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IComparable Interface

Updated: July 2008

Defines a generalized type-specific comparison method that a value type or class implements to order or sort its instances.

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

'Declaration
<ComVisibleAttribute(True)> _
Public Interface IComparable
'Usage
Dim instance As IComparable

This interface is implemented by types whose values can be ordered or sorted. It requires that implementing types define a single method, CompareTo, that indicates whether the position of the current instance in the sort order is before, after, or the same as a second object of the same type. The instance's IComparable implementation is called automatically by methods such as Array.Sort and ArrayList.Sort.

All numeric types (such as Int32 and Double) implement IComparable, as do String, Char, and DateTime. Custom types should also provide their own implementation of IComparable to enable object instances to be ordered or sorted.

The following code sample illustrates the implementation of IComparable and the requisite CompareTo method.

Imports System.Collections

Public Class Temperature
    Implements IComparable
    ' The temperature value 
    Protected temperatureF As Double 

    Public Overloads Function CompareTo(ByVal obj As Object) As Integer _
        Implements IComparable.CompareTo

        If obj Is Nothing Then Return 1

        Dim otherTemperature As Temperature = TryCast(obj, Temperature)
        If otherTemperature IsNot Nothing Then 
            Return Me.temperatureF.CompareTo(otherTemperature.temperatureF)
        Else 
           Throw New ArgumentException("Object is not a Temperature")
        End If    
    End Function 

    Public Property Fahrenheit() As Double 
        Get 
            Return temperatureF
        End Get 
        Set(ByVal Value As Double)
            Me.temperatureF = Value
        End Set 
    End Property 

    Public Property Celsius() As Double 
        Get 
            Return (temperatureF - 32) * (5/9)
        End Get 
        Set(ByVal Value As Double)
            Me.temperatureF = (Value * 9/5) + 32
        End Set 
    End Property 
End Class 

Public Module CompareTemperatures
   Public Sub Main()
      Dim temperatures As New ArrayList
      ' Initialize random number generator. 
      Dim rnd As New Random()

      ' Generate 10 temperatures between 0 and 100 randomly. 
      For ctr As Integer = 1 To 10
         Dim degrees As Integer = rnd.Next(0, 100)
         Dim temp As New Temperature
         temp.Fahrenheit = degrees
         temperatures.Add(temp)   
      Next 

      ' Sort ArrayList.
      temperatures.Sort()

      For Each temp As Temperature In temperatures
         Console.WriteLine(temp.Fahrenheit)
      Next       
   End Sub 
End Module 
' The example displays the following output to the console (individual 
' values may vary because they are randomly generated): 
'       2 
'       7 
'       16 
'       17 
'       31 
'       37 
'       58 
'       66 
'       72 
'       95

Windows 7, Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP Starter Edition, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98, Windows CE, Windows Mobile for Smartphone, Windows Mobile for Pocket PC, Xbox 360, Zune

The .NET Framework and .NET Compact Framework do not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

.NET Framework

Supported in: 3.5, 3.0, 2.0, 1.1, 1.0

.NET Compact Framework

Supported in: 3.5, 2.0, 1.0

XNA Framework

Supported in: 3.0, 2.0, 1.0

Date

History

Reason

July 2008

Added detail on usage by the common language runtime and information for implementors.

Customer feedback.

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