The quick answer
Don’t forget the ~/ (tilde) before the page name.
The lengthy explanation
Assume that we want to redirect to the default.aspx that’s on the site root:
If we’re on a page that’s also on the root, it will map to: http://www.mysite.com/default.aspx and work correctly.
The problem comes when we’re not on the site root but insite a child folder.
Lets say we’re on a page that’s insite the /Administration folder, the same line will map to http://www.mysite.com/Administration/default.aspx that ofcourse won’t resolve and return a glorious 404: Page Not Found.
Solving this is easy using the ResolveUrl method:
Where this won’t work
ResolveUrl is only available withing the context of a Page or a UserControl, if you’re, for instance, inside an ASHX this method isn’t available.
In these cases you have to use the followinf line to do the job:
We must include the tilde before the page name to make it a relative path, otherwise this method will throw an exception.