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.NET Framework 2.0
System.Collections
Comparer Class

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This page is specific to
Microsoft Visual Studio 2005/.NET Framework 2.0

Other versions are also available for the following:
Comparer Class
Compares two objects for equivalence, where string comparisons are case-sensitive.

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

Visual Basic (Declaration)
<SerializableAttribute> _
<ComVisibleAttribute(True)> _
Public NotInheritable Class Comparer
    Implements IComparer, ISerializable
Visual Basic (Usage)
Dim instance As Comparer
C#
[SerializableAttribute] 
[ComVisibleAttribute(true)] 
public sealed class Comparer : IComparer, ISerializable
C++
[SerializableAttribute] 
[ComVisibleAttribute(true)] 
public ref class Comparer sealed : IComparer, ISerializable
J#
/** @attribute SerializableAttribute() */ 
/** @attribute ComVisibleAttribute(true) */ 
public final class Comparer implements IComparer, ISerializable
JScript
SerializableAttribute 
ComVisibleAttribute(true) 
public final class Comparer implements IComparer, ISerializable
XAML
Not applicable.

This class is the default implementation of the IComparer interface. The CaseInsensitiveComparer class is the implementation of the IComparer interface that performs case-insensitive string comparisons.

Comparison procedures use the Thread.CurrentCulture of the current thread unless otherwise specified. String comparisons might have different results depending on the culture. For more information on culture-specific comparisons, see the System.Globalization namespace and Encoding and Localization.

The following code example shows how Compare returns different values depending on the culture associated with the Comparer.

Visual Basic
Imports System
Imports System.Collections
Imports System.Globalization

Public Class SamplesComparer

   Public Shared Sub Main()

      ' Creates the strings to compare.
      Dim str1 As [String] = "llegar"
      Dim str2 As [String] = "lugar"
      Console.WriteLine("Comparing ""{0}"" and ""{1}"" ...", str1, str2)

      ' Uses the DefaultInvariant Comparer.
      Console.WriteLine("   Invariant Comparer: {0}", Comparer.DefaultInvariant.Compare(str1, str2))

      ' Uses the Comparer based on the culture "es-ES" (Spanish - Spain, international sort).
      Dim myCompIntl As New Comparer(New CultureInfo("es-ES", False))
      Console.WriteLine("   International Sort: {0}", myCompIntl.Compare(str1, str2))

      ' Uses the Comparer based on the culture identifier 0x040A (Spanish - Spain, traditional sort).
      Dim myCompTrad As New Comparer(New CultureInfo(&H40A, False))
      Console.WriteLine("   Traditional Sort  : {0}", myCompTrad.Compare(str1, str2))

   End Sub 'Main 

End Class 'SamplesComparer


'This code produces the following output.
'
'Comparing "llegar" and "lugar" ...
'   Invariant Comparer: -1
'   International Sort: -1
'   Traditional Sort  : 1
C#
using System;
using System.Collections;
using System.Globalization;


public class SamplesComparer  {

   public static void Main()  {

      // Creates the strings to compare.
      String str1 = "llegar";
      String str2 = "lugar";
      Console.WriteLine( "Comparing \"{0}\" and \"{1}\" ...", str1, str2 );

      // Uses the DefaultInvariant Comparer.
      Console.WriteLine( "   Invariant Comparer: {0}", Comparer.DefaultInvariant.Compare( str1, str2 ) );

      // Uses the Comparer based on the culture "es-ES" (Spanish - Spain, international sort).
      Comparer myCompIntl = new Comparer( new CultureInfo( "es-ES", false ) );
      Console.WriteLine( "   International Sort: {0}", myCompIntl.Compare( str1, str2 ) );

      // Uses the Comparer based on the culture identifier 0x040A (Spanish - Spain, traditional sort).
      Comparer myCompTrad = new Comparer( new CultureInfo( 0x040A, false ) );
      Console.WriteLine( "   Traditional Sort  : {0}", myCompTrad.Compare( str1, str2 ) );

   }

}

/*
This code produces the following output.

