Bug 3862 - Linker removes certain attributes on members with [Preserve]
Summary: Linker removes certain attributes on members with [Preserve]
Alias: None
Product: iOS
Classification: Xamarin
Component: Tools ()
Version: 5.2
Hardware: PC Mac OS
: --- normal
Target Milestone: Untriaged
Assignee: Sebastien Pouliot
Depends on:
Reported: 2012-03-12 15:45 UTC by David Merriman
Modified: 2012-03-13 08:42 UTC (History)
2 users (show)

Is this bug a regression?: ---
Last known good build:

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Description David Merriman 2012-03-12 15:45:41 UTC
Consider the following code snippet:

using System;
using System.ComponentModel;
using System.Drawing;
using System.Reflection;

using MonoTouch.Foundation;
using MonoTouch.UIKit;

namespace AttributeRemovalBug

    public partial class AttributeRemovalBugViewController : UIViewController
        public AttributeRemovalBugViewController(IntPtr handle) : base (handle)

        public override void ViewDidLoad()

            Type t = GetType().Assembly.GetType("AttributeRemovalBug.IAmNotStaticallyReferenced");
            Console.WriteLine("typeof(IAmNotStaticallyReferenced): {0}", t);
            var props = t.GetProperties();
            foreach (var prop in props)
                var attrs = prop.GetCustomAttributes(true);
                Console.WriteLine("{0} has {1} attributes: ", prop.Name, attrs.Length);
                foreach (var att in attrs)
                    Console.WriteLine("\t{0}", att.GetType());

    public interface IAmNotStaticallyReferenced
        string Property { get; }

The interface has no static references to it anywhere in code, but it is used via reflection - therefore, it has a [Preserve] attribute.  Without it, the code snippet above would crash with a NullReferenceException in ViewDidLoad().  With linking off, I get the output I would expect:

typeof(IAmNotStaticallyReferenced): AttributeRemovalBug.IAmNotStaticallyReferenced
Property has 1 attributes:

With the linker on, I would expected the same output, but instead I get:

typeof(IAmNotStaticallyReferenced): AttributeRemovalBug.IAmNotStaticallyReferenced
Property has 0 attributes:

It seems as though the linker is removing this attribute, despite the [Preserve] on the interface.  The problem is not limited to [DefaultValue]; it occurs with [Preserve] itself and [ReadOnly], but not [Description], which is quite confusing because [ReadOnly] and [DefaultValue] are in the same namespace as [Description], System.ComponentModel.  

The reason this matters especially to me is that MEF (http://mef.codeplex.com/) depends on the [DefaultValue] attribute to signal whether certain metadata attributes are optional or required.  This is preventing the project I am working on from turning on the linker for all code and reducing our application size significantly.  Everything works with the linker off, but turning it on breaks everything.
Comment 1 Sebastien Pouliot 2012-03-12 16:14:01 UTC
The linker removes some attributes [1] that are not generally used at runtime. That includes System.ComponentModel attributes that are often used for design-time.

The best, immediate workaround is to skip linking (--linkskip) for the specific assemblies where the attribute is present (and needed) and continue to use --linkall (for all others).

[1] https://github.com/mono/mono/blob/master/mcs/tools/tuner/Mono.Tuner/RemoveAttributesBase.cs
Comment 2 David Merriman 2012-03-12 18:08:43 UTC
Using --linkskip is not going to be a viable option for us, as it would increase our application size by 50%, and subsequently push us over the 3G download size cap.

My larger concern is that MEF (which is now part of the .NET 4.0 runtime) relies on [DefaultValue] being available at runtime.  Regardless of its usage at design time, it is not expected behavior for *any* attribute to disappear at runtime - with the possible exception of linker specific attributes such as [Preserve].  This could potentially affect many other reflection-based frameworks as well; so long as they rely on any of the attributes that get removed by the linker (something which is not yet easily deduced), they will be off-limits for MonoTouch.
Comment 3 Sebastien Pouliot 2012-03-13 08:42:57 UTC
one step at the time :)

- "immediate workaround" means what you can do, today, using the released (stable) releases of MonoTouch;

- [DefaultValue] won't be removed in future releases (but not the ones already being built/tested);

- All the linker gains are made by twisting the "expected behaviour" and it works fairly well, even if it can't be perfect wrt reflection-usage. To offset the "unexpected" we offers several ways to workaround issues (up to "don't link"). A new one is planned that should allow even more control from users (for cases such as this one).

master: 8ee7ac571edcb82b8b26cf0d21441e09e4b42abb
5.2-series: d0677892124705a2e27a05e6f6ee2d4fde0c7367
QA: unit tests added in the 'linkall' project