title: Conditional compilation depending on the framework version in C#
body: <p>Are there any preprocessor symbols which allow something like</p>
  
  <pre><code>#if CLR_AT_LEAST_3.5
  // use ReaderWriterLockSlim
  #else
  // use ReaderWriterLock
  #endif
  </code></pre>
  
  <p>or some other way to do this?</p>