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

Question

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

The only way that worked for me - add the assembly to web.config

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

@PeterMajeed's comment in the accepted answer helped me out with a related problem. I am not using the portable library, but have the same build error on a fresh Windows Server 2012 install, where I'm running TeamCity.

Installing the Microsoft .NET Framework 4.5.1 Developer Pack took care of the issue (after having separately installed the MS Build Tools).

I had this problem in some solutions on VS 2015 (not MVC though), and even in the same solution on one workstation but not on another. The errors started appeared after changing .NET version to 4.6 and referencing PCL.

The solution is simple: Close the solution and delete the hidden .vs folder in the same folder as the solution.

Adding the missing references as suggested in other answers also solves the problem, but the error remains solved even after you remove the references again.

As for TeamCity, I cannot say since my configuration never had a problem. But make sure that you reset the working catalog as a part of your debugging effort.

It's an old issue but I faced it today in order to fix a build pipeline on our continuous integration server. Adding

<Reference Include="System.Runtime" />

to my .csproj file solved the problem for me.

A bit of context: the interested project is a full .NET Framework 4.6.1 project, without build problem on the development machines. The problem appears only on the build server, which we can't control, may be due to a different SDK version or something similar.

Adding the proposed <Reference solved the build error, at the price of a missing reference warning (yellow triangle on the added entry in the references tree) in Visual Studio.

I was also facing this problem trying to run an ASP .NET MVC project after a minor update to our codebase, even though it compiled without errors:

Compiler Error Message: CS0012: The type 'System.Object' is defined in an assembly that is not referenced. You must add a reference to assembly 'System.Runtime, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'.

Our project had never run into this problem, so I was skeptical about changing configuration files before finding out the root cause. From the error logs I was able to locate this detailed compiler output which pointed out to what was really happening:

warning CS1685: The predefined type 'System.Runtime.CompilerServices.ExtensionAttribute' is defined in multiple assemblies in the global alias; using definition from 'c:\Windows\Microsoft.NET\Framework64\v4.0.30319\mscorlib.dll'

c:\Users\Admin\Software Development\source-control\Binaries\Publish\WebApp\Views\Account\Index.cshtml(35,20): error CS0012: The type 'System.Object' is defined in an assembly that is not referenced. You must add a reference to assembly 'System.Runtime, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'.

c:\Windows\Microsoft.NET\Framework64\v4.0.30319\Temporary ASP.NET Files\meseems.webapp\68e2ea0f\8c5ee951\assembly\dl3\52ad4dac\84698469_3bb3d401\System.Collections.Immutable.DLL: (Location of symbol related to previous error)

Apparently a new package added to our project was referencing an older version of the .NET Framework, causing the "definition in multiple assemblies" issue (CS1685), which led to the razor view compiler error at runtime.

I removed the incompatible package (System.Collections.Immutable.dll) and the problem stopped occurring. However, if the package cannot be removed in your project you will need to try Baahubali's answer.