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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 2678dc63-c7f9-4590-9ddc-0a4df684d42e.

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

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(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.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 Demo(ByVal outputBlock As System.Windows.Controls.TextBlock)

      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(outputBlock, dinosaurs)

      outputBlock.Text &= vbLf & "Sort with generic Comparison(Of String) delegate:" & vbCrLf
      dinosaurs.Sort(AddressOf CompareDinosByLength)
      Display(outputBlock, dinosaurs)

   End Sub

   Private Shared Sub Display(ByVal outputBlock As System.Windows.Controls.TextBlock, ByVal lis As List(Of String))
      outputBlock.Text &= vbCrLf
      For Each s As String In lis
         If s Is Nothing Then
            outputBlock.Text &= "(Nothing)" & vbCrLf
         Else
            outputBlock.Text += String.Format("""{0}""", s) & vbCrLf
         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"


Silverlight

Supported in: 5, 4, 3

Silverlight for Windows Phone

Supported in: Windows Phone OS 7.1, Windows Phone OS 7.0

XNA Framework

Supported in: Xbox 360, Windows Phone OS 7.0

For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.

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