CS0012: The type ‘System.Object’ is defined in an assembly that is not referenced.

Gotcha, , , ,

I was setting up my Sitecore instance(s) on a new laptop, when this error reared its ugly head and kept me stuck for a good hour.

Looking online there are many reasons why this showed up, but in my case, all I had to do was adding this line under configuration/system.web/compilation/assemblies:

<add assembly="netstandard, Version=2.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51"/>

So, from

<compilation defaultLanguage="c#" debug="true" targetFramework="4.7.2">
<assemblies>
<add assembly="System.Web.Abstractions, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/>
<add assembly="System.Web.Routing, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/>
<add assembly="System.Data.Linq, Version=4.0.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089"/>
<add assembly="System.Web.Helpers, Version=3.0.0.0, Culture=neutral,PublicKeyToken=31BF3856AD364E35"/>
<add assembly="System.Web.Mvc, Version=5.2.3.0, Culture=neutral,PublicKeyToken=31BF3856AD364E35"/>
<add assembly="System.Web.WebPages, Version=3.0.0.0, Culture=neutral,PublicKeyToken=31BF3856AD364E35"/>
<add assembly="System.Web.WebPages.Razor, Version=3.0.0.0, Culture=neutral,PublicKeyToken=31BF3856AD364E35"/>
<add assembly="System.Web.Http, Version=5.2.3.0, Culture=neutral,PublicKeyToken=31BF3856AD364E35"/>
<add assembly="System.Web.Http.WebHost, Version=5.2.3.0, Culture=neutral,PublicKeyToken=31BF3856AD364E35"/>
<add assembly="System.Net.Http.Formatting, Version=5.2.3.0, Culture=neutral,PublicKeyToken=31BF3856AD364E35"/>
</assemblies>
</compilation>

To:

<compilation defaultLanguage="c#" debug="true" targetFramework="4.7.2">
<assemblies>
<add assembly="System.Web.Abstractions, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/>
<add assembly="System.Web.Routing, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/>
<add assembly="System.Data.Linq, Version=4.0.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089"/>
<add assembly="System.Web.Helpers, Version=3.0.0.0, Culture=neutral,PublicKeyToken=31BF3856AD364E35"/>
<add assembly="System.Web.Mvc, Version=5.2.3.0, Culture=neutral,PublicKeyToken=31BF3856AD364E35"/>
<add assembly="System.Web.WebPages, Version=3.0.0.0, Culture=neutral,PublicKeyToken=31BF3856AD364E35"/>
<add assembly="System.Web.WebPages.Razor, Version=3.0.0.0, Culture=neutral,PublicKeyToken=31BF3856AD364E35"/>
<add assembly="System.Web.Http, Version=5.2.3.0, Culture=neutral,PublicKeyToken=31BF3856AD364E35"/>
<add assembly="System.Web.Http.WebHost, Version=5.2.3.0, Culture=neutral,PublicKeyToken=31BF3856AD364E35"/>
<add assembly="System.Net.Http.Formatting, Version=5.2.3.0, Culture=neutral,PublicKeyToken=31BF3856AD364E35"/>
<add assembly="netstandard, Version=2.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51"/>
</assemblies>
</compilation>

Just hoping this might help someone with the same problem

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