: 1 is zipped with letter: 'A' and emoji: 🤓
Number: 2 is zipped with letter: 'B' and emoji: 🔥
Number: 3 is zipped with letter: 'C' and emoji: 🎉
Number: 4 is zipped with letter: 'D' and emoji: 👀
Number: 5 is zipped with letter: 'E' and emoji: ⭐
Number: 6 is zipped with letter: 'F' and emoji: 💜

Overload 3 — Using a ‘results selector’

The third Zip overload accepts a Func<T1, T2, T3> argument that acts as a “results selector.” To project a new resulting sequence from the sequences being zipped:

foreach (string result in numbers.Zip(letters, (number, letter) => $"{number} = {letter} ({(int)letter})"))
    Console.WriteLine(result);

Output:

1 = A (65)
2 = B (66)
3 = C (67)
4 = D (68)
5 = E (69)
6 = F (70)