IComparable(Of T) Interface

Defines a generalized comparison method that a value type or class implements to create a type-specific comparison method for ordering or sorting its instances.

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

'Declaration
Public Interface IComparable(Of In T)

Type Parameters

In T

The type of object to compare.

This type parameter is contravariant. That is, you can use either the type you specified or any type that is less derived. For more information about covariance and contravariance, see Covariance and Contravariance in Generics.

The IComparable(Of T) type exposes the following members.

  NameDescription
Public methodSupported by the XNA FrameworkSupported by Portable Class LibraryCompareToCompares 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.
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This interface is implemented by types whose values can be ordered or sorted and provides a strongly typed comparison method for ordering members of a generic collection object. For example, one number can be larger than a second number, and one string can appear in alphabetical order before another. It requires that implementing types define a single method, CompareTo(T), 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. Typically, the method is not called directly from developer code. Instead, it is called automatically by methods such as List(Of T).Sort and Add.

Typically, types that provide an IComparable(Of T) implementation also implement the IEquatable(Of T) interface. The IEquatable(Of T) interface defines the Equals method, which determines the equality of instances of the implementing type.

The implementation of the CompareTo(T) method must return an Int32 that has one of three values, as shown in the following table.

Value

Meaning

Less than zero

This object precedes the object specified by the CompareTo method in the sort order.

Zero

This current instance occurs in the same position in the sort order as the object specified by the CompareTo method argument.

Greater than zero

This current instance follows the object specified by the CompareTo method argument in the sort order.

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

Notes to Implementers

Replace the type parameter of the IComparable(Of T) interface with the type that is implementing this interface.

If you implement IComparable(Of T), you should overload the op_GreaterThan, op_GreaterThanOrEqual, op_LessThan, and op_LessThanOrEqual operators to return values that are consistent with CompareTo. In addition, you should also implement IEquatable(Of T). See the IEquatable(Of T) article for complete information.

The following code example illustrates the implementation of IComparable(Of T) 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: 4.6, 4.5, 4, 3.5, 3.0, 2.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

XNA Framework

Supported in: 3.0, 2.0, 1.0

Portable Class Library

Supported in: Portable Class Library

Supported in: Windows Phone 8.1

Supported in: Windows Phone Silverlight 8.1

Supported in: Windows Phone Silverlight 8
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