title: Why can't my PCL use System.Runtime?
body: <p>I have a PCL that targets the following platforms:</p>
  
  <ul>
  <li>.NET Framework 4.5</li>
  <li>Windows 8</li>
  <li>Windows Phone 8.1</li>
  </ul>
  
  <p>I need to add a reference to <a href="https://www.nuget.org/packages/System.Runtime" rel="nofollow"><code>System.Runtime</code></a> because I want to use another package that depends on it. From what I can see, <a href="https://www.nuget.org/packages/System.Runtime/4.0.0" rel="nofollow">version 4.0.0</a> appears to include the target platforms mentioned above. However, when I try installing it via NuGet, I get this stack trace:</p>
  
  <pre><code>PM&gt; Install-Package System.Runtime -Version 4.0.0
  # ...
  Install failed. Rolling back...
  Package 'System.Runtime 4.0.0' does not exist in project 'PclDummy'
  Package 'System.Runtime 4.0.0' does not exist in folder 'C:\Users\James\Documents\Visual Studio 2015\Projects\PclDummy\packages'
  Install-Package : Could not install package 'System.Runtime 4.0.0'. You are trying to install this package into a project that targets '.NETPortable,Version=v4.5,Profile=Profile111', but the package does not 
  contain any assembly references or content files that are compatible with that framework. For more information, contact the package author.
  </code></pre>
  
  <p>Why does this happen if <code>System.Runtime</code> supports all of the platforms my PCL does?</p>