Question

Monotouch 5.0.2.

Running the simple code below I get this in the output panel:

objc[20283]: Object 0x9c08110 of class __NSCFString autoreleased with no pool in place - just leaking - break on objc_autoreleaseNoPool() to debug
objc[20283]: Object 0x9c07840 of class __NSCFData autoreleased with no pool in place - just leaking - break on objc_autoreleaseNoPool() to debug

Which NSStrings are leaking here and why? It seems to be enough to just create a new Uri() and a new WebClient() objetc. The event handler doesn’t even have to be attached.

using System;
using MonoTouch.Foundation;
using MonoTouch.UIKit;
using System.Net;
using System.IO;

namespace DownloadTest
{
    [Register ("AppDelegate")]
    public partial class AppDelegate : UIApplicationDelegate
    { 
        UIWindow window;

        public override bool FinishedLaunching (UIApplication app, NSDictionary options)
        {
            window = new UIWindow (UIScreen.MainScreen.Bounds);

            UIButton btn = UIButton.FromType (UIButtonType.RoundedRect);
            btn.Frame = new System.Drawing.RectangleF (40, 40, 100, 30);
            btn.TouchUpInside += delegate {
                // These two lines seem to caus the leak.
                Uri uri = new Uri ("http://silverlightinaction.com/video3.wmv");
                WebClient webClient = new WebClient ();
                // This line does not matter, the info about the leaking NSString alsp appears without an event being attached.
                webClient.OpenReadAsync (uri);                             
            };
            window.AddSubview (btn);

            window.MakeKeyAndVisible ();

            return true;
        }

        void HandleWebClientOpenReadCompleted (object sender, OpenReadCompletedEventArgs e)
        {
            using (var pool = new NSAutoreleasePool())
            {
                using (Stream stream = e.Result)
                using (FileStream fileStream = File.OpenWrite(Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.Personal), "download.bin")))
                {
                    stream.CopyTo (fileStream, 30000);
                    stream.Close ();
                    fileStream.Close ();
                }
            }
        }
    }
}

Answer 1

This is a MonoTouch bug. WebClient uses Thread which does not automagically creates an NSAutoreleasePool around the code being executed (e.g. like you did in your delegate). That can cause some leaks – just like you’re seeing with the warning messages.

Note that using (threads from) ThreadPool is already ensuring that an NSAutoreleasePool covers the thread’s execution.