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Comparison<T> Delegate

Represents the method that compares two objects of the same type.

Namespace:  System
Assembly:  mscorlib (in mscorlib.dll)
public delegate int Comparison<T>(
	T x,
	T y
)

Type Parameters

T

The type of the objects to compare.

Parameters

x
Type: T

The first object to compare.

y
Type: T

The second object to compare.

Return Value

Type: System.Int32

Value

Condition

Less than 0

x is less than y.

0

x equals y.

Greater than 0

x is greater than y.

This delegate is used by the Sort<T>(T[], Comparison<T>) method overload of the Array class and the Sort(Comparison<T>) method overload of the List<T> class to sort the elements of an array or list.

The following code example demonstrates the use of the Comparison<T> delegate with the Sort(Comparison<T>) method overload.

The code example defines an alternative comparison method for strings, named CompareDinosByLength. This method works as follows: First, the comparands are tested for null, and a null reference is treated as less than a non-null. Second, the string lengths are compared, and the longer string is deemed to be greater. Third, if the lengths are equal, ordinary string comparison is used.

A List<T> of strings is created and populated with four strings, in no particular order. The list also includes an empty string and a null reference. The list is displayed, sorted using a Comparison<T> generic delegate representing the CompareDinosByLength method, and displayed again.

using System;
using System.Collections.Generic;

public class Example
{
    private static int CompareDinosByLength(string x, string y)
    {
        if (x == null)
        {
            if (y == null)
            {
                // If x is null and y is null, they're 
                // equal.  
                return 0;
            }
            else
            {
                // If x is null and y is not null, y 
                // is greater.  
                return -1;
            }
        }
        else
        {
            // If x is not null... 
            // 
            if (y == null)
                // ...and y is null, x is greater.
            {
                return 1;
            }
            else
            {
                // ...and y is not null, compare the  
                // lengths of the two strings. 
                // 
                int retval = x.Length.CompareTo(y.Length);

                if (retval != 0)
                {
                    // If the strings are not of equal length, 
                    // the longer string is greater. 
                    // 
                    return retval;
                }
                else
                {
                    // If the strings are of equal length, 
                    // sort them with ordinary string comparison. 
                    // 
                    return x.CompareTo(y);
                }
            }
        }
    }

    public static void Main()
    {
        List<string> dinosaurs = new List<string>();
        dinosaurs.Add("Pachycephalosaurus");
        dinosaurs.Add("Amargasaurus");
        dinosaurs.Add("");
        dinosaurs.Add(null);
        dinosaurs.Add("Mamenchisaurus");
        dinosaurs.Add("Deinonychus");
        Display(dinosaurs);

        Console.WriteLine("\nSort with generic Comparison<string> delegate:");
        dinosaurs.Sort(CompareDinosByLength);
        Display(dinosaurs);

    }

    private static void Display(List<string> list)
    {
        Console.WriteLine();
        foreach( string s in list )
        {
            if (s == null)
                Console.WriteLine("(null)");
            else
                Console.WriteLine("\"{0}\"", s);
        }
    }
}

/* This code example produces the following output:

"Pachycephalosaurus"
"Amargasaurus"
""
(null)
"Mamenchisaurus"
"Deinonychus"

Sort with generic Comparison<string> delegate:

(null)
""
"Deinonychus"
"Amargasaurus"
"Mamenchisaurus"
"Pachycephalosaurus"
 */

Windows 7, Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP Starter Edition, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98, Windows CE, Windows Mobile for Smartphone, Windows Mobile for Pocket PC, Xbox 360, Zune

The .NET Framework and .NET Compact Framework do not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

.NET Framework

Supported in: 3.5, 3.0, 2.0

.NET Compact Framework

Supported in: 3.5, 2.0

XNA Framework

Supported in: 3.0, 2.0, 1.0
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