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IComparable(Of T).CompareTo Method

Compares the current object with another object of the same type.

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

'Declaration
Function CompareTo ( _
	other As T _
) As Integer

Parameters

other
Type: T

An object to compare with this object.

Return Value

Type: System.Int32
A value that indicates the relative order of the objects being compared. The return value has the following meanings:

Value

Meaning

Less than zero

This object is less than the other parameter.

Zero

This object is equal to other.

Greater than zero

This object is greater than other.

CompareTo provides a strongly typed comparison method for ordering members of a generic collection object. Because of this, it is usually not called directly from developer code. Instead, it is called automatically by methods such as List(Of T).Sort and Add.

This method is only a definition and must be implemented by a specific class or value type to have effect. The meaning of the comparisons, "less than," "equal to," and "greater than," depends on the particular implementation.

By definition, any object compares greater than Nothing, and two null references compare equal to each other.

Notes to Implementers

For objects A, B, and C, the following must be true:

A.CompareTo(A) is required to return zero.

If A.CompareTo(B) returns zero, then B.CompareTo(A) is required to return zero.

If A.CompareTo(B) returns zero and B.CompareTo(C) returns zero, then A.CompareTo(C) is required to return zero.

If A.CompareTo(B) returns a value other than zero, then B.CompareTo(A) is required to return a value of the opposite sign.

If A.CompareTo(B) returns a value x that is not equal to zero, and B.CompareTo(C) returns a value y of the same sign as x, then A.CompareTo(C) is required to return a value of the same sign as x and y.

Notes to Callers

Use the CompareTo method to determine the ordering of instances of a class.

The following code example illustrates the implementation of IComparable for a simple Temperature object. The example creates a SortedList(Of TKey, TValue) collection of strings with Temperature object keys, and adds several pairs of temperatures and strings to the list out of sequence. In the call to the Add method, the SortedList(Of TKey, TValue) collection uses the IComparable(Of T) implementation to sort the list entries, which are then displayed in order of increasing temperature.

Imports System.Collections.Generic

Public Class Temperature
    Implements IComparable(Of Temperature)

    ' Implement the generic CompareTo method with the Temperature class  
    ' as the type parameter.  
    
    Public Overloads Function CompareTo(ByVal other As Temperature) As Integer _
        Implements IComparable(Of Temperature).CompareTo

        ' If other is not a valid object reference, this instance is greater. 
        If other Is Nothing Then Return 1

        ' The temperature comparison depends on the comparison of the 
        ' the underlying Double values.  
        Return m_value.CompareTo(other.m_value)
    End Function 

    ' The underlying temperature value. 
    Protected m_value As Double = 0.0

    Public ReadOnly Property Celsius() As Double 
        Get 
            Return m_value - 273.15
        End Get 
    End Property 

    Public Property Kelvin() As Double 
        Get 
            Return m_value
        End Get 
        Set(ByVal Value As Double)
            If value < 0.0 Then  
                Throw New ArgumentException("Temperature cannot be less than absolute zero.")
            Else
                m_value = Value
            End If 
        End Set 
    End Property 

    Public Sub New(ByVal kelvins As Double)
        Me.Kelvin = kelvins 
    End Sub 
End Class 

Public Class Example
    Public Shared Sub Main()
        Dim temps As New SortedList(Of Temperature, String)

        ' Add entries to the sorted list, out of order.
        temps.Add(New Temperature(2017.15), "Boiling point of Lead")
        temps.Add(New Temperature(0), "Absolute zero")
        temps.Add(New Temperature(273.15), "Freezing point of water")
        temps.Add(New Temperature(5100.15), "Boiling point of Carbon")
        temps.Add(New Temperature(373.15), "Boiling point of water")
        temps.Add(New Temperature(600.65), "Melting point of Lead")

        For Each kvp As KeyValuePair(Of Temperature, String) In temps
            Console.WriteLine("{0} is {1} degrees Celsius.", kvp.Value, kvp.Key.Celsius)
        Next 
    End Sub 
End Class 

' The example displays the following output: 
'      Absolute zero is -273.15 degrees Celsius. 
'      Freezing point of water is 0 degrees Celsius. 
'      Boiling point of water is 100 degrees Celsius. 
'      Melting point of Lead is 327.5 degrees Celsius. 
'      Boiling point of Lead is 1744 degrees Celsius. 
'      Boiling point of Carbon is 4827 degrees Celsius. 
'

.NET Framework

Supported in: 4.5, 4, 3.5, 3.0, 2.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

.NET for Windows Store apps

Supported in: Windows 8

.NET for Windows Phone apps

Supported in: Windows Phone 8.1, Windows Phone Silverlight 8.1, Windows Phone Silverlight 8

Windows Phone 8.1, Windows Phone 8, Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

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

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