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Array.Sort<T> Method (T[], IComparer<T>)


Sorts the elements in an Array using the specified IComparer<T> generic interface.

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

public static void Sort<T>(
	T[] array,
	IComparer<T> comparer


Type: T[]

The one-dimensional, zero-base Array to sort

Type: System.Collections.Generic.IComparer<T>

The IComparer<T> generic interface implementation to use when comparing elements, or null to use the IComparable<T> generic interface implementation of each element.

Type Parameters


The type of the elements of the array.

Exception Condition

array is null.


comparer is null, and one or more elements in array do not implement the IComparable<T> generic interface.


The implementation of comparer caused an error during the sort. For example, comparer might not return 0 when comparing an item with itself.

If comparer is null, each element of array must implement the IComparable<T> generic interface to be capable of comparisons with every other element in array.

If the sort is not successfully completed, the results are undefined.

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 the Length of array.

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.

The following code example demonstrates the Sort<T>(T[], IComparer<T>) generic method overload and the BinarySearch<T>(T[], T, IComparer<T>) generic method overload.

The code example defines an alternative comparer for strings, named ReverseCompare, which implements the IComparer<string> (IComparer(Of String) in Visual Basic, IComparer<String^> in Visual C++) generic interface. The comparer calls the CompareTo(String) method, reversing the order of the comparands so that the strings sort high-to-low instead of low-to-high.

The array is displayed, sorted, and displayed again. Arrays must be sorted in order to use the BinarySearch<T> method.


The calls to the Sort<T>(T[], IComparer<T>) and BinarySearch<T>(T[], T, IComparer<T>) generic methods do not look any different from calls to their nongeneric counterparts, because Visual Basic, C#, and C++ infer the type of the generic type parameter from the type of the first argument. If you use the Ildasm.exe (IL Disassembler) to examine the Microsoft intermediate language (MSIL), you can see that the generic methods are being called.

The BinarySearch<T>(T[], T, IComparer<T>) generic method overload is then used to search for two strings, one that is not in the array and one that is. The array and the return value of the BinarySearch<T>(T[], T, IComparer<T>) method are passed to the ShowWhere generic method, which displays the index value if the string is found, and otherwise the elements the search string would fall between if it were in the array. The index is negative if the string is not n the array, so the ShowWhere method takes the bitwise complement (the ~ operator in C# and Visual C++, Xor -1 in Visual Basic) to obtain the index of the first element in the list that is larger than the search string.

using System;
using System.Collections.Generic;

public class ReverseComparer: IComparer<string>
    public int Compare(string x, string y)
        // Compare y and x in reverse order.
        return y.CompareTo(x);

public class Example
    public static void Main()
        string[] dinosaurs = {"Pachycephalosaurus", 

        foreach( string dinosaur in dinosaurs )

        ReverseComparer rc = new ReverseComparer();

        Array.Sort(dinosaurs, rc);

        foreach( string dinosaur in dinosaurs )

        Console.WriteLine("\nBinarySearch for 'Coelophysis':");
        int index = Array.BinarySearch(dinosaurs, "Coelophysis", rc);
        ShowWhere(dinosaurs, index);

        Console.WriteLine("\nBinarySearch for 'Tyrannosaurus':");
        index = Array.BinarySearch(dinosaurs, "Tyrannosaurus", rc);
        ShowWhere(dinosaurs, index);

    private static void ShowWhere<T>(T[] array, int index)
        if (index<0)
            // If the index is negative, it represents the bitwise
            // complement of the next larger element in the array.
            index = ~index;

            Console.Write("Not found. Sorts between: ");

            if (index == 0)
                Console.Write("beginning of array and ");
                Console.Write("{0} and ", array[index-1]);

            if (index == array.Length)
                Console.WriteLine("end of array.");
                Console.WriteLine("{0}.", array[index]);
            Console.WriteLine("Found at index {0}.", index);

/* This code example produces the following output:




BinarySearch for 'Coelophysis':
Not found. Sorts between: Deinonychus and Amargasaurus.

BinarySearch for 'Tyrannosaurus':
Found at index 0.

Universal Windows Platform
Available since 8
.NET Framework
Available since 2.0
Portable Class Library
Supported in: portable .NET platforms
Available since 2.0
Windows Phone Silverlight
Available since 7.0
Windows Phone
Available since 8.1
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