web.config

<configuration>

<configSections>

<connectionStrings>

<appSettings>

<add key="webpages:Version" value="3.0.0.0" />

Visual Studio will load Razor v3 dlls to edit the project’s webpage files (i.e. cshtml or vbhtml files) .

If the key is missing from web.config file, Visual Studio will use bin directory’s razor dll version to determine the design time razor engine dll to load.

How does VS determine which version of Razor engine to use when editing razor webpage files