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Which sorting algorithm is used by .NET's Array.Sort() method?

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  • Related thread related to stability of the algorithm used by sort method. Commented Aug 27, 2017 at 9:35
  • Related thread: When you use LINQ based sorting instead. Commented Aug 27, 2017 at 9:36

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Array.Sort() chooses one of three sorting algorithm, depending on the size of the input:

  1. If the size is fewer than 16 elements, it uses an insertion sort algorithm.
  2. If the size exceeds 2 * log^N, where N is the range of the input array, it uses a Heap Sort algorithm.
  3. Otherwise, it uses a Quicksort algorithm

Source: Array.Sort(Array) Method on MSDN.

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2 Comments

You need to re-read that topic. In particular, your item #2 is incorrect. It doesn't base that decision on the size of the input, but rather on the number of partitions. Array.Sort implements Introsort, which is designed to use Quicksort in most cases (with the added optimization of using Insertion sort for small partitions), but if it detects that the item ordering results in a pathological case for Quicksort, it changes to Heapsort.
This answer is incorrect. It doesn't choose one sorting algorithm.
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Actually, It's not that easy as it's seems. It looks like .NET is implementing a set of different sorting algorithms depending on the input and his size. I used to decompile Array.Sort() from CLR and it seems that they are using both Heap, Insertion and Quicksort. enter image description here

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...as it is clearly documented here msdn.microsoft.com/en-us/library/6tf1f0bc.aspx
Not quite right. It uses the Introsort algorithm, which is a hybrid sorting algorithm of those 3 (all three algorithms could be used when sorting a single array)
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Some more notes from MSDN

This method uses the introspective sort (introsort) algorithm as follows:

If the partition size is fewer than 16 elements, it uses an insertion sort algorithm.

If the number of partitions exceeds 2 * LogN, where N is the range of the input array, it uses a Heapsort algorithm.

Otherwise, it uses a Quicksort algorithm.

This implementation performs an unstable sort; that is, if two elements are equal, their order might not be preserved. In contrast, a stable sort preserves the order of elements that are equal.

For arrays that are sorted by using the Heapsort and Quicksort algorithms, in the worst case, this method is an O(n log n) operation, where n is length.

Notes to Callers The .NET Framework 4 and earlier versions used only the Quicksort algorithm. Quicksort identifies invalid comparers in some situations in which the sorting operation throws an IndexOutOfRangeException exception, and throws an ArgumentException exception to the caller. Starting with the .NET Framework 4.5, it is possible that sorting operations that previously threw ArgumentException will not throw an exception, because the insertion sort and heapsort algorithms do not detect an invalid comparer. For the most part, this applies to arrays with fewer than 16 elements.

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3

It uses the QuickSort algorithm.

Source:

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Question: unless the OP asked specifically about .NET 1.1, why give him a link to .NET 1.1 documentation?
.Net framework uses Introsort instead of simple quick sort from version 4.5. en.wikipedia.org/wiki/Introsort
Actually, it uses three variants depending in the length of the input array, as @badgujar describes. (hence my downvote, since your answer is (no longer) correct, at least not for .NET 4.5/4.6)
The answer is old. The algorithm has changed since this was posted.
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Quick sort as mentioned. But it does not equally well for all data!

By using reflector: it does sort in a native dll -> for the most common case of 1D arrays, ascending order. However, other cases are sorted in managed code - with very little optimizations applied. Hence their speed is usually much slower.

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Yes. No mention of TrySZSort above.
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It really uses the Introsort algorithm, which is a hybrid sorting algorithm, consisting of the following:

  • Insertion sort
  • Heapsort
  • Quicksort

The performance is O(n log n) in the average/worst cases. Introsort is in-place (doesn't require lots of RAM) and non-stable (can't predict the order of equal elements) algorithm

From MSDN

This method uses the introspective sort (introsort) algorithm as follows:

If the partition size is less than or equal to 16 elements, it uses an insertion sort algorithm.

If the number of partitions exceeds 2 * LogN, where N is the range of the input array, it uses a Heapsort algorithm.

Otherwise, it uses a Quicksort algorithm.

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-1

It uses quick sort for sorting purpose

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