How do I sort an ArrayList of objects by a property value

  • Thread starter Thread starter Paulers
  • Start date Start date
P

Paulers

Hello I have an ArrayList full of Person objects. I would like to sort
the object array by the objects property 'Name' so when I loop through
it and populate a combobox they are in order. How do I attack something
like this? Any help is greatly appreciated.

thanks!
 
Hello I have an ArrayList full of Person objects. I would like to sort
the object array by the objects property 'Name' so when I loop through
it and populate a combobox they are in order. How do I attack
something like this? Any help is greatly appreciated.

thanks!
Click to expand...


Make the person class implement the IComparable interface, Write code
for the Compare method that appears. Then it will all magically work
when you call the sort method. If this is a 2005 version then you can
use IComparable(Of Person). And use a list(Of Person) instead of
arraylist too!

If you have 2003, then change this to plain IComparable. The parameter
of the Compare method will be an object, so cast it to Person.

Public Class Person
Implements IComparable(Of Person)

Private _name As String

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

' called by the arraylist.sort method.
Public Function CompareTo(ByVal other As Person) As Integer _
Implements System.IComparable(Of Person).CompareTo
' Compare this ones name with the other ones name.
' return <0 if this one comes first,
' 0 if they are equal,
' >0 if the other one comes first.
' String.Compare does this anyway
Return String.Compare(Me.Name, other.Name)
End Function
End Class
 
Paulers,

One way is to have your Person class implement IComparable.CompareTo. For
Example:

Public Class Person
Implements IComparable

Public Overloads Function CompareTo(ByVal obj As Object) As Integer _
Implements IComparable.CompareTo

Dim person As Person = DirectCast(obj, Person)

If Me.Name < person.Name Then
Return -1
ElseIf Me.Name = person.Name Then
Return 0
Else
Return 1
End If
End Function

Then, when you call the arraylist's Sort method, the objects will be sorted
by Name.
 
Here's how I've done it.. If ArrayList implements the IBindingList interface
you can use something like this... If it doesn't you can implement the
IBindingList interface in one of your base classes, if you want an example of
how to do that look at Rocky Lhotka's CSLA object framework, it's open
source... Or just use his base classes, it takes a little bit more time to
implement but it's worth it with the funtionality you achieve by doing so,
there's also some code generation tools that make generating these object
hiearchies trivial..

Public WriteOnly Property Sort() As String

Set(ByVal Value As String)
Dim str() As String = Split(Value, " ")
Dim mSortPropertyName As String = "", mSortDirection As String =
""

'parse the input string
If str.Length = 1 Then 'direction not specified, use default
mSortPropertyName = str(0).Trim
ElseIf str.Length = 2 Then
mSortPropertyName = str(0).Trim & ""
mSortDirection = str(1).Trim & ""
Else
Throw New ApplicationException("Too many parameters
specified for the sort.")
End If
Dim childType As Type = GetType(Me)
Dim mSortProperty As PropertyDescriptor = _

TypeDescriptor.GetProperties(childType).Item(mSortPropertyName)
Select Case mSortDirection.ToUpper.Trim
Case Is = "DESC"
DirectCast(Me, IBindingList).ApplySort(mSortProperty,
ListSortDirection.Descending)
Case Is = "ASC"
DirectCast(Me, IBindingList).ApplySort(mSortProperty,
ListSortDirection.Ascending)
Case Else
DirectCast(Me, IBindingList).ApplySort(mSortProperty,
ListSortDirection.Ascending)
End Select
End Set
End Property


HTH
 
Kerry said:
One way is to have your Person class implement IComparable.CompareTo. For
Example:

Public Class Person
Implements IComparable

Public Overloads Function CompareTo(ByVal obj As Object) As Integer _
Implements IComparable.CompareTo

Dim person As Person = DirectCast(obj, Person)

If Me.Name < person.Name Then
Return -1
ElseIf Me.Name = person.Name Then
Return 0
Else
Return 1
End If
End Function

Then, when you call the arraylist's Sort method, the objects will be sorted
by Name.
Click to expand...

Assuming the Name property is a String, you can shorten that code to
this:

Public Class Person
Implements IComparable

Public Overloads Function CompareTo(ByVal obj As Object) As Integer
_
Implements IComparable.CompareTo

Dim person As Person = DirectCast(obj, Person)

Return Me.Name.CompareTo(person.Name)

End Function
 
Kerry said:
Chris,

That's really nice. I had not noticed that CompareTo method of a string.
Click to expand...

That should work for all primitive types. Of course if you have custom
comparison logic, such as last name AND first name, then you would have
to implement like you showed.

Cheers!

Chris
 
Thanks everyone! I really appreciate your help.
Chris said:
That should work for all primitive types. Of course if you have custom
comparison logic, such as last name AND first name, then you would have
to implement like you showed.

Cheers!

Chris
Click to expand...
 
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