SelectMany for flattening in C Sharp (C#) - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does SelectMany do in LINQ?

SelectMany takes a collection of collections and flattens it into a single collection by combining all inner collections into one.

beginner

How is SelectMany different from Select?

Select projects each element into a new form but keeps the nested collections, while SelectMany flattens those nested collections into one sequence.

beginner

Example: What is the output of this code?<br>

var numbers = new List<List<int>> { new() {1, 2}, new() {3, 4} };
var flat = numbers.SelectMany(x => x);
foreach(var n in flat) Console.Write(n + " ");

The output is: 1 2 3 4

Because SelectMany flattens the list of lists into a single list of numbers.

beginner

Can SelectMany be used with arrays or only lists?

SelectMany works with any IEnumerable<T>, so it works with arrays, lists, and other collections.

beginner

Why is SelectMany useful in real life?

It helps when you have nested data like a list of orders where each order has many items, and you want to work with all items in one list.

What does SelectMany do in LINQ?

A Groups elements by a key

B Filters elements based on a condition

C Sorts elements in ascending order

D Flattens nested collections into a single collection

SelectMany combines nested collections into one flat collection.

Which method keeps nested collections without flattening?

A SelectMany

B Where

C Select

D OrderBy

Select projects elements but keeps nested collections intact.

Given var data = new List<List<int>> { new() {1,2}, new() {3,4} };, what does data.SelectMany(x => x) return?

A A flat list: 1, 2, 3, 4

B A list of lists

C An empty list

D A list with two elements

SelectMany flattens the nested lists into one list with all elements.

Can SelectMany be used on arrays?

A Yes, on any IEnumerable&lt;T&gt;

B No, only on lists

C Only on dictionaries

D Only on strings

SelectMany works on any collection implementing IEnumerable<T>, including arrays.

Why use SelectMany when working with nested data?

A To remove duplicates

B To flatten nested collections for easier processing

C To sort nested collections

D To convert collections to strings

SelectMany helps flatten nested data so you can work with all elements easily.

Explain in your own words what SelectMany does and give a simple example.

Think about turning many small boxes into one big box.

Describe a real-life scenario where using SelectMany would make your code simpler.

Imagine you have many shopping bags and want to see all items at once.