title: Illegal redeclaration of property in class extension 'AppDelegate' (attribute must be 'readwrite', while its primary must be 'readonly')
body: <p>I moved this line:</p>
  
  <pre><code>@property (nonatomic, retain) IBOutlet UIWindow *window;
  </code></pre>
  
  <p>into a class extension in my App Delegate but now get this error:</p>
  
  <pre><code>Illegal redeclaration of property in class extension 'AppDelegate' (attribute must be 'readwrite', while its primary must be 'readonly')
  </code></pre>
  
  <p>The full code from the <code>.m</code> file looks like this:</p>
  
  <pre><code>@interface AppDelegate()
  @property (nonatomic, retain) IBOutlet UIWindow *window;
  @end
  </code></pre>
  
  <p>What's the problem?</p>