title: How to resolve 'unrecognized selector sent to instance'?
body: <p>In the AppDelegate, I'm alloc'ing an instance defined in a static library.  This instance has an NSString property set a "copy".  When I access the string property on this instance, the app crashes with 'unrecognized selector sent to instance'.  Xcode provides a code hint for the property, which means it is known in the calling app. The particular class is compiled into the static library target. What am I missing?</p>
  
  <p>Adding some code.</p>
  
  <pre><code>//static library 
  //ClassA.h
  @interface ClassA : NSObject {
  ...
  NSString *downloadUrl;
  }
  @property(nonatomic, copy) NSString *downloadUrl;
  
  //ClassA.m
  @synthesize downloadUrl;
  </code></pre>
  
  <p>In the calling app's appDelegate.</p>
  
  <pre><code>//app delegate header file
  @interface myApp : NSObject &lt;UIApplicationDelegate&gt; {
  ClassA *classA;
  }
  @property (nonatomic, retain) ClassA *classA;
  
  //app delegate .m file
  @synthesize classA;
  
  - (void)applicationDidFinishLaunching:(UIApplication *)application {
  classA = [[ClassA alloc] init];
  //exception occurs here.  downloadUrl is of type NSCFNumber
  classA.downloadUrl = @"http://www.abc.com/";
  ...}
  </code></pre>
  
  <p>Other classes in the app will get a reference to the delegate and call classA.downloadUrl.</p>