使用JetBrains的DotPeek工具可以方便地查看.net的部分源码。于是看了一下.NET的内部是如何实现排序的算法。

在System.Collections.Generic 命名空间下可以看到ArraySortHelper<T>的实现。

public void Sort(T[] keys, int index, int length, IComparer<T> comparer)  
    {  
      try  
      {  
        if (comparer == null)  
          comparer = (IComparer<T>) Comparer<T>.Default;  
        if (BinaryCompatibility.TargetsAtLeast_Desktop_V4_5)  
          ArraySortHelper<T>.IntrospectiveSort(keys, index, length, comparer);  
        else  
          ArraySortHelper<T>.DepthLimitedQuickSort(keys, index, length + index - 1, comparer, 32);  
      }  
      catch (IndexOutOfRangeException ex)  
      {  
        IntrospectiveSortUtilities.ThrowOrIgnoreBadComparer((object) comparer);  
      }  
      catch (Exception ex)  
      {  
        throw new InvalidOperationException(Environment.GetResourceString("InvalidOperation_IComparerFailed"), ex);  
      }  
    } 

发现在.NET4.5以上的版本，开始使用一种叫做 Introspective Sort的排序方法。

 internal static void IntrospectiveSort(T[] keys, int left, int length, IComparer<T> comparer)  
    {  
      if (length < 2)  
        return;  
      ArraySortHelper<T>.IntroSort(keys, left, length + left - 1, 2 * IntrospectiveSortUtilities.FloorLog2(keys.Length), comparer);  
    }  
  
    private static void IntroSort(T[] keys, int lo, int hi, int depthLimit, IComparer<T> comparer)  
    {  
      for (; hi > lo; {  
        int num;  
        hi = num - 1;  
      }  
      )  
      {  
        int num = hi - lo + 1;  
        if (num <= 16)  
        {  
          if (num == 1)  
            break;  
          if (num == 2)  
          {  
            ArraySortHelper<T>.SwapIfGreater(keys, comparer, lo, hi);  
            break;  
          }  
          else if (num == 3)  
          {  
            ArraySortHelper<T>.SwapIfGreater(keys, comparer, lo, hi - 1);  
            ArraySortHelper<T>.SwapIfGreater(keys, comparer, lo, hi);  
            ArraySortHelper<T>.SwapIfGreater(keys, comparer, hi - 1, hi);  
            break;  
          }  
          else  
          {  
            ArraySortHelper<T>.InsertionSort(keys, lo, hi, comparer);  
            break;  
          }  
        }  
        else if (depthLimit == 0)  
        {  
          ArraySortHelper<T>.Heapsort(keys, lo, hi, comparer);  
          break;  
        }  
        else  
        {  
          --depthLimit;  
          num = ArraySortHelper<T>.PickPivotAndPartition(keys, lo, hi, comparer);  
          ArraySortHelper<T>.IntroSort(keys, num + 1, hi, depthLimit, comparer);  
        }  
      }  
    }  
  
    private static int PickPivotAndPartition(T[] keys, int lo, int hi, IComparer<T> comparer)  
    {  
      int index = lo + (hi - lo) / 2;  
      ArraySortHelper<T>.SwapIfGreater(keys, comparer, lo, index);  
      ArraySortHelper<T>.SwapIfGreater(keys, comparer, lo, hi);  
      ArraySortHelper<T>.SwapIfGreater(keys, comparer, index, hi);  
      T obj = keys[index];  
      ArraySortHelper<T>.Swap(keys, index, hi - 1);  
      int i = lo;  
      int j = hi - 1;  
      while (i < j)  
      {  
        do  
          ;  
        while (comparer.Compare(keys[++i], obj) < 0);  
        do  
          ;  
        while (comparer.Compare(obj, keys[--j]) < 0);  
        if (i < j)  
          ArraySortHelper<T>.Swap(keys, i, j);  
        else  
          break;  
      }  
      ArraySortHelper<T>.Swap(keys, i, hi - 1);  
      return i;  
    }  

