List<T>.Sort Method

May 11, 2014

Sorts the elements in the entire List<T> using the default comparer.

Namespace:  System.Collections.Generic
Assembly:  mscorlib (in mscorlib.dll)

public void Sort()

ExceptionCondition
InvalidOperationException

The default comparer Comparer<T>.Default cannot find an implementation of the IComparable<T> generic interface or the IComparable interface for type T.

This method uses the default comparer Comparer<T>.Default for type T to determine the order of list elements. The Comparer<T>.Default property checks whether type T implements the IComparable<T> generic interface and uses that implementation, if available. If not, Comparer<T>.Default checks whether type T implements the IComparable interface. If type T does not implement either interface, Comparer<T>.Default throws an InvalidOperationException.

This method uses Array.Sort, which uses the 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.

On average, this method is an O(n log n) operation, where n is Count; in the worst case it is an O(n ^ 2) operation.

The following code example demonstrates the Sort() method overload and the BinarySearch(T) method overload. A List<T> of strings is created and populated with four strings, in no particular order. The list is displayed, sorted, and displayed again.

The BinarySearch(T) method overload is then used to search for two strings that are not in the list, and the Insert method is used to insert them. The return value of the BinarySearch method is negative in each case, because the strings are not in the list. Taking the bitwise complement (the ~ operator in C# and Visual C++, Xor -1 in Visual Basic) of this negative number produces the index of the first element in the list that is larger than the search string, and inserting at this location preserves the sort order. The second search string is larger than any element in the list, so the insertion position is at the end of the list.


using System;
using System.Collections.Generic;

public class Example
{
   public static void Demo(System.Windows.Controls.TextBlock outputBlock)
   {
      List<string> dinosaurs = new List<string>();

      dinosaurs.Add("Pachycephalosaurus");
      dinosaurs.Add("Amargasaurus");
      dinosaurs.Add("Mamenchisaurus");
      dinosaurs.Add("Deinonychus");

      outputBlock.Text += "\n";
      foreach (string dinosaur in dinosaurs)
      {
         outputBlock.Text += dinosaur + "\n";
      }

      outputBlock.Text += "\nSort" + "\n";
      dinosaurs.Sort();

      outputBlock.Text += "\n";
      foreach (string dinosaur in dinosaurs)
      {
         outputBlock.Text += dinosaur + "\n";
      }

      outputBlock.Text += "\nBinarySearch and Insert \"Coelophysis\":" + "\n";
      int index = dinosaurs.BinarySearch("Coelophysis");
      if (index < 0)
      {
         dinosaurs.Insert(~index, "Coelophysis");
      }

      outputBlock.Text += "\n";
      foreach (string dinosaur in dinosaurs)
      {
         outputBlock.Text += dinosaur + "\n";
      }

      outputBlock.Text += "\nBinarySearch and Insert \"Tyrannosaurus\":" + "\n";
      index = dinosaurs.BinarySearch("Tyrannosaurus");
      if (index < 0)
      {
         dinosaurs.Insert(~index, "Tyrannosaurus");
      }

      outputBlock.Text += "\n";
      foreach (string dinosaur in dinosaurs)
      {
         outputBlock.Text += dinosaur + "\n";
      }
   }
}

/* This code example produces the following output:

Pachycephalosaurus
Amargasaurus
Mamenchisaurus
Deinonychus

Sort

Amargasaurus
Deinonychus
Mamenchisaurus
Pachycephalosaurus

BinarySearch and Insert "Coelophysis":

Amargasaurus
Coelophysis
Deinonychus
Mamenchisaurus
Pachycephalosaurus

BinarySearch and Insert "Tyrannosaurus":

Amargasaurus
Coelophysis
Deinonychus
Mamenchisaurus
Pachycephalosaurus
Tyrannosaurus
 */


Windows Phone OS

Supported in: 8.1, 8.0, 7.1, 7.0

Windows Phone

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