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IComparable.CompareTo Method

Compares the current instance with another object of the same type and returns an integer that indicates whether the current instance precedes, follows, or occurs in the same position in the sort order as the other object.

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

'Declaration
Function CompareTo ( _
	obj As Object _
) As Integer

Parameters

obj
Type: System.Object

An object to compare with this instance.

Return Value

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

Value

Meaning

Less than zero

This instance precedes obj in the sort order.

Zero

This instance occurs in the same position in the sort order as obj.

Greater than zero

This instance follows obj in the sort order.

ExceptionCondition
ArgumentException

obj is not the same type as this instance.

The CompareTo method is implemented by types whose values can be ordered or sorted. It is called automatically by methods of non-generic collection objects, such as Array.Sort, to order each member of the array. If a custom class or structure does not implement IComparable, its members cannot be ordered and the sort operation can throw an InvalidOperationException.

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 specified in the Return Value section ("precedes", "occurs in the same position as", and "follows") depends on the particular implementation.

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

The parameter, obj, must be the same type as the class or value type that implements this interface; otherwise, an ArgumentException is thrown.

Notes to Implementers

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

A.CompareTo(A) must return zero.

If A.CompareTo(B) returns zero, then B.CompareTo(A) must return zero.

If A.CompareTo(B) returns zero and B.CompareTo(C) returns zero, then A.CompareTo(C) must return zero.

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

If A.CompareTo(B) returns a value x not equal to zero, and B.CompareTo(C) returns a value y of the same sign as x, then A.CompareTo(C) must 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 example illustrates the use of CompareTo to compare a Temperature object implementing IComparable with another object. The Temperature object implements CompareTo by simply wrapping a call to the Int32.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

.NET Framework

Supported in: 4.5.2, 4.5.1, 4.5, 4, 3.5, 3.0, 2.0, 1.1, 1.0

.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 8, Silverlight 8.1

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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