How does Enumerate work in MonoTouch?

Question

In MonoTouch I need to process each object in an NSSet. My attempt, using Enumerate, is as follows:

public override void ReturnResults ( BarcodePickerController picker, NSSet results )
{
    var n = results.Count;  // Debugging - value is 3
    results.Enumerate( delegate( NSObject obj, ref bool stop ) 
    {
        var foundCode = ( obj as BarcodeResult ); // Executed only once, not 3 times
        if ( foundCode != null )
        {
            controller.BarcodeScannedResult (foundCode);
        }
    });
// Etc
}

Although the method is invoked with three objects in results, only one object is processed in the delegate. I would have expected the delegate to be executed three times, but I must have the wrong idea of how it works.

Unable to find any documentation or examples. Any suggestion much appreciated.

Answer 1

You have to set the ref parameter to false. This instructs the handler to continue enumerating:

if ( foundCode != null )
{
    controller.BarcodeScannedResult (foundCode);
    stop = false; // inside the null check
}

Here is the ObjC equivalent from Apple documentation.

Answer 2

Or you could try this extension method to make it easier..

public static class MyExtensions {
    public static IEnumerable<T> ItemsAs<T>(this NSSet set) where T : NSObject {
        List<T> res = new List<T>();
        set.Enumerate( delegate( NSObject obj, ref bool stop ) {
            T item = (T)( obj ); // Executed only once, not 3 times
            if ( item != null ) {
                res.Add (item);
                stop = false; // inside the null check
            }
         });

         return res;
    }
}   

Then you can do something like:

foreach(BarcodeResult foundCode in results.ItemsAs<BarcodeResult>()) {
    controller.BarcodeScannedResult (foundCode);
}

Note: Keep in mind this creates another list and copies everything to it, which is less efficient. I did this because "yield return" isn't allowed in anonymous methods, and the alternative ways I could think of to make it a real enumerator without the copy were much much more code. Most of the sets I deal with are tiny so this doesn't matter, but if you have a big set this isn't ideal.