Build error: You must add a reference to System.Runtime

To implement the fix, first expand out the existing web.config compilation section that looks like this by default:

<compilation debug="true" targetFramework="4.5"/>

Once expanded, I then added the following new configuration XML as I was instructed:

  <assemblies>     
    <add assembly="System.Runtime, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />   
  </assemblies>

The final web.config tags should look like this:

<compilation debug="true" targetFramework="4.5">
  <assemblies>     
    <add assembly="System.Runtime, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />   
  </assemblies>
</compilation>

Adding a reference to this System.Runtime.dll assembly fixed the issue:

C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETFramework\v4.5.1\Facades\System.Runtime.dll

Though that file in that explicit path doesn't exist on the build server.

I will post back with more information once I've found some documentation on PCL and these Facades.

Update

Yeah pretty much nothing on facade assemblies on the whole internet.

Google:

(Facades OR Facade) Portable Library site:microsoft.com