.NET Framework Class Library
IEqualityComparer Interface

Defines methods to support the comparison of objects for equality.

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

Visual Basic (Declaration)
<ComVisibleAttribute(True)> _
Public Interface IEqualityComparer
Visual Basic (Usage)
Dim instance As IEqualityComparer
C#
[ComVisibleAttribute(true)]
public interface IEqualityComparer
Visual C++
[ComVisibleAttribute(true)]
public interface class IEqualityComparer
JScript
public interface IEqualityComparer
Remarks

This interface allows the implementation of customized equality comparison for collections. That is, you can create your own definition of equality, and specify that this definition be used with a collection type that accepts the IEqualityComparer interface. In the .NET Framework, constructors of the Hashtable, NameValueCollection, and OrderedDictionary collection types accept this interface.

This interface supports only equality comparisons. Customization of comparisons for sorting and ordering is provided by the IComparer interface.

For the generic version of this interface, see System.Collections.Generic..::.IEqualityComparer<(Of <(T>)>).

Examples

The following code example demonstrates the implementation of a case-insensitive IEqualityComparer. In this example, the CaseInsensitiveComparer..::.Compare method is used to determine whether two objects are equal, based on the provided CultureInfo.

Visual Basic
Public Class myCultureComparer
    Implements IEqualityComparer

    Dim myComparer As CaseInsensitiveComparer

    Public Sub New()
        myComparer = CaseInsensitiveComparer.DefaultInvariant
    End Sub

    Public Sub New(ByVal myCulture As CultureInfo)
        myComparer = New CaseInsensitiveComparer(myCulture)
    End Sub

    Public Function Equals1(ByVal x As Object, ByVal y As Object) _
        As Boolean Implements IEqualityComparer.Equals

        If (myComparer.Compare(x, y) = 0) Then
            Return True
        Else
            Return False
        End If
    End Function

    Public Function GetHashCode1(ByVal obj As Object) _
        As Integer Implements IEqualityComparer.GetHashCode
        Return obj.ToString().ToLower().GetHashCode()
    End Function
End Class
C#
class myCultureComparer : IEqualityComparer
{
    public CaseInsensitiveComparer myComparer;

    public myCultureComparer()
    {
        myComparer = CaseInsensitiveComparer.DefaultInvariant;
    }

    public myCultureComparer(CultureInfo myCulture)
    {
        myComparer = new CaseInsensitiveComparer(myCulture);
    }

    public new bool Equals(object x, object y)
    {
        if (myComparer.Compare(x, y) == 0)
        {
            return true;
        }
        else
        {
            return false;
        }
    }

    public int GetHashCode(object obj)
    {
        return obj.ToString().ToLower().GetHashCode();
    }
}
Visual C++
ref class myCultureComparer : IEqualityComparer
{
private:
    CaseInsensitiveComparer^ myComparer;

public:
    myCultureComparer()
    {
        myComparer = CaseInsensitiveComparer::DefaultInvariant;
    }

    myCultureComparer(CultureInfo^ myCulture)
    {
        myComparer = gcnew CaseInsensitiveComparer(myCulture);
    }

    virtual bool Equals(Object^ x, Object^ y) 
    {
        if (myComparer->Compare(x, y) == 0)
        {
            return true;
        }
        else
        {
            return false;
        }
    }

    virtual int GetHashCode(Object^ obj)
    {
        return obj->ToString()->ToLower()->GetHashCode();
    }
};
Platforms

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.
Version Information

.NET Framework

Supported in: 3.5, 3.0, 2.0

.NET Compact Framework

Supported in: 3.5, 2.0

XNA Framework

Supported in: 3.0, 2.0, 1.0
See Also

Reference

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