How to Flatten an `IEnumerable` into a `List` using LINQ?

Streamlining Nested LINQ Queries: Flattening IEnumerable<List<int>> to List<int>

Working with nested collections in LINQ often requires flattening them into a single list for easier processing. The SelectMany() method provides an elegant solution for this common task.

Scenario:

Your LINQ query returns an IEnumerable<List<int>>—a collection of integer lists. The goal is to transform this into a single List<int>.

Solution:

The SelectMany() method efficiently flattens the nested structure. Here's how:

<code class="language-csharp">var result = iList.SelectMany(i => i).ToList();</code>

The SelectMany() method iterates through each inner list (i) within iList and projects its elements into a single sequence. The .ToList() extension method then converts this sequence into a List<int>.

Illustration:

Let's say your input is:

<code>[1, 2, 3, 4]
[5, 6, 7]</code>

Applying SelectMany() yields:

<code>[1, 2, 3, 4, 5, 6, 7]</code>

This approach simplifies nested data structures, making your LINQ queries more concise and readable. It's a valuable technique for handling various scenarios involving hierarchical data.

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