title: How to understand "?." symbol in Entity Framework source code?
body: <p>I'm studying Entity Framework source code and find a lot of <code>?.</code> symbols in this project. I can't understand the function of <code>?.</code>. </p>
  
  <p>For example, we can find <code>?.</code> symbol in <code>DbContext.cs</code> like below:</p>
  
  <pre><code>public virtual void Dispose()
  {
      _disposed = true;
      _serviceScope?.Dispose();
      _setInitializer = null;
      _changeTracker = null;
      _stateManager = null;
      _changeDetector = null;
      _graphAttacher = null;
      _model = null;
  }
  </code></pre>