Comparison<T> Delegate

Definition

generic <typename T>
public delegate int Comparison(T x, T y);

public delegate int Comparison<in T>(T x, T y);

public delegate int Comparison<in T>(T x, T y) where T : allows ref struct;

public delegate int Comparison<T>(T x, T y);

type Comparison<'T> = delegate of 'T * 'T -> int

Public Delegate Function Comparison(Of In T)(x As T, y As T) As Integer 

Public Delegate Function Comparison(Of T)(x As T, y As T) As Integer 

Type Parameters

Parameters

Return Value

A signed integer that indicates the relative values of x and y, as shown in the following table.

Examples

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"
 */

open System

let compareDinosByLength (x: string) (y: string) =
 match x with 
 | null when isNull y ->
 // If x is null and y is null, they're equal.
 0
 | null ->
 // If x is null and y is not null, y is greater.
 -1
 | _ when isNull y->
 // If x is not null and y is null, x is greater.
 1
 | _ ->
 // If x is not null and y is not null, compare the lengths of the two strings.
 let retval = x.Length.CompareTo y.Length

 if retval <> 0 then
 // If the strings are not of equal length, the longer string is greater.
 retval
 else
 // If the strings are of equal length, sort them with ordinary string comparison.
 x.CompareTo y

let display list =
 printfn ""
 for s in list do
 if isNull s then
 printfn "(null)"
 else
 printfn $"\"%s{s}\""


let dinosaurs = ResizeArray()
dinosaurs.Add "Pachycephalosaurus"
dinosaurs.Add "Amargasaurus"
dinosaurs.Add ""
dinosaurs.Add null
dinosaurs.Add "Mamenchisaurus"
dinosaurs.Add "Deinonychus"
display dinosaurs

printfn "\nSort with generic Comparison<string> delegate:"
dinosaurs.Sort compareDinosByLength
display dinosaurs


// This code example produces the following output:
//
// "Pachycephalosaurus"
// "Amargasaurus"
// ""
// (null)
// "Mamenchisaurus"
// "Deinonychus"
//
// Sort with generic Comparison<string> delegate:
//
// (null)
// ""
// "Deinonychus"
// "Amargasaurus"
// "Mamenchisaurus"
// "Pachycephalosaurus"

Imports System.Collections.Generic

Public Class Example

 Private Shared Function CompareDinosByLength( _
 ByVal x As String, ByVal y As String) As Integer

 If x Is Nothing Then
 If y Is Nothing Then 
 ' If x is Nothing and y is Nothing, they're
 ' equal. 
 Return 0
 Else
 ' If x is Nothing and y is not Nothing, y
 ' is greater. 
 Return -1
 End If
 Else
 ' If x is not Nothing...
 '
 If y Is Nothing Then
 ' ...and y is Nothing, x is greater.
 Return 1
 Else
 ' ...and y is not Nothing, compare the 
 ' lengths of the two strings.
 '
 Dim retval As Integer = _
 x.Length.CompareTo(y.Length)

 If retval <> 0 Then 
 ' 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)
 End If
 End If
 End If

 End Function

 Public Shared Sub Main()

 Dim dinosaurs As New List(Of String)
 dinosaurs.Add("Pachycephalosaurus")
 dinosaurs.Add("Amargasaurus")
 dinosaurs.Add("")
 dinosaurs.Add(Nothing)
 dinosaurs.Add("Mamenchisaurus")
 dinosaurs.Add("Deinonychus")
 Display(dinosaurs)

 Console.WriteLine(vbLf & "Sort with generic Comparison(Of String) delegate:")
 dinosaurs.Sort(AddressOf CompareDinosByLength)
 Display(dinosaurs)

 End Sub

 Private Shared Sub Display(ByVal lis As List(Of String))
 Console.WriteLine()
 For Each s As String In lis
 If s Is Nothing Then
 Console.WriteLine("(Nothing)")
 Else
 Console.WriteLine("""{0}""", s)
 End If
 Next
 End Sub
End Class

