URI for pages on the site

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I'm developing a Web App using C#. I want a page in the app to refer to
another page in the app. I tried to create a web request with the page
address below:

pageAddress = "http://~/clientDBQuery.aspx";
WebRequest request = WebRequest.Create (pageAddress);

and got the following error:

Exception Details: System.UriFormatException: Invalid URI: The hostname
could not be parsed.

What is the syntax for referring to a page on the current site so I don't
have to change it when it's published?
When I used the following page address:

pageAddress = "http://clientDBQuery.aspx";

I didn't get the parsing error but the debugger didn't stop on a break point
I set in the LoadPage method. This seems to occurr because it goes to a page
on my ISP which doesn't exist.

What's the right way to handle this situation?

Thanks, Neil