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Tip #001

Flatten a collection of collections

C# 12

Code card showing SelectMany flattening three arrays; a C# 12 spread places the values in a list. Output: 1 through 9.

About this tip

SelectMany flattens nested sequences into one. Spread puts the result into a list. It is available since C# 12. The key line is List<int> modern = [.. numbers.SelectMany(n => n)] in flatten.cs. Run it and the output reads 1, 2, 3, 4, 5, 6, 7, 8, 9.

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Code

flatten.cs
int[][] numbers = [[1, 2, 3], [4, 5, 6], [7, 8, 9]];


// classic LINQ
var values = numbers.SelectMany(n => n).ToList();


// C# 12: collection expression + spread
List<int> modern = [.. numbers.SelectMany(n => n)];


Console.WriteLine(string.Join(", ", modern));

Output

Output
$ dotnet run flatten.cs
1, 2, 3, 4, 5, 6, 7, 8, 9