The type ‘System.Web.Mvc.ModelClientValidationRule’ exists in both ‘c:\Program Files (x86)\Microsoft ASP.NET\ASP.NET MVC 3\Assemblies\System.Web.Mvc.dll’ and ‘c:\Program Files (x86)\Microsoft ASP.NET\ASP.NET Web Pages\v2.0\Assemblies\System.Web.WebPages.dll’

 

http://stackoverflow.com/questions/9471263/mvc-4-beta-side-by-side-installation-error

After installing MVC4 beta today, a few of my MVC 3 projects would not compile. (ModelClientValidationRule conflict) The fix was:

Edit

ProjectName.csproj

Change

<Reference Include="System.Web.WebPages"/>

To

<Reference Include="System.Web.WebPages, Version=1.0.0.0,
Culture=neutral, PublicKeyToken=31bf3856ad364e35, processorArchitecture=MSIL "/>