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IComparable.CompareTo Method

July 28, 2014

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 32-bit signed integer 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 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) 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 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 sample 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 Example
   Public Sub Demo(ByVal outputBlock As System.Windows.Controls.TextBlock)
      Dim temperatures(9) As Temperature
      ' 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(ctr - 1) = temp
      Next

      ' Sort Array.
      Array.Sort(temperatures)

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


Windows Phone OS

Supported in: 8.1, 8.0, 7.1, 7.0

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