Comparer<T> Class

Definition

generic <typename T>
public ref class Comparer abstract : System::Collections::Generic::IComparer<T>, System::Collections::IComparer

public abstract class Comparer<T> : 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<'T>
 interface IComparer

[<System.Serializable>]
type Comparer<'T> = class
 interface IComparer
 interface IComparer<'T>

Public MustInherit Class Comparer(Of T)
Implements IComparer, IComparer(Of T)

Type Parameters

Inheritance

Object

Comparer<T>

Attributes

SerializableAttribute

Implements

IComparer<T> IComparer

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;
 }
 }
}

open System
open System.Collections.Generic

type Box(h, l, w) =
 member val Height: int = h with get
 member val Length: int = l with get
 member val Width: int = w with get

 interface IComparable<Box> with
 member this.CompareTo(other) =
 // Compares Height, Length, and Width.
 if this.Height.CompareTo other.Height <> 0 then
 this.Height.CompareTo other.Height
 elif this.Length.CompareTo other.Length <> 0 then
 this.Length.CompareTo other.Length
 elif this.Width.CompareTo other.Width <> 0 then
 this.Width.CompareTo other.Width
 else
 0

type BoxLengthFirst() =
 inherit Comparer<Box>()

 // Compares by Length, Height, and Width.
 override _.Compare(x: Box, y: Box) =
 if x.Length.CompareTo y.Length <> 0 then
 x.Length.CompareTo y.Length
 elif x.Height.CompareTo y.Height <> 0 then
 x.Height.CompareTo y.Height
 elif x.Width.CompareTo y.Width <> 0 then
 x.Width.CompareTo y.Width
 else
 0

let boxes = ResizeArray<Box>()
boxes.Add(Box(4, 20, 14))
boxes.Add(Box(12, 12, 12))
boxes.Add(Box(8, 20, 10))
boxes.Add(Box(6, 10, 2))
boxes.Add(Box(2, 8, 4))
boxes.Add(Box(2, 6, 8))
boxes.Add(Box(4, 12, 20))
boxes.Add(Box(18, 10, 4))
boxes.Add(Box(24, 4, 18))
boxes.Add(Box(10, 4, 16))
boxes.Add(Box(10, 2, 10))
boxes.Add(Box(6, 18, 2))
boxes.Add(Box(8, 12, 4))
boxes.Add(Box(12, 10, 8))
boxes.Add(Box(14, 6, 6))
boxes.Add(Box(16, 6, 16))
boxes.Add(Box(2, 8, 12))
boxes.Add(Box(4, 24, 8))
boxes.Add(Box(8, 6, 20))
boxes.Add(Box(18, 18, 12))

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

printfn "H - L - W"
printfn "=========="

for bx in boxes do
 printfn $"{bx.Height}\t{bx.Length}\t{bx.Width}"

printfn ""
printfn "H - L - W"
printfn "=========="

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

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

for bx in boxes do
 printfn $"{bx.Height}\t{bx.Length}\t{bx.Width}"


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

let bc = LengthFirst :> Comparer<Box>

let BoxA = Box(2, 6, 8)
let BoxB = Box(10, 12, 14)
let x = LengthFirst.Compare(BoxA, BoxB)
printfn $"\n{x}"

// 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>.
type BoxComp() =
 interface IComparer<Box> with
 // Compares by Height, Length, and Width.
 member _.Compare(x: Box, y: Box) =
 if x.Height.CompareTo(y.Height) <> 0 then
 x.Height.CompareTo(y.Height)
 elif x.Length.CompareTo(y.Length) <> 0 then
 x.Length.CompareTo(y.Length)
 elif x.Width.CompareTo(y.Width) <> 0 then
 x.Width.CompareTo(y.Width)
 else
 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 Implementers

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.

See also

• IComparer<T>
• IComparable<T>
• StringComparer
• CultureInfo