What is the difference between `Select()` and `SelectMany()`?

Select()

• Projects each element into a new form (1-to-1 transformation)
• Returns IEnumerable<T>
• Maintains the structure of the collection

SelectMany()

• Flattens nested collections into a single sequence (1-to-many transformation)
• Returns flattened IEnumerable<T>
• Useful for hierarchical or nested data

// Sample data
var schools = new List
{
    new School 
    { 
        Name = "Lincoln High", 
        Students = new[] { "Alice", "Bob" } 
    },
    new School 
    { 
        Name = "Jefferson High", 
        Students = new[] { "Charlie", "Diana" } 
    }
};

// Select() - Returns IEnumerable
var selectResult = schools.Select(s => s.Students);
// Result: [ ["Alice", "Bob"], ["Charlie", "Diana"] ]
// You get a collection of collections

// SelectMany() - Returns IEnumerable
var selectManyResult = schools.SelectMany(s => s.Students);
// Result: ["Alice", "Bob", "Charlie", "Diana"]
// You get a flattened single collection

// Another example
var numbers = new List<List>
{
    new List { 1, 2, 3 },
    new List { 4, 5, 6 },
    new List { 7, 8, 9 }
};

// Select - nested structure preserved
var selected = numbers.Select(list => list);
// Type: IEnumerable<List>

// SelectMany - flattened
var flattened = numbers.SelectMany(list => list);
// Type: IEnumerable
// Result: [1, 2, 3, 4, 5, 6, 7, 8, 9]

// Practical example: Get all words from sentences
var sentences = new[] { "Hello world", "LINQ is powerful" };
var allWords = sentences.SelectMany(s => s.Split(' '));
// Result: ["Hello", "world", "LINQ", "is", "powerful"]