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.

Note Passing CultureInfo.InvariantCulture to a comparison method does perform a culture-insensitive comparison. However, it does not cause a non-linguistic comparison, for example, for file paths, registry keys, and environment variables. Neither does it support security decisions based on the comparison result. For a non-linguistic comparison or support for result-based security decisions, the application should use a comparison method that accepts a StringComparison value. The application should then pass StringComparison.

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 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 comparer 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 implementation. For the comparer 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

CaseInsensitiveComparer
CaseInsensitiveHashCodeProvider
ArrayList.Sort
SortedList
Hashtable
IComparer
Performing Culture-Insensitive String Operations
CollectionsUtil.CreateCaseInsensitiveHashtable