Clean Up Your C# Loops: Flattening Nested Collections with LINQ SelectMany

Have you ever dealt with a "List within a List" and found yourself writing messy nested foreach loops just to get to the data?

As a developer who loves puzzles (and LEGO Technic!), I’m always looking for ways to simplify complex structures. In C#, the .SelectMany operator is one of the most elegant ways to "flatten" these nested collections into a single, usable stream.

The Problem

Imagine you have a list of Orders, and each order contains a list of LineItems. If you want a single list of every item sold across all orders, the traditional way looks like this:

var allItems = new List<LineItem>();
foreach (var order in orders)
{
    foreach (var item in order.LineItems)
    {
        allItems.Add(item);
    }
}

This works, but it's "noisy." It’s hard to read at a glance.

The Solution: SelectMany

With LINQ, we can achieve the same result in a single line of code. Think of SelectMany as a way to reach into each parent object, grab the child collection, and spread it out on the table.

var allItems = orders.SelectMany(o => o.LineItems).ToList();

Why this is better

1. Readability: You state what you want to do, not how to loop through it.
2. Chainability: You can immediately add a .Where() or .OrderBy()after the .SelectMany to further refine your data without adding more loops.

If you’re moving from a "loop-heavy" style to a more functional approach in .NET, SelectMany is a tool you'll reach for constantly!