Comparer<T> Comparer<T> Comparer<T> Comparer<T> Class

Definition

Provides a base class for implementations of the IComparer<T> generic interface.

generic <typename T>
public ref class Comparer abstract : System::Collections::Generic::IComparer<T>, System::Collections::IComparer
[System.Serializable]
public abstract class Comparer<T> : System.Collections.Generic.IComparer<T>, System.Collections.IComparer
type Comparer<'T> = class
    interface IComparer
    interface IComparer<'T>
Public MustInherit Class Comparer(Of T)
Implements IComparer, IComparer(Of T)

Type Parameters

T

The type of objects to compare.

Inheritance
Comparer<T>Comparer<T>Comparer<T>Comparer<T>
Attributes
Implements

Examples

The following example derives a class, BoxLengthFirst, from the Comparer<T> class. This comparer compares two objects of type Box. It sorts them first by length, then by height, and then by width. The Box class implements the IComparable<T> interface to control the default comparison between two Box objects. This default implementation sorts first by height, then by length, and then by width. The example shows the differences between the two comparisons by sorting a list of Box objects first by using the BoxLengthFirst comparer and then by using the default comparer.

using System;
using System.Collections;
using System.Collections.Generic;

class Program
{
    static void Main(string[] args)
    {
        List<Box> Boxes = new List<Box>();
        Boxes.Add(new Box(4, 20, 14));
        Boxes.Add(new Box(12, 12, 12));
        Boxes.Add(new Box(8, 20, 10));
        Boxes.Add(new Box(6, 10, 2));
        Boxes.Add(new Box(2, 8, 4));
        Boxes.Add(new Box(2, 6, 8));
        Boxes.Add(new Box(4, 12, 20));
        Boxes.Add(new Box(18, 10, 4));
        Boxes.Add(new Box(24, 4, 18));
        Boxes.Add(new Box(10, 4, 16));
        Boxes.Add(new Box(10, 2, 10));
        Boxes.Add(new Box(6, 18, 2));
        Boxes.Add(new Box(8, 12, 4));
        Boxes.Add(new Box(12, 10, 8));
        Boxes.Add(new Box(14, 6, 6));
        Boxes.Add(new Box(16, 6, 16));
        Boxes.Add(new Box(2, 8, 12));
        Boxes.Add(new Box(4, 24, 8));
        Boxes.Add(new Box(8, 6, 20));
        Boxes.Add(new Box(18, 18, 12));

        // Sort by an Comparer<T> implementation that sorts
        // first by the length.
        Boxes.Sort(new BoxLengthFirst());

        Console.WriteLine("H - L - W");
        Console.WriteLine("==========");
        foreach (Box bx in Boxes)
        {
            Console.WriteLine("{0}\t{1}\t{2}",
                bx.Height.ToString(), bx.Length.ToString(), 
                bx.Width.ToString());
        }

        Console.WriteLine();
        Console.WriteLine("H - L - W"); 
        Console.WriteLine("==========");

        // Get the default comparer that 
        // sorts first by the height.
        Comparer<Box> defComp = Comparer<Box>.Default;

        // Calling Boxes.Sort() with no parameter
        // is the same as calling Boxs.Sort(defComp)
        // because they are both using the default comparer.
        Boxes.Sort();

        foreach (Box bx in Boxes)
        {
            Console.WriteLine("{0}\t{1}\t{2}",
                bx.Height.ToString(), bx.Length.ToString(), 
                bx.Width.ToString());
        }


        // This explicit interface implementation
        // compares first by the length.
        // Returns -1 because the length of BoxA
        // is less than the length of BoxB.
        BoxLengthFirst LengthFirst = new BoxLengthFirst(); 

        Comparer<Box> bc = (Comparer<Box>) LengthFirst;

        Box BoxA = new Box(2, 6, 8);
        Box BoxB = new Box(10, 12, 14);
        int x = LengthFirst.Compare(BoxA, BoxB);
        Console.WriteLine();
        Console.WriteLine(x.ToString());

    

