Page Inspector Runtime must be registred in root web.config. Warning - asp.net-mvc

Question

In VS 2012 update 1,
I am getting a "Page Inspector Runtime must be registred in root web.config" Warning, when running the page inspector.
Can anyone assist me in what I have to put in the root web.config to fix this . Or some other solution.
Thanks

Answer 1

I realize this is an old question, but since it comes up on a search for the same issue which is still relevant to VS 2019, I thought I'd provide the actual answer:
The error mentioned here is referring to the registration of a file that is in GAC that needs to happen in the machine-level web.config. Not, as the other answers mentioned, in the project's web.config file.
The machine-level web.config file is located (for .net 4+, which I'm assuming is all anyone is using right now) at c:\Windows\Microsoft.NET\Framework\v4.0.30319\Config\web.config
And here is what you'll need to add:
<configuration>
..
<system.web>
...
<compilation>
<assemblies>
...
<add assembly="Microsoft.VisualStudio.Web.PageInspector.Loader, Version=1.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a"/>
</assemblies>
</compilation>
</system.web>
</configuration>
There's obviously a lot more in your machine-level web.config file - I'm just showing it as above so you see the hierarchy it needs to be in.
Normally this is added by the VS installer. But there are circumstances in which the installer fails to add it or even removes it..

Answer 2

To generate the metadata that is required for some of its features, Page Inspector must instrument the project by adding the following setting to the element in the Web.config file. You can use Page Inspector even if this setting is disabled or it is missing. However, some features like the source selection mapping will not work under these conditions.
Web.config
<add key="VisualStudioDesignTime:Enabled" value="true" />

Answer 3

Add to 'web.config' the key/value suggested by #Vicky_Raj_Sharma above and the "PageInspector:ServerCodeMappingSupport" under , like this:
<appSettings>
<add key="webpages:Version" value="2.0.0.0" />
<add key="webpages:Enabled" value="false" />
<add key="PreserveLoginUrl" value="true" />
<add key="ClientValidationEnabled" value="true" />
<add key="UnobtrusiveJavaScriptEnabled" value="true" />
<!-- Keys above came with Hot Towel SPA template for VS2012 -->
<!-- Keys below where manually added -->
<add key="VisualStudioDesignTime:Enabled" value="true" />
<add key="PageInspector:ServerCodeMappingSupport" value="Enabled"/>
</appSettings>
When browsing the page (Browse with menu) from within VS it will show the page in your preferred browser.
Still haven´t figured out how to run this directly on IIS8 though...