using System;

using System.Linq;

using System.Collections.Generic;

​

public class Program

{

    private static int[, ] _matrix = new int[2, 3];

    public static void Add(List<int> lst)

    {

        var height = _matrix.GetLength(0);

        var width = _matrix.GetLength(1);

        if (lst.Count != width * height)

            throw new ArgumentException($"The number of elements in lst ({lst.Count}) is not equal to the matrix {width * height}");

        for (var y = 0; y < height; y++)

            for (var x = 0; x < width; x++)

                _matrix[y, x] = lst[x + width * y];

    }

​

    public static void Add2(List<int> lst)

    {

        var width = _matrix.GetLength(0);

        var height = _matrix.GetLength(1);

        if (lst.Count != width * height)

            throw new ArgumentException($"The number of elements in lst ({lst.Count}) is not equal to the matrix {width * height}");

        Buffer.BlockCopy(lst.ToArray(), 0, _matrix, 0, lst.Count * sizeof(int));

    }

​

    public static void Write()

    {

        var height = _matrix.GetLength(0);

        var width = _matrix.GetLength(1);

        for (var y = 0; y < height; y++)

        {

            for (var x = 0; x < width; x++)

                Console.Write(_matrix[y, x]);

            Console.WriteLine();

        }

    }

​

    public static void Main()

    {

        Add(new List<int>{1, 2, 3, 4, 5, 6});

        Write();

        Console.WriteLine();

        Add2(new List<int>{1, 2, 3, 4, 5, 6});

        Write();

    }

}