    }

}

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

public class Box : IComparable<Box>
{

    public Box(int h, int l, int w)
    {
        this.Height = h;
        this.Length = l;
        this.Width = w;
    }
    public int Height { get; private set; }
    public int Length { get; private set; }
    public int Width { get; private set; }

    public int CompareTo(Box other)
    {
        // Compares Height, Length, and Width.
        if (this.Height.CompareTo(other.Height) != 0)
        {
            return this.Height.CompareTo(other.Height);
        }
        else if (this.Length.CompareTo(other.Length) != 0)
        {
            return this.Length.CompareTo(other.Length);
        }
        else if (this.Width.CompareTo(other.Width) != 0)
        {
            return this.Width.CompareTo(other.Width);
        }
        else
        {
            return 0;
        }
    }

}

Imports System.Collections.Generic

Friend Class Program
	Shared Sub Main(ByVal args() As String)
		Dim Boxes As New List(Of Box)()
		Boxes.Add(New Box(4, 20, 14))
		Boxes.Add(New Box(12, 12, 12))
		Boxes.Add(New Box(8, 20, 10))
		Boxes.Add(New Box(6, 10, 2))
		Boxes.Add(New Box(2, 8, 4))
		Boxes.Add(New Box(2, 6, 8))
		Boxes.Add(New Box(4, 12, 20))
		Boxes.Add(New Box(18, 10, 4))
		Boxes.Add(New Box(24, 4, 18))
		Boxes.Add(New Box(10, 4, 16))
		Boxes.Add(New Box(10, 2, 10))
		Boxes.Add(New Box(6, 18, 2))
		Boxes.Add(New Box(8, 12, 4))
		Boxes.Add(New Box(12, 10, 8))
		Boxes.Add(New Box(14, 6, 6))
		Boxes.Add(New Box(16, 6, 16))
		Boxes.Add(New Box(2, 8, 12))
		Boxes.Add(New Box(4, 24, 8))
		Boxes.Add(New Box(8, 6, 20))
		Boxes.Add(New Box(18, 18, 12))

		' Sort by an Comparer<T> implementation that sorts
		' first by the length.
		Boxes.Sort(New BoxLengthFirst())

		Console.WriteLine("H - L - W")
		Console.WriteLine("==========")
		For Each bx As Box In Boxes
            Console.WriteLine("{0}" & vbTab & "{1}" & vbTab & "{2}", _
                              bx.Height.ToString(), bx.Length.ToString(), _
                              bx.Width.ToString())
		Next bx

		Console.WriteLine()
		Console.WriteLine("H - L - W")
		Console.WriteLine("==========")

		' Get the default comparer that 
		' sorts first by the height.
		Dim defComp As Comparer(Of Box) = Comparer(Of Box).Default

		' Calling Boxes.Sort() with no parameter
		' is the same as calling Boxs.Sort(defComp)
		' because they are both using the default comparer.
		Boxes.Sort()

		For Each bx As Box In Boxes
            Console.WriteLine("{0}" & vbTab & "{1}" & vbTab & "{2}", _
                              bx.Height.ToString(), _
                              bx.Length.ToString(), _
                              bx.Width.ToString())
		Next bx


		' This explicit interface implementation
		' compares first by the length.
		' Returns -1 because the length of BoxA
		' is less than the length of BoxB.
		Dim LengthFirst As New BoxLengthFirst()

		Dim bc As Comparer(Of Box) = CType(LengthFirst, Comparer(Of Box))

		Dim BoxA As New Box(2, 6, 8)
		Dim BoxB As New Box(10, 12, 14)
		Dim x As Integer = LengthFirst.Compare(BoxA, BoxB)
		Console.WriteLine()
		Console.WriteLine(x.ToString())



	End Sub

End Class

Public Class BoxLengthFirst
	Inherits Comparer(Of Box)
	' Compares by Length, Height, and Width.
	Public Overrides Function Compare(ByVal x As Box, ByVal y As Box) As Integer
		If x.Length.CompareTo(y.Length) <> 0 Then
			Return x.Length.CompareTo(y.Length)
		ElseIf x.Height.CompareTo(y.Height) <> 0 Then
			Return x.Height.CompareTo(y.Height)
		ElseIf x.Width.CompareTo(y.Width) <> 0 Then
			Return x.Width.CompareTo(y.Width)
		Else
			Return 0
		End If
	End Function

End 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)
        Else
            Return 0
        End If
    End Function
End Class

