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Example: Troubleshooting Dynamic Programming

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Note

This topic refers to the .NET Native Developer Preview, which is pre-release software. You can download the preview from the Microsoft Connect website (requires registration).

Not all metadata lookup failures in apps developed using the .NET Native tool chain result in an exception. Some can manifest in unpredictable ways in an app. The following example shows an access violation caused by referencing a null object:

Access violation - code c0000005 (first chance)
App!$3_App::Core::Util::NavigationArgs.Setup
App!$3_App::Core::Util::NavigationArgs..ctor
App!$0_App::Gibbon::Util::DesktopNavigationArgs..ctor
App!$0_App::ViewModels::DesktopAppVM.NavigateToPage
App!$3_App::Core::ViewModels::AppViewModel.NavigateToFirstPage
App!$3_App::Core::ViewModels::AppViewModel::<HandleLaunch>d__a.MoveNext
App!$43_System::Runtime::CompilerServices::AsyncMethodBuilderCore.CallMoveNext
App!System::Action.InvokeClosedStaticThunk
App!System::Action.Invoke
App!$43_System::Threading::Tasks::AwaitTaskContinuation.InvokeAction
App!$43_System::Threading::SendOrPostCallback.InvokeOpenStaticThunk
[snip]

Let's try to troubleshoot this exception by using the three-step approach outlined in the "Manually resolve missing metadata" section of Getting Started.

What was the app doing?

The first thing to note is the async keyword machinery at the base of the stack. Determining what the app was really doing in an async method can be problematic, because the stack has lost the context of the originating call and has run the async code on a different thread. However, we can deduce that the app is trying to load its first page. In the implementation for NavigationArgs.Setup, the following code caused the access violation:

AppViewModel.Current.LayoutVM.PageMap

In this instance, the LayoutVM property on AppViewModel.Current was null. Some absence of metadata caused a subtle behavior difference and resulted in a property being uninitialized instead of set, as the app expected. Setting a breakpoint in the code where LayoutVM should have been initialized might throw light on the situation. However, note that LayoutVM's type is App.Core.ViewModels.Layout.LayoutApplicationVM. The only metadata directive present so far in the rd.xml file is:

<Namespace Name="App.ViewModels" Browse="Required Public" Dynamic="Required Public" />

A likely candidate for the failure is that App.Core.ViewModels.Layout.LayoutApplicationVM is missing metadata because it's in a different namespace.

In this case, adding a runtime directive for App.Core.ViewModels resolved the issue. The root cause was an API call to the Type.GetType(String) method that returned null, and the app silently ignored the problem until a crash occurred.

In dynamic programming, a good practice when using reflection APIs under .NET Native is to use the Type.GetType overloads that throw an exception on failure.

Is this an isolated case?

Other issues might also arise when using App.Core.ViewModels. You must decide whether it's worth identifying and fixing each missing metadata exception, or saving time and adding directives for a larger class of types. Here, adding dynamic metadata for App.Core.ViewModels might be the best approach if the resulting size increase of the output binary isn't an issue.

Could the code be rewritten?

If the app had used typeof(LayoutApplicationVM) instead of Type.GetType("LayoutApplicationVM"), the tool chain could have preserved browse metadata. However, it still wouldn't have created invoke metadata, which would have led to a MissingMetadataException exception when instantiating the type. To prevent the exception, you'd still have to add a runtime directive for the namespace or the type that specifies the dynamic policy. For information on runtime directives, see the Runtime Directives (rd.xml) Configuration File Reference.

See also