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Comparison(Of T) Delegate

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

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

'Declaration
Public Delegate Function Comparison(Of In T) ( _
	x As T, _
	y As T _
) As Integer

Type Parameters

In T

The type of the objects to compare.

This type parameter is contravariant. That is, you can use either the type you specified or any type that is less derived. For more information about covariance and contravariance, see Covariance and Contravariance in Generics.

Parameters

x
Type: T

The first object to compare.

y
Type: T

The second object to compare.

Return Value

Type: System.Int32
A signed integer that indicates the relative values of x and y, as shown in the following table.

Value

Meaning

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(Of T)(T(), Comparison(Of T)) method overload of the Array class and the Sort(Comparison(Of T)) method overload of the List(Of T) class to sort the elements of an array or list.

The following code example demonstrates the use of the Comparison(Of T) delegate with the Sort(Comparison(Of 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 Nothing, 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(Of 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(Of T) generic delegate representing the CompareDinosByLength method, and displayed again.

Imports System
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(Of 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(Of T)(T(), Comparison(Of T)) method.

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", "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                   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

.NET Framework

Supported in: 4.5.2, 4.5.1, 4.5, 4, 3.5, 3.0, 2.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

.NET for Windows Store apps

Supported in: Windows 8

.NET for Windows Phone apps

Supported in: Windows Phone 8.1, Windows Phone 8, Silverlight 8.1

Windows Phone 8.1, Windows Phone 8, Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

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

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