C# LINQ SelectMany

last modified July 5, 2023

In this article we show how to flatten sequences into a single sequence with LINQ’s SelectMany method.

Language-Integrated Query (LINQ) is a domain-specific language for querying data from various data sources, including arrays, lists, XML files, or databases.

The SelectMany method flattens a number of sequences into a single sequence.

C# LINQ SelectMany array of arrays

The first example is a simple program which uses LINQ SelectMany method.

Program.cs

int[][] vals = { new[] {1, 2, 3}, new[] {4}, new[] {5, 6, 6, 2, 7, 8}, };

var res = vals.SelectMany(a => a).OrderBy(e => e); Console.WriteLine(string.Join(", “, res));

In the program, we have an array of arrays. With the SelectMany method, we flatten the two-dimensional array into an one-dimensional array values. The values are also ordered.

$ dotnet run 1, 2, 2, 3, 4, 5, 6, 6, 7, 8

C# LINQ SelectMany Distinct

The Distinct method is used to get unique values.

Program.cs

var vals = new List<List<int>> { new List<int> {1, 2, 3, 3}, new List<int> {4}, new List<int> {5, 6, 6, 7, 7} };

var res = vals.SelectMany(list => list) .Distinct() .OrderByDescending(e => e);

Console.WriteLine(string.Join(”, “, res));

In the program, we flatten all integers from a list of lists into a single sequence. We pick up only unique values and order them in descending order.

$ dotnet run 7, 6, 5, 4, 3, 2, 1

C# LINQ SelectMany string list

In the next example, we apply the SelectMany method on a list of strings.

Program.cs

var ru_words = new List<string> { “осторожность”, “собака”, “облако”, “чашка” };

var res = ru_words.SelectMany(e => e); Console.WriteLine(string.Join(”,", res));

We define a list of Russian words. The SelectMany cuts the words into letters and forms a single sequence from them.

$ dotnet run о,с,т,о,р,о,ж,н,о,с,т,ь,с,о,б,а,к,а,о,б,л,а,к,о,ч,а,ш,к,а

C# LINQ SelectMany on record field

The next example applies the SelectMany method on a record field.

Program.cs

var users = new List<User> { new (“John”, “Doe”, new List<string> { “red”, “blue” }), new (“Roger”, “Roe”, new List<string> { “black”, “yellow” }), new (“Jerry”, “Dane”, new List<string> { “blue”, “orange”, “white” }), new (“Thomas”, “Green”, new List<string> { “brown” }), };

List<string> res = users.SelectMany(e => e.colours).ToList(); Console.WriteLine(string.Join(",", res));

record User(string fname, string lname, List<string> colours);

We have a list of users, who have their favourite colours specified. The colours are defined as a list of strings inside a record type.

List<string> res = users.SelectMany(e => e.colours).ToList();

The flattening operation is applied on the colours fields of the records.

$ dotnet run red,blue,black,yellow,blue,orange,white,brown

C# LINQ Select

We can perform the same operations with Select; the SelectMany is more convenient.

Program.cs

var users = new List<User> { new (“John”, “Doe”, new List<string> { “red”, “blue” }), new (“Roger”, “Roe”, new List<string> { “black”, “yellow” }), new (“Jerry”, “Dane”, new List<string> { “blue”, “orange”, “white” }), new (“Thomas”, “Green”, new List<string> { “brown” }), };

IEnumerable<List<string>> res = users.Select(e => e.colours).ToList();

List<string> colours = new List<string>();

foreach(var cols in res) { foreach (var col in cols) { colours.Add(col); } }

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

colours.Clear();

foreach (var data in res) { colours.AddRange(data); }

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

record User(string fname, string lname, List<string> colours);

In the example we flatten all favourite colours into a list of strings.

foreach(var cols in res) { foreach (var col in cols) { colours.Add(col); } }

In the first case, we use two foreach loops.

foreach (var data in res) { colours.AddRange(data); }

In the second case, we use a foreach loop and the AddRange method.

$ dotnet run red,blue,black,yellow,blue,orange,white,brown red,blue,black,yellow,blue,orange,white,brown

Source

Enumerable.SelectMany method

In this article we have presented the LINQ SelectMany method.

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

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