Comparing "llegar" and "lugar" ...
   Invariant Comparer: -1
   International Sort: -1
   Traditional Sort  : 1

*/
C++
using namespace System;
using namespace System::Collections;
using namespace System::Globalization;
int main()
{
   
   // Creates the strings to compare.
   String^ str1 = "llegar";
   String^ str2 = "lugar";
   Console::WriteLine( "Comparing \"{0}\" and \"{1}\" ...", str1, str2 );
   
   // Uses the DefaultInvariant Comparer.
   Console::WriteLine( "   Invariant Comparer: {0}", Comparer::DefaultInvariant->Compare( str1, str2 ) );
   
   // Uses the Comparer based on the culture "es-ES" (Spanish - Spain, international sort).
   Comparer^ myCompIntl = gcnew Comparer( gcnew CultureInfo(  "es-ES",false ) );
   Console::WriteLine( "   International Sort: {0}", myCompIntl->Compare( str1, str2 ) );
   
   // Uses the Comparer based on the culture identifier 0x040A (Spanish - Spain, traditional sort).
   Comparer^ myCompTrad = gcnew Comparer( gcnew CultureInfo( 0x040A,false ) );
   Console::WriteLine( "   Traditional Sort  : {0}", myCompTrad->Compare( str1, str2 ) );
}

/*
This code produces the following output.

Comparing "llegar" and "lugar" ...
   Invariant Comparer: -1
   International Sort: -1
   Traditional Sort  : 1

*/
J#
import System.* ;
import System.Collections.*;
import System.Globalization.*;

public class SamplesComparer
{
    public static void main(String[] args)
    {
        // Creates the strings to compare.
        String str1 = "llegar";
        String str2 = "lugar";
        Console.WriteLine("Comparing \"{0}\" and \"{1}\" ...", str1, str2);
      
        // Uses the DefaultInvariant Comparer.
        Console.WriteLine("   Invariant Comparer: {0}", 
            (Int32)Comparer.DefaultInvariant.Compare(str1, str2));

        // Uses the Comparer based on the culture "es-ES" (Spanish - Spain, 
        // international sort).
        Comparer myCompIntl = new Comparer(new CultureInfo("es-ES", false));
        Console.WriteLine("   International Sort: {0}", 
            (Int32)myCompIntl.Compare(str1, str2));

        // Uses the Comparer based on the culture identifier 0x040A 
        // (Spanish - Spain, traditional sort).
        Comparer myCompTrad = new Comparer(new CultureInfo(0x40A, false));
        Console.WriteLine("   Traditional Sort  : {0}",
            (Int32)myCompTrad.Compare(str1, str2));
    } //main
} //SamplesComparer

/*
This code produces the following output.

Comparing "llegar" and "lugar" ...
   Invariant Comparer: -1
   International Sort: -1
   Traditional Sort  : 1

*/
System.Object
  System.Collections.Comparer
Any public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.

Windows 98, Windows Server 2000 SP4, Windows CE, Windows Millennium Edition, Windows Mobile for Pocket PC, Windows Mobile for Smartphone, Windows Server 2003, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP SP2, Windows XP Starter Edition

The Microsoft .NET Framework 3.0 is supported on Windows Vista, Microsoft Windows XP SP2, and Windows Server 2003 SP1.

.NET Framework

Supported in: 3.0, 2.0, 1.1, 1.0

.NET Compact Framework

Supported in: 2.0, 1.0

XNA Framework

Supported in: 1.0
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Sample Sort of ArrayList of Integers      digioz   |   Edit   |   Show History

Bellow is a sample sorting of an arraylist of integers in ascending order using IComparer:

Public Class Comparer : Implements IComparer
    Public Function Compare(ByVal a As Object, ByVal b As Object) As Integer Implements IComparer.Compare
        Dim i1 As Integer = CInt(a)
        Dim i2 As Integer = CInt(b)
        If (i1 < i2) Then
            Return -1
        End If
        If (i1 > i2) Then
            Return 1
        Else
            Return 0
        End If
    End Function
End Class

In the form where this implementation is to be used, the following shared function is needed:

    Public Shared Function Comparer() As IComparer
        Return CType(New Comparer(), IComparer)
    End Function

So a sample usage of the above Class would be:

        Dim aList As New ArrayList
        aList.Add(1)
        aList.Add(10)
        aList.Add(11)
        aList.Add(12)
        aList.Add(2)
        aList.Add(3)
        aList.Add(4)
        aList.Add(5)
        aList.Add(6)
        aList.Add(7)
        aList.Add(8)
        aList.Add(9)
        aList.Sort(Comparer)

The ArrayList will now contain the numbers 1 through 12 in ascending order.

For questions, please visit us at www.digioz.com .

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