CompareTo Method
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IStructuralComparable.CompareTo Method (Object, IComparer)

 

Determines whether the current collection object precedes, occurs in the same position as, or follows another object in the sort order.

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

Function CompareTo (
	other As Object,
	comparer As IComparer
) As Integer

Parameters

other
Type: System.Object

The object to compare with the current instance.

comparer
Type: System.Collections.IComparer

An object that compares members of the current collection object with the corresponding members of other.

Return Value

Type: System.Int32

An integer that indicates the relationship of the current collection object to other, as shown in the following table.

Return value

Description

-1

The current instance precedes other.

0

The current instance and other are equal.

1

The current instance follows other.

Exception Condition
ArgumentException

This instance and other are not the same type.

The CompareTo method supports custom structural comparison and sorting of array and tuple objects. The CompareTo method calls the comparer object's IComparer.Compare method to compare individual array elements or tuple components, starting with the first element or component. The individual calls to IComparer.Compare end and the CompareTo method returns a value when one of the following conditions becomes true:

The following example creates an array of Tuple(Of T1, T2, T3, T4, T5, T6) objects that contains population data for three U.S. cities from 1960 to 2000. The sextuple's first component is the city name. The remaining five components represent the population at ten-year intervals from 1960 to 2000.

The PopulationComparer class provides an IComparer implementation that allows the array of sextuples to be sorted by any one of its components. Two values are provided to the PopulationComparer class in its constructor: the position of the component that defines the sort order, and a Boolean value that indicates whether the tuple objects should be sorted in ascending or descending order.

The example then displays the elements in the array in unsorted order, sorts them by the third component (the population in 1970) and displays them, and then sorts them by the sixth component (the population in 2000) and displays them. Note that the example does not directly call the IStructuralComparable.CompareTo implementation. The method is called implicitly by the Sort(Array, IComparer) method for each tuple object in the array.

Imports System.Collections
Imports System.Collections.Generic

Public Class PopulationComparer(Of T1, T2, T3, T4, T5, T6) : Implements IComparer
   Private itemPosition As Integer
   Private multiplier As Integer = -1

   Public Sub New(component As Integer)
      Me.New(component, True)
   End Sub

   Public Sub New(component As Integer, descending As Boolean)
      If Not descending Then multiplier = 1

      If component <= 0 Or component > 6 Then 
         Throw New ArgumentException("The component argument is out of range.")
      End If
      itemPosition = component
   End Sub 

   Public Function Compare(x As Object, y As Object) As Integer _
                   Implements IComparer.Compare

      Dim tX = TryCast(x, Tuple(Of T1, T2, T3, T4, T5, T6))
      If tX Is Nothing Then
         Return 0
      Else
         Dim tY = DirectCast(y, Tuple(Of T1, T2, T3, T4, T5, T6))
         Select Case itemPosition
            Case 1
               Return Comparer(Of T1).Default.Compare(tX.Item1, tY.Item1) * multiplier
            Case 2
               Return Comparer(Of T2).Default.Compare(tX.Item2, tY.Item2) * multiplier
            Case 3
               Return Comparer(Of T3).Default.Compare(tX.Item3, tY.Item3) * multiplier
            Case 4
               Return Comparer(Of T4).Default.Compare(tX.Item4, tY.Item4) * multiplier
            Case 5
               Return Comparer(Of T5).Default.Compare(tX.Item5, tY.Item5) * multiplier
            Case 6
               Return Comparer(Of T6).Default.Compare(tX.Item6, tY.Item6) * multiplier
            ' This should never happen.
            Case Else
               Return 0
         End Select      
      End If
   End Function
End Class

Module Example
   Public Sub Main()
      ' Create array of sextuple with population data for three U.S. 
      ' cities, 1960-2000.
      Dim cities() = 
          { Tuple.Create("Los Angeles", 2479015, 2816061, 2966850, 3485398, 3694820),
            Tuple.Create("New York", 7781984, 7894862, 7071639, 7322564, 8008278),  
            Tuple.Create("Chicago", 3550904, 3366957, 3005072, 2783726, 2896016) } 

      ' Display array in unsorted order.
      Console.WriteLine("In unsorted order:")
      For Each city In cities
         Console.WriteLine(city.ToString())
      Next
      Console.WriteLine()

      Array.Sort(cities, New PopulationComparer(Of String, Integer, Integer, Integer, Integer, Integer)(3)) 

      ' Display array in sorted order.
      Console.WriteLine("Sorted by population in 1970:")
      For Each city In cities
         Console.WriteLine(city.ToString())
      Next
      Console.WriteLine()

      Array.Sort(cities, New PopulationComparer(Of String, Integer, Integer, Integer, Integer, Integer)(6))

      ' Display array in sorted order.
      Console.WriteLine("Sorted by population in 2000:")
      For Each city In cities
         Console.WriteLine(city.ToString())
      Next
   End Sub
End Module
' The example displays the following output:
'    In unsorted order:
'    (Los Angeles, 2479015, 2816061, 2966850, 3485398, 3694820)
'    (New York, 7781984, 7894862, 7071639, 7322564, 8008278)
'    (Chicago, 3550904, 3366957, 3005072, 2783726, 2896016)
'    
'    Sorted by population in 1970:
'    (New York, 7781984, 7894862, 7071639, 7322564, 8008278)
'    (Chicago, 3550904, 3366957, 3005072, 2783726, 2896016)
'    (Los Angeles, 2479015, 2816061, 2966850, 3485398, 3694820)
'    
'    Sorted by population in 2000:
'    (New York, 7781984, 7894862, 7071639, 7322564, 8008278)
'    (Los Angeles, 2479015, 2816061, 2966850, 3485398, 3694820)
'    (Chicago, 3550904, 3366957, 3005072, 2783726, 2896016)

Universal Windows Platform
Available since 8
.NET Framework
Available since 4.0
Portable Class Library
Supported in: portable .NET platforms
Silverlight
Available since 4.0
Windows Phone Silverlight
Available since 8.0
Windows Phone
Available since 8.1
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