Changing the command argument on a button

Hi,
I've come across a minor issue but I'm not sure how to correct it.  I've Googled... and Googled but still fot no answer.  What's more I'm sure it's a schoolboy error but haven't managed to sort it yet.
Any help therefore woulf be appreciated.
I have a button which has a command argument set to "open" in the aspx page.
The button click event has the following code:
Button btn = (Button)sender
if(btn.commandargument == "open")
{
    // do something
    btn.commandargument = "close"
}
else
{
    // do something else
    btn.commandargument = "open"
}
 
The issue is that the button opens (the first time), but doesn't close.  Do I need to do something with the event handler?
 
Thanks
 
John
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