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CaseInsensitiveComparer Class

Compares two objects for equivalence, ignoring the case of strings.

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

[SerializableAttribute] 
[ComVisibleAttribute(true)] 
public class CaseInsensitiveComparer : IComparer
/** @attribute SerializableAttribute() */ 
/** @attribute ComVisibleAttribute(true) */ 
public class CaseInsensitiveComparer implements IComparer
SerializableAttribute 
ComVisibleAttribute(true) 
public class CaseInsensitiveComparer implements IComparer
Not applicable.

CaseInsensitiveComparer implements the IComparer interface supporting case-insensitive comparisons on strings, just as CaseInsensitiveHashCodeProvider implements the IHashCodeProvider interface supporting case-insensitive comparisons on strings.

The Comparer class is the default implementation of the IComparer interface and performs case-sensitive string comparisons.

The objects used as keys by a Hashtable are required to override the Object.GetHashCode method (or the IHashCodeProvider interface) and the Object.Equals method (or the IComparer interface). The implementation of both methods or interfaces must handle case sensitivity the same way; otherwise, the Hashtable might behave incorrectly. For example, when creating a Hashtable, you must use this class with the CaseInsensitiveHashCodeProvider class or any case-insensitive IHashCodeProvider implementation.

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 creates a case-sensitive hash table and a case-insensitive hash table and demonstrates the difference in their behavior, even if both contain the same elements.

using System;
using System.Collections;
using System.Globalization;

public class SamplesHashtable  {

   public static void Main()  {

      // Create a Hashtable using the default hash code provider and the default comparer.
      Hashtable myHT1 = new Hashtable();
      myHT1.Add("FIRST", "Hello");
      myHT1.Add("SECOND", "World");
      myHT1.Add("THIRD", "!");

      // Create a Hashtable using a case-insensitive code provider and a case-insensitive comparer,
      // based on the culture of the current thread.
      Hashtable myHT2 = new Hashtable( new CaseInsensitiveHashCodeProvider(), new CaseInsensitiveComparer() );
      myHT2.Add("FIRST", "Hello");
      myHT2.Add("SECOND", "World");
      myHT2.Add("THIRD", "!");

      // Create a Hashtable using a case-insensitive code provider and a case-insensitive comparer,
      // based on the InvariantCulture.
      Hashtable myHT3 = new Hashtable( CaseInsensitiveHashCodeProvider.DefaultInvariant, CaseInsensitiveComparer.DefaultInvariant );
      myHT3.Add("FIRST", "Hello");
      myHT3.Add("SECOND", "World");
      myHT3.Add("THIRD", "!");

      // Create a Hashtable using a case-insensitive code provider and a case-insensitive comparer,
      // based on the Turkish culture (tr-TR), where "I" is not the uppercase version of "i".
      CultureInfo myCul = new CultureInfo( "tr-TR" );
      Hashtable myHT4 = new Hashtable( new CaseInsensitiveHashCodeProvider( myCul ), new CaseInsensitiveComparer( myCul ) );
      myHT4.Add("FIRST", "Hello");
      myHT4.Add("SECOND", "World");
      myHT4.Add("THIRD", "!");

      // Search for a key in each hashtable.
      Console.WriteLine( "first is in myHT1: {0}", myHT1.ContainsKey( "first" ) );
      Console.WriteLine( "first is in myHT2: {0}", myHT2.ContainsKey( "first" ) );
      Console.WriteLine( "first is in myHT3: {0}", myHT3.ContainsKey( "first" ) );
      Console.WriteLine( "first is in myHT4: {0}", myHT4.ContainsKey( "first" ) );

   }

}


/* 
This code produces the following output.  Results vary depending on the system's culture settings.

first is in myHT1: False
first is in myHT2: True
first is in myHT3: True
first is in myHT4: False

*/


import System.*;
import System.Collections.*;
import System.Globalization.*;

public class SamplesHashtable
{
    public static void main(String[] args)
    {
        // Create a Hashtable using the default hash code provider and the
        // default comparer.
        Hashtable myHT1 = new Hashtable();
        myHT1.Add("FIRST", "Hello");
        myHT1.Add("SECOND", "World");
        myHT1.Add("THIRD", "!");
          
        // Create a Hashtable using a case-insensitive code provider and a 
        // case-insensitive comparer, based on the culture of the 
        // current thread.
        Hashtable myHT2 = new Hashtable(new CaseInsensitiveHashCodeProvider(),
            new CaseInsensitiveComparer());
        myHT2.Add("FIRST", "Hello");
        myHT2.Add("SECOND", "World");
        myHT2.Add("THIRD", "!");
          
        // Create a Hashtable using a case-insensitive code provider and 
        // a case-insensitive comparer, based on the InvariantCulture.
        Hashtable myHT3 =  
            new Hashtable(CaseInsensitiveHashCodeProvider.get_DefaultInvariant(),
            CaseInsensitiveComparer.get_DefaultInvariant());
        myHT3.Add("FIRST", "Hello");
        myHT3.Add("SECOND", "World");
        myHT3.Add("THIRD", "!");
          
        // Create a Hashtable using a case-insensitive code provider and a 
        // case-insensitive comparer, based on the Turkish culture (tr-TR), 
        // where "I" is not the uppercase version of "i".
        CultureInfo myCul =  new CultureInfo("tr-TR");
        Hashtable myHT4 =  
            new Hashtable(new CaseInsensitiveHashCodeProvider(myCul), 
            new CaseInsensitiveComparer(myCul));
        myHT4.Add("FIRST", "Hello");
        myHT4.Add("SECOND", "World");
        myHT4.Add("THIRD", "!");
          
        // Search for a key in each hashtable.
        Console.WriteLine("first is in myHT1: {0}", 
            (System.Boolean)myHT1.ContainsKey("first"));
        Console.WriteLine("first is in myHT2: {0}", 
            (System.Boolean)myHT2.ContainsKey("first"));
        Console.WriteLine("first is in myHT3: {0}", 
            (System.Boolean)myHT3.ContainsKey("first"));
        Console.WriteLine("first is in myHT4: {0}", 
            (System.Boolean)myHT4.ContainsKey("first"));
    } //main
} //SamplesHashtable

/* 
This code produces the following output.  Results vary depending on the system's 
culture settings.

first is in myHT1: False
first is in myHT2: True
first is in myHT3: True
first is in myHT4: False

*/

System.Object
  System.Collections.CaseInsensitiveComparer
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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