Custom dialog

I have a problem with my code...
VB Code:
Public Class frmViewEditBoundary
    Inherits System.Windows.Forms.Form
 
'some code omitted for clarity
 
'constructor
    Public Sub New(ByVal data As Boundary)
        MyBase.New()
 
        InitializeComponent()
 
        itsBoundary = data
    End Sub
 
    Public Shadows Function ShowDialog(ByRef owner As IWin32Window) As Boundary
        MyBase.ShowDialog(owner)       'call the base class version and ignore its return value
        Return DialogResult       'return our own data
    End Function
 
    Public Shadows Property DialogResult() As Boundary
        Get
            Return itsDialogResult
        End Get
        Set(ByVal Value As Boundary)
            itsDialogResult = Value
        End Set
    End Property
 
'other code omitted for clarity
 
    Private Sub cmdCancel_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles cmdCancel.Click
        DialogResult = Nothing
        Me.Close()
    End Sub
 
    Private Sub cmdOK_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles cmdOK.Click
        DialogResult = itsBoundary
        Me.Close()
    End Sub
 
End Class
What it should do:
A Boundary object is passed in when declaring a NEW frmViewEditBoundary instance. This boundary's data is examined and displayed in the form's controls. This part works OK. But when the user has finished viewing or editing the data he clicks either OK or CANCEL to close the form.

If OK is clicked then the boundary object is returned via the DialogResult property that I have shadowed to use class Boundary as its data type. It should be returned regardless of any changes made.

If CANCEL is clicked then the value NOTHING is passed back instead.

However, the problem is that no matter which button I hit the boundary is passed back. Why is this? The code for the buttons is right at the bottom of the class block above.

Have I shadowed the 2 things properly (DialogResult and ShowDialog())?

Any ideas?

Thanks,
Adam.