Unable to start debugging on the web server. Could not start ASP.NET debugging VS 2010, II7, Win 7 x64

Try going to IIS and checking to make sure the App Pool you are using is started. A lot of times, you will produce an error that shuts down the app pool. You just need to right click and Start and you should be good to go.

Turns out that the culprit was the IIS Url Rewrite module. I had defined a rule that redirected calls to Default.aspx (which was set as the start page of the web site) to the root of the site so that I could have a canonical home URL. However, apparently VS had a problem with this and got confused. This problem did not happen when I was using Helicon ISAPI_Rewrite so it didn't even occur to me to check.

I ended up creating a whole new web site from scratch and porting projects/files over little by little into my solution and rebuilding my web.config until I found this out!Well, at least now I have a slightly cleaner site using .NET 4.0 (so far, hopefully I won't run into any walls)--but what a pain!

Visual Studio, when starting up, will (for some reason) attempt to access the URL:

/debugattach.aspx

If you have a rewrite rule that redirects (or otherwise catches), say, .aspx files, somewhere else then you will get this error. The solution is to add this section to the beginning of your web.config's <system.webServer>/<rewrite>/<rules> section:

<rule name="Ignore Default.aspx" enabled="true" stopProcessing="true">    <match url="^debugattach\.aspx" />    <conditions logicalGrouping="MatchAll" trackAllCaptures="false" />    <action type="None" /></rule>

This will make sure to catch this one particular request, do nothing, and, most importantly, stop execution so none of your other rules will get run. This is a robust solution, so feel free to keep this in your config file for production.