If I have a custom .browser file, and I want to evaluate what will happen if it is given a particular User Agent, is there any way to do that through the .NET API?
I was attempting to use HttpBrowserCapabilites, but I'm not sure how to load a custom .browser file into that class.
Normally, you should not have to explicitly load the HttpBrowserCapabilities class; ASP.NET will load it for you, as long as you have your .browser file in the right place (in App_Browsers).
However, testing it will be another problem. You can't modify the "User-Agent" HTTP Header from within either the HttpApplication (global.asax) or a custom HttpModule.
This leaves only awkward techniques, such as using Reflection to force the value, or using an external tool (such as Fiddler). Alternately, if you're good with C/C++, you could take a simple example for an ISAPI filter and modify it, then install it in IIS.
Install the User Agent Switcher Firefox extension. It will let you adjust the user agent the browser sends to the web server.