SelectMany for flattening in C Sharp (C#) - Step-by-Step Execution

Concept Flow - SelectMany for flattening

Start with collection of collections

↓

Apply SelectMany

↓

Flatten inner collections

↓

Create single flat collection

↓

Use or output flat collection

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

Execution Sample

C Sharp (C#)

var numbers = new List<List<int>> {
    new List<int> {1, 2},
    new List<int> {3, 4},
    new List<int> {5}
};
var flat = numbers.SelectMany(x => x).ToList();

This code flattens a list of lists of integers into one list of integers.

Execution Table

Step	Current Outer Element	Inner Elements	Action	Flat Collection State
1	[1, 2]	1, 2	Add 1, 2 to flat collection	[1, 2]
2	[3, 4]	3, 4	Add 3, 4 to flat collection	[1, 2, 3, 4]
3	[5]	5	Add 5 to flat collection	[1, 2, 3, 4, 5]
Exit	No more outer elements	-	Stop flattening	[1, 2, 3, 4, 5]

💡 All inner lists processed, flat collection complete

Variable Tracker

Variable	Start	After 1	After 2	After 3	Final
flat	empty	[1, 2]	[1, 2, 3, 4]	[1, 2, 3, 4, 5]	[1, 2, 3, 4, 5]

Key Moments - 2 Insights

Why does SelectMany flatten the inner lists instead of keeping them nested?

SelectMany takes each inner collection and adds its elements directly to the result, so the output is a single flat list, as shown in execution_table rows 1-3.

What happens if the inner collections are empty?

If an inner collection is empty, SelectMany adds nothing for that step, so the flat collection stays the same, similar to skipping that row in the execution_table.

Visual Quiz - 3 Questions

Look at the execution table, what is the flat collection after step 2?

A [1, 2]

B [1, 2, 3, 4]

C [3, 4]

D [1, 2, 3]

💡 Hint

Check the 'Flat Collection State' column at step 2 in the execution_table.

At which step does the flat collection first contain the number 5?

A Step 1

B Step 2

C Step 3

D Exit step

💡 Hint

Look at the 'Inner Elements' and 'Flat Collection State' columns in the execution_table.

If the second inner list was empty, how would the flat collection look after step 3?

A [1, 2, 5]

B [1, 2, 3, 4, 5]

C [5]

D [1, 2]

💡 Hint

Consider that an empty inner list adds no elements, so the flat collection skips those.

Concept Snapshot

SelectMany flattens collections of collections into one collection.
Syntax: collection.SelectMany(inner => inner)
It takes each inner collection and adds its elements to a single flat list.
Useful to simplify nested lists into one list.
Remember: output has no nested structure after flattening.

Full Transcript

This visual execution shows how SelectMany works in C#. We start with a list of lists of integers. SelectMany takes each inner list and adds its elements to a new flat list. Step by step, the flat list grows as elements from each inner list are added. When all inner lists are processed, the flat list contains all elements in one sequence. This helps beginners see how nested collections become flat using SelectMany.