int[,] arrayBidimensional = new int[3, 3]
{
    { 1, 2, 3 },
    { 4, 5, 6 },
    { 7, 8, 9 }
};

using System;

class MainClass {
    public static void Main (string[] args) {
        int[,] arrayBidimensional = new int[3, 3]
        {
            { 1, 2, 3 },
            { 4, 5, 6 },
            { 7, 8, 9 }
        };

        var novoArray = new int[arrayBidimensional.GetLength(0) * arrayBidimensional.GetLength(1)];

        int colunas = arrayBidimensional.GetLength(1);
        for(int i = 0; i < arrayBidimensional.GetLength(0); i++) {
            for(int j = 0; j < colunas; j++) {
                var indice = colunas * i + j;
                novoArray[indice] = arrayBidimensional[i, j];

                Console.WriteLine($"{indice} = {novoArray[indice]}");
            }
        }
    }
}