' This code example produces the following output:
'
'"Pachycephalosaurus"
'"Amargasaurus"
'""
'(Nothing)
'"Mamenchisaurus"
'"Deinonychus"
'
'Sort with generic Comparison(Of String) delegate:
'
'(Nothing)
'""
'"Deinonychus"
'"Amargasaurus"
'"Mamenchisaurus"
'"Pachycephalosaurus"

The following example uses the Comparison<T> delegate to sort the elements of a collection of CityInfo objects. CityInfo is an application-defined class that contains information about a city and its population. The example defines three methods, CompareByName, CompareByPopulation, and CompareByNames, that offer three different ways of ordering the CityInfo objects. Each method is assigned to the comparison argument of the Array.Sort<T>(T[], Comparison<T>) method.

using System;

public class CityInfo
{
 string cityName;
 string countryName;
 int pop2010;

 public CityInfo(string name, string country, int pop2010)
 {
 this.cityName = name;
 this.countryName = country;
 this.pop2010 = pop2010;
 }

 public string City
 { get { return this.cityName; } }

 public string Country
 { get { return this.countryName; } }

 public int Population
 { get { return this.pop2010; } }

 public static int CompareByName(CityInfo city1, CityInfo city2)
 {
 return String.Compare(city1.City, city2.City);
 }

 public static int CompareByPopulation(CityInfo city1, CityInfo city2)
 {
 return city1.Population.CompareTo(city2.Population);
 }

 public static int CompareByNames(CityInfo city1, CityInfo city2)
 {
 return String.Compare(city1.Country + city1.City, city2.Country + city2.City);
 }
}

public class Example
{
 public static void Main()
 {
 CityInfo NYC = new CityInfo("New York City", "United States of America", 8175133 );
 CityInfo Det = new CityInfo("Detroit", "United States of America", 713777);
 CityInfo Paris = new CityInfo("Paris", "France", 2193031);
 CityInfo[] cities = { NYC, Det, Paris };
 // Display ordered array.
 DisplayArray(cities);

 // Sort array by city name.
 Array.Sort(cities, CityInfo.CompareByName);
 DisplayArray(cities);

 // Sort array by population.
 Array.Sort(cities, CityInfo.CompareByPopulation);
 DisplayArray(cities);

 // Sort array by country + city name.
 Array.Sort(cities, CityInfo.CompareByNames);
 DisplayArray(cities);
 }

 private static void DisplayArray(CityInfo[] cities)
 {
 Console.WriteLine("{0,-20} {1,-25} {2,10}", "City", "Country", "Population");
 foreach (var city in cities)
 Console.WriteLine("{0,-20} {1,-25} {2,10:N0}", city.City,
 city.Country, city.Population);

 Console.WriteLine();
 }
}
// The example displays the following output:
// City Country Population
// New York City United States of America 8,175,133
// Detroit United States of America 713,777
// Paris France 2,193,031
//
// City Country Population
// Detroit United States of America 713,777
// New York City United States of America 8,175,133
// Paris France 2,193,031
//
// City Country Population
// Detroit United States of America 713,777
// Paris France 2,193,031
// New York City United States of America 8,175,133
//
// City Country Population
// Paris France 2,193,031
// Detroit United States of America 713,777
// New York City United States of America 8,175,133

open System

type CityInfo =
 { City: string
 Country: string
 Population: int }

 static member CompareByName city1 city2 =
 String.Compare(city1.City, city2.City)

 static member CompareByPopulation city1 city2 =
 city1.Population.CompareTo city2.Population 

 static member CompareByNames city1 city2 =
 String.Compare(city1.Country + city1.City, city2.Country + city2.City)

let display cities =
 printfn $"""{"City",-20} {"Country",-25} {"Population",10}"""
 for city in cities do
 printfn $"{city.City,-20} {city.Country,-25} {city.Population,10:N0}"
 printfn ""

let NYC = { City = "New York City"; Country = "United States of America"; Population = 8175133 }
let Det = { City = "Detroit"; Country = "United States of America"; Population = 713777 }
let Paris = { City = "Paris"; Country = "France"; Population = 2193031 }
let cities = [| NYC; Det; Paris |]
// Display ordered array.
display cities

