Wednesday, July 11, 2007

Compare Just About Any Property of Any Object

Archimedes once claimed that given a lever long enough, a fulcrum strong enough, and a place to stand, he could move the world. (Archimedes also reportedly ran through the streets of Athens naked shouting Eureka when he discovered displacement, but that's another story.) Advanced techniques such as interfaces, reflection, and generics may not enable you to move the world, but they are sufficient tools for doing a lot of work, which is what Archimedes meant.

One such powerful tool that the .NET Framework provides is the generic interface IComparer. Implement IComparer and provide the type of object to compare, and .NET supports sorting any kind of collection of objects. Implement IComparer and fold in some reflection, and your IComparer will compare just about any property of any object. Using reflection won't yield a lightening fast sort, but the tradeoff for speed is flexibility.

Implementing IComparer

IComparer takes a parameter type T and two arguments of type T. Your job is to implement the comparison behavior and return -1 for x < y, 0 for x = y, and >0 for x > y. You can slightly enhance this basic comparison behavior by passing in a property name to the constructor, passing in an enumerated value to indicate sort direction, and using reflection to obtain access to the property value.

Listing 1 shows how you can specify the value of enumerated fields to reverse the results of the sort behavior:

Listing 1: An Enumeration with Specific Values

Public Enum SortDirection
Descending = -1
Ascending = 1

By multiplying the return value of a comparison by 1 or -1, you change the direction of the sort.

Listing 2 shows the implementation of the PropertyComparer class, which uses reflection to ensure that the field either implements IComparable or has a CompareTo method:

Listing 2: The PropertyComparer

Public Class PropertyComparer(Of T)
Implements IComparer(Of T)
Private FPropertyName As String = ""
Private FDirection As SortDirection

Public Sub New(ByVal propertyName As String)
FPropertyName = propertyName
FDirection = SortDirection.Ascending
End Sub

Public Sub New(ByVal propertyName As String, _
ByVal Direction As SortDirection)
FPropertyName = propertyName
FDirection = Direction
End Sub

' Try to sort based on type using CompareTo method
' Multiple by FDirection to alternate sort direction
Public Function Compare(ByVal x As T, ByVal y As T) _
As Integer Implements System.Collections.Generic. _
IComparer(Of T).Compare

Dim propertyX As PropertyInfo = _
Dim propertyY As PropertyInfo = _
Dim px As Object = propertyX.GetValue(x, Nothing)
Dim py As Object = propertyY.GetValue(y, Nothing)

If (TypeOf px Is Integer) Then
Return Compare(Of Integer)(CType(px, Integer), _
CType(py, Integer)) * FDirection
End If

If (TypeOf px Is Decimal) Then
Return Compare(Of Decimal)(CType(px, Decimal), _
CType(py, Decimal)) * FDirection
End If

If (TypeOf px Is DateTime) Then
Return Compare(Of DateTime)(CType(px, DateTime), _
CType(py, DateTime)) * FDirection
End If

If (TypeOf px Is Double) Then
Return Compare(Of Double)(CType(px, Double), _
CType(py, Double)) * FDirection
End If

If (TypeOf px Is String) Then
Return Compare(Of String)(CType(px, String), _
CType(py, String)) * FDirection
End If

If (TypeOf px Is Decimal) Then
Return Compare(Of Decimal)(CType(px, Decimal), _
CType(py, Decimal)) * FDirection
End If

Dim methodX As MethodInfo = _
If (methodX Is Nothing = False) Then
Return CType(methodX.Invoke(px, New Object() {py}), _
Integer) * FDirection
Return 0
End If
End Function

Private Function Compare(Of K As IComparable)(ByVal x As K, _
ByVal y As K) As Integer
Return x.CompareTo(y)
End Function

End Class

The PropertyComparer class uses reflection to get the type information for the property passed into the PropertyComparer's constructor and attempts to call the property's CompareTo method (if it exists). If the property has a CompareTo method, the comparison will work. If no CompareTo method exists, the Compare method returns 0, which has a benign effect.

The key to understanding the PropertyComparer class lies in the boldfaced code of Listing 2. The first thing Compare does is get the PropertyInfo record for the x and y arguments. Next, it obtains the value of the property using the argument's x and y and the PropertyInfo record. Finally, it inspects the type of the property to determine how to call the Compare method implemented at the end of the class. The Compare method implemented has a where predicate that limits the types of the parameter K to those that implement IComparable. The reason for this is that IComparable types implement CompareTo.


Defining Something to Sort

You can sort just about anything. To make the demonstration reflect something you may be familiar with, Listing 3 contains an implementation of a simple Customer class:


Listing 3: A Simple Customer Class

Public Class Customer
Private FCustomerNumber As Integer
Private FName As String
Public Sub New(ByVal customerNumber As Integer, _
ByVal name As String)
FCustomerNumber = customerNumber
FName = name
End Sub

Public ReadOnly Property CustomerNumber() As Integer
Return FCustomerNumber
End Get
End Property

Public Property Name() As String
Return FName
End Get
Set(ByVal value As String)
FName = value
End Set
End Property
End Class

You can sort a list of Customer objects by the Name or CustomerNumber.

Invoking the Sort Behavior

Listing 4 demonstrates how to create a generic list of strongly typed Customer objects, add some Customers to the list, and sort the Customers by Name:

Listing 4: Code to Demonstrate the PropertyComparer

Imports System
Imports System.Collections.Generic
Imports System.Text
Imports System.Reflection

Module Module1
Sub Main()
Dim list As List(Of Customer) = New List(Of Customer)
list.Add(New Customer("Paul"))
list.Add(New Customer("Noah"))
list.Add(New Customer("Alex"))
list.Add(New Customer("Jim"))
list.Sort(New PropertyComparer(Of Customer)("Name"))
Dim o As Customer
For Each o In list
End Sub
End Module

If you changed the construction of the PropertyComparer to initialize the PropertyComparer with the CustomerNumber property name, you would get a completely different sort result.

A General Technique for Sorting Objects

Advanced techniques are the lever, fulcrum, and place to stand that can help you move mountains. This example combined reflection, interfaces, and generics to create a general technique for sorting objects based on any field.

I use a variation of the PropertyComparer in production, and it's a nice addition to the sorting behavior in .NET.

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