Performing Culture-Insensitive String Operations in Collections 

There are classes and members in the System.Collections namespace that provide culture-sensitive behavior by default. The default constructors for the CaseInsensitiveComparer and CaseInsensitiveHashCodeProvider classes initialize a new instance using the Thread.CurrentCulture property. All overloads of the CollectionsUtil.CreateCaseInsensitiveHashTable method create a new instance of the Hashtable class using the Thread.CurrentCulture property by default. Overloads of the ArrayList.Sort method perform culture-sensitive sorts by default using Thread.CurrentCulture. Sorting and lookup in a SortedList can be affected by Thread.CurrentCulture when strings are used as the keys. Follow the usage recommendations provided in this section to obtain culture-insensitive results from these classes and methods in the Collections namespace.

Using the CaseInsensitiveComparer and CaseInsensitiveHashCodeProvider Classes

The default constructors for CaseInsensitiveHashCodeProvider and CaseInsensitiveComparer initialize a new instance of the class using the Thread.CurrentCulture, resulting in culture-sensitive behavior. The following code example demonstrates the constructor for a Hashtable that is culture-sensitive because it uses the default constructors for CaseInsensitiveHashCodeProvider and CaseInsensitiveComparer.

internalHashtable = New Hashtable(CaseInsensitiveHashCodeProvider.Default, CaseInsensitiveComparer.Default)

internalHashtable = new Hashtable(CaseInsensitiveHashCodeProvider.Default, CaseInsensitiveComparer.Default);

If you want to create a culture-insensitive Hashtable using the CaseInsensitiveComparer and CaseInsensitiveHashCodeProvider classes, initialize new instances of these classes using the constructors that accept a culture parameter. For the culture parameter, specify CultureInfo.InvariantCulture. The following code example demonstrates the constructor for a culture-insensitive Hashtable.

internalHashtable = New Hashtable(New
    CaseInsensitiveHashCodeProvider(CultureInfo.InvariantCulture),
    New CaseInsensitiveComparer(CultureInfo.InvariantCulture))

internalHashtable = new Hashtable(new CaseInsensitiveHashCodeProvider
    (CultureInfo.InvariantCulture), 
    new CaseInsensitiveComparer(CultureInfo.InvariantCulture));

Using the CollectionsUtil.CreateCaseInsensitiveHashTable Method

The CollectionsUtil.CreateCaseInsensitiveHashTable method is a useful shortcut for creating a new instance of the Hashtable class that ignores the case of strings. However, all overloads of the CollectionsUtil.CreateCaseInsensitiveHashTable method are culture-sensitive because they use the Thread.CurrentCulture property. You cannot create a culture-insensitive Hashtable using this method. To create a culture-insensitive Hashtable, use the Hashtable constructor that accepts a culture parameter. For the culture parameter, specify CultureInfo.InvariantCulture. The following code example demonstrates the constructor for a culture-insensitive Hashtable.

internalHashtable = New Hashtable(New
    CaseInsensitiveHashCodeProvider(CultureInfo.InvariantCulture),
    New CaseInsensitiveComparer(CultureInfo.InvariantCulture))

internalHashtable = new Hashtable(new CaseInsensitiveHashCodeProvider
    (CultureInfo.InvariantCulture), 
    new CaseInsensitiveComparer(CultureInfo.InvariantCulture));

Using the SortedList Class

A SortedList represents a collection of key-and-value pairs that are sorted by the keys and are accessible by key and by index. When you use a SortedList where strings are the keys, the sorting and lookup can be affected by the Thread.CurrentCulture property. To obtain culture-insensitive behavior from a SortedList, create a SortedList using one of the constructors that accepts a comparer parameter. The comparer parameter specifies the IComparer implementation to use when comparing keys. For the IComparer parameter, specify a custom comparer class that uses CultureInfo.InvariantCulture to compare keys. The following example illustrates a custom culture-insensitive comparer class that you can specify as the IComparer parameter to a SortedList constructor.

Imports System
Imports System.Collections
Imports System.Globalization

Friend Class InvariantComparer
    Implements IComparer 
    Private m_compareInfo As CompareInfo
    Friend Shared [Default] As New InvariantComparer()
    
    Friend Sub New()
        m_compareInfo = CultureInfo.InvariantCulture.CompareInfo
    End Sub   
    
    Public Function Compare(a As Object, b As Object) As Integer _
            Implements IComparer.Compare
        Dim sa As String = CType(a, String)
        Dim sb As String = CType(b, String)
        If Not (sa Is Nothing) And Not (sb Is Nothing) Then
            Return m_compareInfo.Compare(sa, sb)
        Else
            Return Comparer.Default.Compare(a, b)
        End If
    End Function
End Class

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

internal class InvariantComparer : IComparer 
{
    private CompareInfo m_compareInfo;
    internal static readonly InvariantComparer Default = new
        InvariantComparer();

    internal InvariantComparer() 
    {
        m_compareInfo = CultureInfo.InvariantCulture.CompareInfo;
    }
    
    public int Compare(Object a, Object b)
    {
        String sa = a as String;
        String sb = b as String;
        if (sa != null && sb != null)
            return m_compareInfo.Compare(sa, sb);
        else
            return Comparer.Default.Compare(a,b);
    }
}

In general, if you use a SortedList on strings without specifying a custom invariant comparer, a change to Thread.CurrentCulture after the list has been populated can invalidate the list.

Using the ArrayList.Sort Method

Overloads of the ArrayList.Sort method perform culture-sensitive sorts by default using the Thread.CurrentCulture property. Results can vary by culture due to different sort orders. To eliminate culture-sensitive behavior, use the overloads of this method that accept an IComparer parameter. For the IComparer parameter, specify a custom invariant comparer class that uses CultureInfo.InvariantCulture. An example of a custom invariant comparer class is provided in the Using the SortedList Class topic.

See Also

Reference

CaseInsensitiveComparer Class
CaseInsensitiveHashCodeProvider Class
CollectionsUtil.CreateCaseInsensitiveHashTable Method
ArrayList.Sort Method
SortedList Class
Hashtable Class
IComparer Interface

Other Resources

Performing Culture-Insensitive String Operations