Comparison(Of T) Delegate
Represents the method that compares two objects of the same type.
Assembly: mscorlib (in mscorlib.dll)
Parameters
- x
-
Type:
T
The first object to compare.
- y
-
Type:
T
The second object to compare.
Return Value
Type: System.Int32A 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. |
Type Parameters
- In T
The type of the objects to compare.
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 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(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
Available since 8
.NET Framework
Available since 2.0
Portable Class Library
Supported in: portable .NET platforms
Silverlight
Available since 2.0
Windows Phone Silverlight
Available since 7.0
Windows Phone
Available since 8.1