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.NET Framework Class Library

IComparer<T> Interface

Defines a method that a type implements to compare two objects.

Namespace:  System.Collections.Generic
Assembly:  mscorlib (in mscorlib.dll)
Public Interface IComparer(Of In T)

Type Parameters

in T

The type of objects to compare.

This type parameter is contravariant. That is, you can use either the type you specified or any type that is less derived. For more information about covariance and contravariance, see Covariance and Contravariance in Generics.

The IComparer<T> type exposes the following members.

  Name Description
Public method Supported by the XNA Framework Supported by Portable Class Library Compare Compares two objects and returns a value indicating whether one is less than, equal to, or greater than the other.

This interface is used with the List<T>.Sort and List<T>.BinarySearch methods. It provides a way to customize the sort order of a collection. Classes that implement this interface include the SortedDictionary<TKey, TValue> and SortedList<TKey, TValue> generic classes.

The default implementation of this interface is the Comparer<T> class. The StringComparer class implements this interface for type String.

This interface supports ordering comparisons. That is, when the Compare method returns 0, it means that two objects sort the same. Implementation of exact equality comparisons is provided by the IEqualityComparer<T> generic interface.

We recommend that you derive from the Comparer<T> class instead of implementing the IComparer<T> interface, because the Comparer<T> class provides an explicit interface implementation of the IComparer.Compare method and the Default property that gets the default comparer for the object.

The following example implements the IComparer<T> interface to compare objects of type Box according to their dimensions. This example is part of a larger example provided for the Comparer<T> class.

' This class is not demonstrated in the Main method 
' and is provided only to show how to implement 
' the interface. It is recommended to derive 
' from Comparer<T> instead of implementing IComparer<T>. 
Public Class BoxComp
	Implements IComparer(Of Box)
	' Compares by Height, Length, and Width. 
    Public Function Compare(ByVal x As Box, ByVal y As Box) As Integer Implements _
                                                IComparer(Of Box).Compare
        If x.Height.CompareTo(y.Height) <> 0 Then 
            Return x.Height.CompareTo(y.Height)
        ElseIf x.Length.CompareTo(y.Length) <> 0 Then 
            Return x.Length.CompareTo(y.Length)
        ElseIf x.Width.CompareTo(y.Width) <> 0 Then 
            Return x.Width.CompareTo(y.Width)
            Return 0
        End If 
    End Function 
End Class

.NET Framework

Supported in: 4.6, 4.5, 4, 3.5, 3.0, 2.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

XNA Framework

Supported in: 3.0, 2.0, 1.0

Portable Class Library

Supported in: Portable Class Library

Supported in: Windows Phone 8.1

Supported in: Windows Phone Silverlight 8.1

Supported in: Windows Phone Silverlight 8