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Showing content from https://learn.microsoft.com/dotnet/api/system.collections.generic.icomparer-1 below:

IComparer<T> Interface (System.Collections.Generic) | Microsoft Learn

IComparer<T> Interface Definition

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

generic <typename T>
public interface class IComparer

public interface IComparer<in T>

public interface IComparer<in T> where T : allows ref struct

public interface IComparer<T>

type IComparer<'T> = interface

Public Interface IComparer(Of In T)

Public Interface IComparer(Of T)

Type Parameters

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

Derived

Examples

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 : IComparer<Box>
{
    // Compares by Height, Length, and Width.
    public int Compare(Box x, Box y)
    {
        if (x.Height.CompareTo(y.Height) != 0)
        {
            return x.Height.CompareTo(y.Height);
        }
        else if (x.Length.CompareTo(y.Length) != 0)
        {
            return x.Length.CompareTo(y.Length);
        }
        else if (x.Width.CompareTo(y.Width) != 0)
        {
            return x.Width.CompareTo(y.Width);
        }
        else
        {
            return 0;
        }
    }
}

' 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)
        Else
            Return 0
        End If
    End Function
End Class

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.

Methods Compare(T, T)

Compares two objects and returns a value indicating whether one is less than, equal to, or greater than the other.