title: Trying to build System.Runtime.Caching from Microsoft/referencesource
body: <p>I'm trying to build on my machine the System.Runtime.Caching project in Visual Studio 2015. The build fails because there is no definition for the classes:</p>
  
  <pre><code>private CacheExpires _expires;
  private CacheUsage _usage;
  </code></pre>
  
  <p>CacheExpires and CacheUsage are not defined in this project and also not in any other project from the .net framework, and even in the Reference Source <a href="http://referencesource.microsoft.com/#System.Runtime.Caching" rel="nofollow">website</a> there is no definition for those classes.</p>
  
  <p>Those classes are used in MemoryCacheEntry.cs and MemoryCacheStore.cs</p>
  
  <p>Has anyone succeed to build it? Am I doing something wrong or is this project not completely open source?</p>
  
  <p>thanks!   </p>