Replace Nested Loops with SelectMany()

Instead of using nested foreach loops to flatten collections, SelectMany() provides a more readable and efficient way to achieve the same result. It flattens nested collections into a single sequence.

var categories = new List < List < string >> {
  new List < string > {
    "Apple",
    "Banana"
  },
  new List < string > {
    "Orange",
    "Grape"
  },
  new List < string > {
    "Mango"
  }
};

// Flatten nested lists using SelectMany()
var flattened = categories.SelectMany(category => category).ToList();

Console.WriteLine(string.Join(", ", flattened));
//Apple, Banana, Orange, Grape, Mango