Listbox SelectionChanged firing when setting to -1 within function, wp7 - windows-phone-7

Question

I am running into a very odd problem in c# and I just wanted to know what is causing this. I have my theories, but not entirely sure and just want to see if it can be reproduced.
Standard Pivot Page in wp7 silverlight 4.
<Pivot>
<PivotItem>
<Listbox Width="400" Height="500" x:Name="box" SelectionChanged="myhandle">
<ListBoxItem x:Name="item1">
<TextBlock Height="40" Width="200" Text="hi everyone!"/>
</ListBoxItem>
<ListBoxItem x:Name="item2">
<TextBlock Height="40" Width="200" Text="No Wai"/>
</ListBoxItem>
<ListBoxItem x:Name="item3">
<TextBlock Height="40" Width="200" Text="Ya Rly!"/>
</ListBoxItem>
</Listbox>
</PivotItem>
</Pivot>
In my C#, I have the following:
private void myhandle(object sender, SelectionChangedEventArgs args)
{
var selection ="";
selection = (sender as Listbox).SelectedIndex.ToString();
box.SelectedIndex = -1;
}
Here is the problem: Whenever I click on one of the three listboxitems, the myhandle code makes selection equal to the proper SelectedIndex, but then it hits the box.SelectedIndex =-1; line and then refires the myhandle function. In doing so, selection is now -1.
I have no idea why it is going back up the stack. This shouldn't be a recursive function.
My goal is to select the item, but then have the SelectedIndex back to -1 so that the person is able to select the item once again if need be, instead of changing to another item and back.
Sure there is an easy fix of throwing a switch function and checking to see if it's -1 already, but that doesn't solve the problem of the recursion.
Thanks for the time.

Answer 1

Everytime the selection is changed, the SelectionChanged event will fire. This includes clearing the selection, which includes setting SelectedIndex = -1 and even if you are already in a SelectionChanged handler.
You can do something like this:
private bool inMyHandle = false;
private void myhandle(object sender, SelectionChangedEventArgs args)
{
if (!this.isMyHandle) {
this.isMyHandle = true;
try {
var selection ="";
selection = (sender as Listbox).SelectedIndex.ToString();
box.SelectedIndex = -1;
}
finally {
this.isMyHandle = false;
}
}
}

Answer 2

The standard MS samples already have this in a standard Listbox selected item event.
Simply use the following in the event handler code:
private void ListBox_SelectionChanged(object sender,System.Windows.Controls.SelectionChangedEventArgs e)
{
// If selected index is -1 (no selection) do nothing
if (ListBox.SelectedIndex == -1)
return;
//Do Something
// Reset selected index to -1 (no selection)
ListBox.SelectedIndex = -1;
}
No need to have any boolean handlers, the function just nothing if "-1" is the current index.
All this is to compensate for the way that the standard listbox opperates.
If you use MVVM and bind to the "Selecteditem" / "SelectedIndex" properties you have to keep the same thing in mind.

Answer 3

You can check args.AddedItems.Count too:
private void myhandle(object sender, SelectionChangedEventArgs args)
{
if (args.AddedItems.Count > 0)
{
....
box.SelectedIndex = -1;
}
}