C# Linq SelectMany Method

The SelectMany method in C# LINQ is used to flatten collections of collections into a single sequence. It projects each element of a sequence to an IEnumerable<T> and then flattens the resulting sequences into one sequence.

Syntax

Following is the basic syntax for SelectMany method −

public static IEnumerable<TResult> SelectMany<TSource, TResult>(
    this IEnumerable<TSource> source,
    Func<TSource, IEnumerable<TResult>> selector
)

Parameters

• source − The input sequence to transform.

• selector − A function that transforms each element into a collection.

Return Value

Returns an IEnumerable<T> whose elements are the result of invoking the one-to-many transform function on each element of the input sequence.

Using SelectMany to Convert Strings to Characters

A common use case is converting an array of strings into individual characters −

using System;
using System.Linq;
using System.Collections.Generic;

public class Demo {
    public static void Main() {
        string[] str = { "Mobile", "Laptop", "Tablet" };
        var res = str.SelectMany(item => item.ToCharArray());
        
        Console.WriteLine("String converted to character array: ");
        foreach (char letter in res) {
            Console.Write(letter + " ");
        }
        Console.WriteLine();
        
        Console.WriteLine("\nTotal characters: " + res.Count());
    }
}

The output of the above code is −

String converted to character array: 
M o b i l e L a p t o p T a b l e t 

Total characters: 18

Using SelectMany with Nested Collections

Here's how SelectMany flattens collections of collections −

using System;
using System.Linq;
using System.Collections.Generic;

public class School {
    public string Name { get; set; }
    public List<string> Students { get; set; }
}

public class Demo {
    public static void Main() {
        var schools = new List<School> {
            new School { 
                Name = "School A", 
                Students = new List<string> { "Alice", "Bob" } 
            },
            new School { 
                Name = "School B", 
                Students = new List<string> { "Charlie", "David", "Eve" } 
            }
        };

        var allStudents = schools.SelectMany(school => school.Students);
        
        Console.WriteLine("All students from all schools:");
        foreach (string student in allStudents) {
            Console.WriteLine(student);
        }
    }
}

The output of the above code is −

All students from all schools:
Alice
Bob
Charlie
David
Eve

SelectMany vs Select

Understanding the difference between Select and SelectMany −

Example Comparing Select and SelectMany

using System;
using System.Linq;
using System.Collections.Generic;

public class Demo {
    public static void Main() {
        string[] words = { "Hello", "World" };
        
        // Using Select - returns IEnumerable<char[]>
        var selectResult = words.Select(word => word.ToCharArray());
        Console.WriteLine("Select result type: IEnumerable<char[]>");
        Console.WriteLine("Count of arrays: " + selectResult.Count());
        
        // Using SelectMany - returns IEnumerable<char>
        var selectManyResult = words.SelectMany(word => word.ToCharArray());
        Console.WriteLine("\nSelectMany result type: IEnumerable<char>");
        Console.WriteLine("Count of characters: " + selectManyResult.Count());
        Console.Write("Characters: ");
        foreach (char c in selectManyResult) {
            Console.Write(c + " ");
        }
    }
}

The output of the above code is −

Select result type: IEnumerable<char[]>
Count of arrays: 2

SelectMany result type: IEnumerable<char>
Count of characters: 10
Characters: H e l l o W o r l d 

Conclusion

The SelectMany method is essential for flattening nested collections into a single sequence. It's particularly useful when working with collections of collections, allowing you to perform operations on all elements across multiple levels of nesting in a streamlined way.