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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 _
	Implements IComparer
Dim instance As CaseInsensitiveComparer

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.

Imports System
Imports System.Collections
Imports System.Globalization

Public Class SamplesHashtable

   Public Shared Sub Main()

      ' Create a Hashtable using the default hash code provider and the default comparer. 
      Dim myHT1 As 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. 
      Dim myHT2 As 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. 
      Dim myHT3 As 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". 
      Dim myCul As New CultureInfo("tr-TR")
      Dim myHT4 As 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"))

   End Sub 'Main 

End Class '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


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 7, Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP Starter Edition, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98, Windows CE, Windows Mobile for Smartphone, Windows Mobile for Pocket PC, Xbox 360, Zune

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

.NET Framework

Supported in: 3.5, 3.0, 2.0, 1.1, 1.0

.NET Compact Framework

Supported in: 3.5, 2.0, 1.0

XNA Framework

Supported in: 3.0, 2.0, 1.0