sorting many arrays according to a sorted index of an array C#

One option:

int[] a = {120, 60, 50, 40, 30, 20};
int[] b = {12, 29, 37, 85, 63, 11};
int[] c = {30, 23, 90 ,110, 21, 34};

var ordered = a.Select((item, index) =>
                       Tuple.Create(item, b[index], c[index]))
               .OrderBy(tuple => tuple.Item1).ToArray();

a = ordered.Select(tuple => tuple.Item1).ToArray();
b = ordered.Select(tuple => tuple.Item2).ToArray();
c = ordered.Select(tuple => tuple.Item3).ToArray();

You could do it with a layer of indirection, like this:

1. Put the values 0, 1, 2, ..., N-1 into an int array, named indices, say.
2. Sort the array indices such that a[indices[i]] <= a[indices[i+1]] for all i. Your compare function will compare a[indices[Left]] with a[indices[Right]].
3. Access elements in the other arrays using the indirection: a[indices[i]] and so on.

You could make new copies of a, b and c if you wish, using the order defined by indicies. But you also have the option of not modifying the original arrays.

This option of not modifying the original arrays is quite interesting. It allows you have have multiple simultaneous orderings active in tandem.

Got bored so I tried to minimise the creation of new objects and sorting.

static void Main(string[] args)
{
    int[] a = { 120, 60, 50, 40, 30, 20 };
    int[] b = { 12, 29, 37, 85, 63, 11 };
    int[] c = { 30, 23, 90, 110, 21, 34 };

    var indexes = Enumerable.Range(0, a.Length).OrderBy(i => a[i]).ToArray();

    var temp = new int[a.Length];
    foreach (var arr in new[] { a, b, c })
    {
        for (int i = 0; i < a.Length; i++) temp[i] = arr[indexes[i]];
        for (int i = 0; i < a.Length; i++) arr[i] = temp[i];
    }

    Console.WriteLine(String.Join(", ", a));
    Console.WriteLine(String.Join(", ", b));
    Console.WriteLine(String.Join(", ", c));
    Console.ReadLine();
}

Not the best it could be (I'm sure you can get rid of the temp array somehow) - but a different solution nonetheless. I'd support sticking with the LINQesque solutions until performance becomes an issue.

You can use SortedList for this. SortedList sorts your items using a Key. Also it allows you to add new items to your collection.

SortedList Class: 
Represents a collection of key/value pairs that are sorted by the keys and are accessible by key and by index.

SortedList by MSDN

I suggest to use LINQ as Eli Arbel provided the answer. But here is an other solution for those who don't know LINQ.

class Program
    {
        public static int get_key(int key , int [,] keylist)
        {
            for (int i = 0; i <= keylist.GetUpperBound(0); ++i)
            {
                if (keylist[i, 0] == key)
                    return keylist[i, 1];
            }
            return -1;
        }
       public static int[] Sort_by_index(int [] arr , int [] key , int [,] index_list)
        {
            int[] _out = new int[arr.Length];

            for (int i = 0; i < key.Length; i++)
            {
                //Get key index
                int key_index = get_key(key[i], index_list);
                _out[i] = arr[key_index];

            }
            return _out;
        }
        static void Main(string[] args)
        {
            int[] a = { 120, 60, 50, 40, 30, 20 };
            int[] b = { 12, 29, 37, 85, 63, 11 };
            int[] c = { 30, 23, 90, 110, 21, 34 };
            int[,] a_index = { { 120, 0 }, { 60, 1 }, { 50, 2 }, { 40, 3 }, { 30, 4 }, { 20, 5 } };
            Array.Sort(a);
            b =Sort_by_index(b, a, a_index);
            c =Sort_by_index(c, a, a_index);
            Console.WriteLine("Result A");
            Console.WriteLine(string.Join(", ",a));
            Console.WriteLine("Result B");
            Console.WriteLine(string.Join(", ",b));
            Console.WriteLine("Result C");
            Console.WriteLine(string.Join(", ",c));
            Console.ReadKey(false);

        }
    }