How create matrix from array in C#

Translating a single dimension array into a multi dimension array is straight forward.

public static T getEntry<T>(this T[] array, int column, int row, int width)
{
  return array[column+row*width];
}

Add wrapper classes and/or validation as desired.

Usage example:

var array=Enumerable.Range(1,25).ToArray();
for (int row = 0; row  < 5; row ++)
    {
        for (int column = 0; column  < 5; column  ++)
        {
            Console.WriteLine("Value in column {0}, row {1} is {2}", column, row, array.getEntry(column,row));
        }
    }

As @Taemyr suggest you can simply use indexing to simulate the structure of the matrix. If you need to access the element at row 2, col 3 in a 5 by 5 matrix simply access index 2*5+3 of your array. (row * # of cols + col)

If you want to split your array into a 2D array you can do so using the following code:

public static T[,] Matrix<T>(T[] arr, int rows) {
    var cols = arr.Length / rows;
    var m = new T[rows, cols];
    for (var i = 0; i < arr.Length; i++)
        m[i / cols, i % cols] = arr[i];
    return m;
} 

You can use Buffer.BlockCopy

using System;

class Test
{
    static double[,] ConvertMatrix(double[] flat, int m, int n)
    {
        if (flat.Length != m * n)
        {
            throw new ArgumentException("Invalid length");
        }
        double[,] ret = new double[m, n];
        // BlockCopy uses byte lengths: a double is 8 bytes
        Buffer.BlockCopy(flat, 0, ret, 0, flat.Length * sizeof(double));
        return ret;
    }

    static void Main()
    {
        double[] d = { 2, 5, 3, 5, 1, 6 };

        double[,] matrix = ConvertMatrix(d, 3, 2);

        for (int i = 0; i < 3; i++)
        {
            for (int j = 0; j < 2; j++)
            {
                Console.WriteLine("matrix[{0},{1}] = {2}", i, j, matrix[i, j]);
            }
        }
    }
}