C# Sort list while also returning the original index positions?

Question

It can be done quite easily using Linq.

Convert your list into a new list of pairs (object, original index of object).
Sort the new list by the first item in the pair
Extract the sorted list and the original indices.

Here’s some code to demonstrate the principle:

List<int> A = new List<int>() { 3, 2, 1 };

var sorted = A
    .Select((x, i) => new KeyValuePair<int, int>(x, i))
    .OrderBy(x => x.Key)
    .ToList();

List<int> B = sorted.Select(x => x.Key).ToList();
List<int> idx = sorted.Select(x => x.Value).ToList();

I think this gives A[idx[i]] = B[i], but that hopefully is good enough for you.

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