// Sort array by city name.
Array.Sort(cities, CityInfo.CompareByName)
display cities

// Sort array by population.
Array.Sort(cities, CityInfo.CompareByPopulation);
display cities

// Sort array by country + city name.
Array.Sort(cities, CityInfo.CompareByNames);
display cities


// The example displays the following output:
// City Country Population
// New York City United States of America 8,175,133
// Detroit United States of America 713,777
// Paris France 2,193,031
//
// City Country Population
// Detroit United States of America 713,777
// New York City United States of America 8,175,133
// Paris France 2,193,031
//
// City Country Population
// Detroit United States of America 713,777
// Paris France 2,193,031
// New York City United States of America 8,175,133
//
// City Country Population
// Paris France 2,193,031
// Detroit United States of America 713,777
// New York City United States of America 8,175,133

Public Class CityInfo
 Dim cityName As String
 Dim countryName As String
 Dim pop2010 As Integer
 
 Public Sub New(name As String, country As String, pop2010 As Integer)
 Me.cityName = name
 Me.countryName = country
 Me.pop2010 = pop2010
 End Sub
 
 Public ReadOnly Property City As String
 Get
 Return Me.cityName
 End Get
 End Property
 
 Public ReadOnly Property Country As String
 Get
 Return Me.countryName
 End Get
 End Property
 
 Public ReadOnly Property Population As Integer
 Get
 Return Me.pop2010
 End Get 
 End Property
 
 Public Shared Function CompareByName(city1 As CityInfo, city2 As CityInfo) As Integer
 Return String.Compare(city1.City, city2.City)
 End Function
 
 Public Shared Function CompareByPopulation(city1 As CityInfo, city2 As CityInfo) As Integer
 Return city1.Population.CompareTo(city2.Population)
 End Function
 
 Public Shared Function CompareByNames(city1 As CityInfo, city2 As CityInfo) As Integer
 Return String.Compare(city1.Country + city1.City, city2.Country + city2.City)
 End Function 
End Class

Module Example
 Public Sub Main()
 Dim NYC As New CityInfo("New York City", "United States of America", 8175133)
 Dim Det As New CityInfo("Detroit", "United States of America", 713777)
 Dim Paris As New CityInfo("Paris", "France", 2193031)
 Dim cities As CityInfo() = { NYC, Det, Paris }
 ' Display ordered array.
 DisplayArray(cities)
 
 ' Sort array by city name.
 Array.Sort(cities, AddressOf CityInfo.CompareByName)
 DisplayArray(cities)
 
 ' Sort array by population.
 Array.Sort(cities, AddressOf CityInfo.CompareByPopulation)
 DisplayArray(cities)
 
 ' Sort array by country + city name.
 Array.Sort(cities, AddressOf CityInfo.CompareByNames)
 DisplayArray(cities)
 End Sub
 
 Private Sub DisplayArray(cities() As CityInfo)
 Console.WriteLine("{0,-20} {1,-25} {2,10}", "City", "Country/Region", "Population")
 For Each city In cities
 Console.WriteLine("{0,-20} {1,-25} {2,10:N0}", city.City, city.Country, city.Population)
 Next
 Console.WriteLine()
 End Sub
End Module
' The example displays the following output:
' City Country/Region Population
' New York City United States of America 8,175,133
' Detroit United States of America 713,777
' Paris France 2,193,031
' 
' City Country/Region Population
' Detroit United States of America 713,777
' New York City United States of America 8,175,133
' Paris France 2,193,031
' 
' City Country/Region Population
' Detroit United States of America 713,777
' Paris France 2,193,031
' New York City United States of America 8,175,133
' 
' City Country/Region Population
' Paris France 2,193,031
' Detroit United States of America 713,777
' New York City United States of America 8,175,133

Remarks

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.

See also

• Sort<T>(T[], Comparison<T>)
• Sort(Comparison<T>)