How to resolve 'unrecognized selector sent to instance'?

Question

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?

Adding some code.

//static library 
//ClassA.h
@interface ClassA : NSObject {
...
NSString *downloadUrl;
}
@property(nonatomic, copy) NSString *downloadUrl;

//ClassA.m
@synthesize downloadUrl;

In the calling app's appDelegate.

//app delegate header file
@interface myApp : NSObject <UIApplicationDelegate> {
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/";
...}

Other classes in the app will get a reference to the delegate and call classA.downloadUrl.

Answer 1

1) Is the synthesize within @implementation block?

2) Should you refer to self.classA = [[ClassA alloc] init]; and self.classA.downloadUrl = @"..." instead of plain classA?

3) In your myApp.m file you need to import ClassA.h, when it's missing it will default to a number, or pointer? (in C variables default to int if not found by compiler):

#import "ClassA.h".