// Example 9: multi or single dimensional arrays?

// We define this extension method
public static void Add<T>(this List<T> list, params T[] items)
=> list.AddRange(items);

// And then we can make a flattened matrix
var matrix = new List<double>
{
  { 1, 0, 0 },
  { 0, 1, 0 },
  { 0, 0, 1 }
};

Console.WriteLine(matrix.Count);