Public Class Box
	Implements IComparable(Of Box)

	Public Sub New(ByVal h As Integer, ByVal l As Integer, ByVal w As Integer)
		Me.Height = h
		Me.Length = l
		Me.Width = w
	End Sub
	Private privateHeight As Integer
	Public Property Height() As Integer
		Get
			Return privateHeight
		End Get
		Private Set(ByVal value As Integer)
			privateHeight = value
		End Set
	End Property
	Private privateLength As Integer
	Public Property Length() As Integer
		Get
			Return privateLength
		End Get
		Private Set(ByVal value As Integer)
			privateLength = value
		End Set
	End Property
	Private privateWidth As Integer
	Public Property Width() As Integer
		Get
			Return privateWidth
		End Get
		Private Set(ByVal value As Integer)
			privateWidth = value
		End Set
	End Property

    Public Function CompareTo(ByVal other As Box) As Integer _
                        Implements IComparable(Of Box).CompareTo
        ' Compares Height, Length, and Width.
        If Me.Height.CompareTo(other.Height) <> 0 Then
            Return Me.Height.CompareTo(other.Height)
        ElseIf Me.Length.CompareTo(other.Length) <> 0 Then
            Return Me.Length.CompareTo(other.Length)
        ElseIf Me.Width.CompareTo(other.Width) <> 0 Then
            Return Me.Width.CompareTo(other.Width)
        Else
            Return 0
        End If
    End Function

End Class

Remarks

Derive from this class to provide a custom implementation of the IComparer<T> interface for use with collection classes such as the SortedList<TKey,TValue> and SortedDictionary<TKey,TValue> generic classes.

The difference between deriving from the Comparer<T> class and implementing the System.IComparable interface is as follows:

  • To specify how two objects should be compared by default, implement the System.IComparable interface in your class. This ensures that sort operations will use the default comparison code that you provided.

  • To define a comparer to use instead of the default comparer, derive from the Comparer<T> class. You can then use this comparer in sort operations that take a comparer as a parameter.

The object returned by the Default property uses the System.IComparable<T> generic interface (IComparable<T> in C#, IComparable(Of T) in Visual Basic) to compare two objects. If type T does not implement the System.IComparable<T> generic interface, the Default property returns a Comparer<T> that uses the System.IComparable interface.

Notes to Inheritors

Compare(T, T) and Equals(T, T) may behave differently in terms of culture-sensitivity and case-sensitivity.

For string comparisons, the StringComparer class is recommended over Comparer<String>. Properties of the StringComparer class return predefined instances that perform string comparisons with different combinations of culture-sensitivity and case-sensitivity. The case-sensitivity and culture-sensitivity are consistent among the members of the same StringComparer instance.

For more information on culture-specific comparisons, see the System.Globalization namespace and Globalization and Localization.

Constructors

Comparer<T>() Comparer<T>() Comparer<T>() Comparer<T>()

Initializes a new instance of the Comparer<T> class.

Properties

Default Default Default Default

Returns a default sort order comparer for the type specified by the generic argument.

Methods

Compare(T, T) Compare(T, T) Compare(T, T) Compare(T, T)

When overridden in a derived class, performs a comparison of two objects of the same type and returns a value indicating whether one object is less than, equal to, or greater than the other.

Create(Comparison<T>) Create(Comparison<T>) Create(Comparison<T>) Create(Comparison<T>)

Creates a comparer by using the specified comparison.

Equals(Object) Equals(Object) Equals(Object) Equals(Object)

Determines whether the specified object is equal to the current object.

(Inherited from Object)
GetHashCode() GetHashCode() GetHashCode() GetHashCode()

Serves as the default hash function.

(Inherited from Object)
GetType() GetType() GetType() GetType()

Gets the Type of the current instance.

(Inherited from Object)
MemberwiseClone() MemberwiseClone() MemberwiseClone() MemberwiseClone()

Creates a shallow copy of the current Object.

(Inherited from Object)
ToString() ToString() ToString() ToString()

Returns a string that represents the current object.

(Inherited from Object)

Explicit Interface Implementations

IComparer.Compare(Object, Object) IComparer.Compare(Object, Object) IComparer.Compare(Object, Object) IComparer.Compare(Object, Object)

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

Applies to

See also