using System;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        var nested = new [] {

            new[] { 1, 2, 3},

            new[] { 4, 5, 6},

            new[] { 7, 8, 9}};

        Console.WriteLine(nested.GetType());

        Console.WriteLine(nested.SelectMany(t => t).ToArray().GetType());

    }

}