Many .Net developers that have experience in presentation-layer development have previous exposure to this one, but to those who haven’t, I must share: Request.ApplicationPath is evil! Request.Application path should never be used. Ever. I hate it!

It’s misgivings are most commonly exposed when concatenating strings to the end of it to create a URL of some sort.

sURL = Request.ApplicationPath & “/someDirectory/somePage.aspx”

This is evil. Neither will work in all situations due to the method’s inconsistency with trailing slashes.

If the application root is in a sub-directory, say “http://server/myRoot/index.aspx”, then the appPath is “/myRoot”. Note: there is no slash at the end, and the above sURL gets a value of “/myRoot/someDirectory/somePage.aspx”.

If the application root is not in a sub-directory, say “http://server/index.aspx”, then the appPath is “/”. Note: there is a slash at the end, and the above sURL gets a value or “//someDirectory/somePage.aspx”. In this case, rather than requesting “http://server/someDirectory/somePage.aspx”, wonderful Internet Explorer requests “http://somedirectory/somePage.aspx”. (I don’t fault IE for this. I commend it. It is actually one of the few cases where IE doesn’t kludge together a band-aid for Developer mistakes.)

So, don’t use Request.ApplicationPath. Ever. With no exception. You could make a method, perhaps MyUtilities.ApplicationPath, that checks to see if the return contains a trailing ‘/’, and if it does, give it the axe. This will turn your domain-root appPath to an empty string. Use your new method rather than Request.ApplicationPath, and all will be well with the world. But, don’t do it! Don’t make a new method. That just continues the evilness.

Use Page.ResolveURL(”~/someDirectory/somePage.aspx”) or the counterpart Control.ResolveURL if you want it to be relative to the control’s location. This is the only scenario you should use. ApplicationPath is evil!