而.NET4.5以下使用的是另一种排序的方案。

在排序的数字小于16个的时候，直接使用插入排序。

private static void InsertionSort(T[] keys, int lo, int hi, IComparer<T> comparer)  
    {  
      for (int index1 = lo; index1 < hi; ++index1)  
      {  
        int index2 = index1;  
        T x;  
        for (x = keys[index1 + 1]; index2 >= lo && comparer.Compare(x, keys[index2]) < 0; --index2)  
          keys[index2 + 1] = keys[index2];  
        keys[index2 + 1] = x;  
      }  
    }  

而如果大于16个的时候，且当递归深度在32次之内的话（也就是数字小于4GB的数量时），使用快速排序。

internal static void DepthLimitedQuickSort(T[] keys, int left, int right, IComparer<T> comparer, int depthLimit)  
    {  
      while (depthLimit != 0)  
      {  
        int index1 = left;  
        int index2 = right;  
        int index3 = index1 + (index2 - index1 >> 1);  
        ArraySortHelper<T>.SwapIfGreater(keys, comparer, index1, index3);  
        ArraySortHelper<T>.SwapIfGreater(keys, comparer, index1, index2);  
        ArraySortHelper<T>.SwapIfGreater(keys, comparer, index3, index2);  
        T obj1 = keys[index3];  
        do  
        {  
          while (comparer.Compare(keys[index1], obj1) < 0)  
            ++index1;  
          while (comparer.Compare(obj1, keys[index2]) < 0)  
            --index2;  
          if (index1 <= index2)  
          {  
            if (index1 < index2)  
            {  
              T obj2 = keys[index1];  
              keys[index1] = keys[index2];  
              keys[index2] = obj2;  
            }  
            ++index1;  
            --index2;  
          }  
          else  
            break;  
        }  
        while (index1 <= index2);  
        --depthLimit;  
        if (index2 - left <= right - index1)  
        {  
          if (left < index2)  
            ArraySortHelper<T>.DepthLimitedQuickSort(keys, left, index2, comparer, depthLimit);  
          left = index1;  
        }  
        else  
        {  
          if (index1 < right)  
            ArraySortHelper<T>.DepthLimitedQuickSort(keys, index1, right, comparer, depthLimit);  
          right = index2;  
        }  
        if (left >= right)  
          return;  
      }  
      ArraySortHelper<T>.Heapsort(keys, left, right, comparer);  
    }  

而如果大于4GB的数量时，使用堆排序。

private static void Heapsort(T[] keys, int lo, int hi, IComparer<T> comparer)  
    {  
      int n = hi - lo + 1;  
      for (int i = n / 2; i >= 1; --i)  
        ArraySortHelper<T>.DownHeap(keys, i, n, lo, comparer);  
      for (int index = n; index > 1; --index)  
      {  
        ArraySortHelper<T>.Swap(keys, lo, lo + index - 1);  
        ArraySortHelper<T>.DownHeap(keys, 1, index - 1, lo, comparer);  
      }  
    }  
  
    private static void DownHeap(T[] keys, int i, int n, int lo, IComparer<T> comparer)  
    {  
      T x = keys[lo + i - 1];  
      for (; i <= n / 2; {  
        int num;  
        i = num;  
      }  
      )  
      {  
        num = 2 * i;  
        if (num < n && comparer.Compare(keys[lo + num - 1], keys[lo + num]) < 0)  
          ++num;  
        if (comparer.Compare(x, keys[lo + num - 1]) < 0)  
          keys[lo + i - 1] = keys[lo + num - 1];  
        else  
          break;  
      }  
      keys[lo + i - 1] = x;  
    }  

private static void SwapIfGreater(T[] keys, IComparer<T> comparer, int a, int b)  
    {  
      if (a == b || comparer.Compare(keys[a], keys[b]) <= 0)  
        return;  
      T obj = keys[a];  
      keys[a] = keys[b];  
      keys[b] = obj;  
    }  
  
    private static void Swap(T[] a, int i, int j)  
    {  
      if (i == j)  
        return;  
      T obj = a[i];  
      a[i] = a[j];  
      a[j] = obj;  
    }