**Error 1 The type 'System.Web.Compilation.WCFBuildProvider' is ambiguous: it could come from assembly 'C:\Windows\Micro

**Error 1 The type 'System.Web.Compilation.WCFBuildProvider' is ambiguous: it could come from assembly 'C:\Windows\Microsoft.Net\assembly\GAC_MSIL\System.Web.Extensions\v4.0_4.0.0.0__31bf3856ad364e35\System.Web.Extensions.dll' or from assembly 'C:\Windows\assembly\GAC_MSIL\System.Web.Extensions\3.5.0.0__31bf3856ad364e35\System.Web.Extensions.dll'. Please specify the assembly explicitly in the type name. App_WebReferences/**

 Today we upgrade our project from vs2008 to vs2010.

 then i get the error.

when i search it by google i don't get the answer.

so i try to slove it by myself. i get it.

 Right click the web site project and choice "Property pages" then remove system.web.extensions.