David Mohundro - DataBinding and Pointers, or "Why is my DataBinding not working?"

Wednesday, February 15, 2006

DataBinding and Pointers, or "Why is my DataBinding not working?"

Let's say you have the following class:

   51 Public Class Person
   52     Private _name As String
   53 
   54     Public Event NameChanged As EventHandler
   55 
   56     Public Property Name() As String
   57         Get
   58             Return _name
   59         End Get
   60         Set(ByVal value As String)
   61             _name = value
   62             OnNameChanged(EventArgs.Empty)
   63         End Set
   64     End Property
   65 
   66     Protected Sub OnNameChanged(ByVal e As EventArgs)
   67         RaiseEvent NameChanged(Me, e)
   68     End Sub
   69 End Class

We've set up the [Property]Changed events so that DataBinding will work correctly. Now, let's assume you want to bind the name property over to the text property of your TextBox control. We'll go the simple data binding route here for simplicity.

    3 Public Class Form1
    4     Private txtName As New TextBox
    5     Private _person As New Person
    6 
    7     Public Sub New()
    8         ' This call is required by the Windows Form Designer.
    9         InitializeComponent()
   10 
   11         ' Add any initialization after the InitializeComponent() call.
   12         Me.Controls.Add(txtName)
   13 
   14         _person.Name = "cool"
   15         txtName.DataBindings.Add("Text", _person, "Name")
   16     End Sub
   17 
   18     Private Sub Form_Click(ByVal sender As Object, ByVal e As EventArgs) _
   19         Handles Me.Click
   20         _person.Name = "clicked"
   21     End Sub
   22 End Class

There! Now the txtName will display "clicked" whenever you click on the form. Magic! Now, let's change that Click event like so to see what happens:

   18     Private Sub Form_Click(ByVal sender As Object, ByVal e As EventArgs) _
   19         Handles Me.Click
   20 
   21         Dim newPerson As New Person
   22         newPerson.Name = "new person"
   23         _person = newPerson
   24     End Sub

Why would I ever want to do is this way? Well, let's say you're remoting somewhere to get your data and your background thread returns a fully populated object. The easiest thing to do is to just point to the new object... except that it doesn't work.

All we did was change our instance over, but the textbox never changed. Why not? Because of REFERENCES, POINTERS, and MEMORY ADDRESSES! That's why! Check this out:

   18     Private Sub Form_Click(ByVal sender As Object, ByVal e As EventArgs) _
   19         Handles Me.Click
   20 
   21         Dim newPerson As New Person
   22         newPerson.Name = "new person"
   23 
   24         Debug.WriteLine("Are references equal? " & Object.ReferenceEquals(txtName.DataBindings("Text").DataSource, _person))
   25         _person = newPerson
   26         Debug.WriteLine("Are references equal? " & Object.ReferenceEquals(txtName.DataBindings("Text").DataSource, _person))
   27     End Sub

Whoa! The first Debug.Writeline will give you TRUE, but the second will give you FALSE. You see, understanding how references and pointers work is important. The textbox's DataSource is holding onto a reference of the old Person instance. You now have two different instances in memory, whether you like it or not.

What do you do about it? Well, as far as I can tell, you can either update your DataSource reference (it did change after all) or you can move your properties over manually so that your references don't change. I'm open to suggestions on other methods here.

.NET | Programming

posted on Wednesday, February 15, 2006 1:05:04 PM (Central Standard Time, UTC-06:00)

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