XAML compilation (XamlC) fails with generic types

[Aquastorm]

Description

There's a bug in the Xamarin XAML compilation code that can be observed when referencing from XAML code a generic type having multiple type parameters and a concrete type is specified for one of them.
I think that the problem will be easier to understand with the example described below.

Steps to Reproduce

1. Create a new cross-platform (Xamarin.Forms) project from VisualStudio. See the screenshot below to check which settings where used. You can also download the attached project.
   Note: the bug appears when compiling xaml code contained in either a .NET Standard Library or PCL project -> You can choose whatever platform you like (Android, iOS or UWP), it doesn't matter.

2. Add the BaseValidationBehavior class to the root of the project.
   Note: the BaseValidationBehavior contains code (not shown here) common to all behaviors that validates user input:

using Xamarin.Forms;

namespace XamBugGenerics
{
    internal abstract class BaseValidationBehavior<TBindable, TModel> : Behavior<TBindable> where TBindable : BindableObject
    {
    }
}

Here both 'TBindable' and 'TModel' are type parameters.

3. Add the EntryValidationBehavior class to the root of the project.
   This class contains code (not shown here) to validate 'Entry' views content.

using Xamarin.Forms;

namespace XamBugGenerics
{
    internal sealed class EntryValidationBehavior<TModel> : BaseValidationBehavior<Entry, TModel>
    {
    }
}

Note: 'TModel' is a type parameter but 'Entry' is a concrete type. Mixing type parameters and concrete types is what triggers the XAML compilation bug.

4. Add the XAML compilation attribute shown below to the MainPage.xaml.cs:

using Xamarin.Forms;
using Xamarin.Forms.Xaml;

namespace XamBugGenerics
{
    [XamlCompilation(XamlCompilationOptions.Compile)]
    public partial class MainPage : ContentPage
    {
        public MainPage()
        {
            InitializeComponent();
        }
    }
}

5. Add an 'Entry' to the MainPage.xaml as shown below:

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:local="clr-namespace:XamBugGenerics"
             x:Class="XamBugGenerics.MainPage">

    <Entry VerticalOptions="Center" 
           HorizontalOptions="Center">
        <Entry.Behaviors>
            <local:EntryValidationBehavior x:TypeArguments="Entry"/>
        </Entry.Behaviors>
    </Entry>
</ContentPage>

Note: normally in a real project we should pass a model type as 'TypeArgument', but to reveal the bug any type can be used.

Expected Behavior

The XamlCTask is able to correctly compile the XAML code without raising an exception.
Actually the same code runs without issues when XAML compilation is deactivated.

Actual Behavior

By changing the logging verbosity of the XamlCTask in the Xamarin.Forms.targets file, you can observe this error in the build output window when building the project:

1>Target XamlC:
1>  Using "XamlCTask" task from assembly "C:\Users\[removed]\.nuget\packages\xamarin.forms\2.5.0.121934\build\netstandard1.0\Xamarin.Forms.Build.Tasks.dll".
1>  Task "XamlCTask"
1>    Compiling Xaml
1>    
1>    Assembly: obj\Debug\netstandard2.0\XamBugGenerics.dll
1>    ReferencePath: 	C:\Program Files\dotnet\sdk\NuGetFallbackFolder\netstandard.library\2.0.0\build\netstandard2.0\ref\Microsoft.Win32.Primitives.dll;
1>		[multiple lines removed]
1>    DebugSymbols:"True"
1>    DebugType:"portable"
1>    Adding searchpath C:\Program Files\dotnet\sdk\NuGetFallbackFolder\netstandard.library\2.0.0\build\netstandard2.0\ref
1>    Adding searchpath C:\Users\[removed]\.nuget\packages\xamarin.forms\2.5.0.121934\lib\netstandard1.0
1>		[multiple lines removed]
1>     Module: XamBugGenerics.dll
1>      Resource: XamBugGenerics.App.xaml... Has XamlCompilationAttribute set to Skip and not Compile... skipped
1>      Resource: XamBugGenerics.MainPage.xaml...
1>       Creating empty MainPage.__InitComponentRuntime ...done.
1>       Copying body of InitializeComponent to __InitComponentRuntime ...done.
1>       Parsing Xaml... done.
1>       Replacing MainPage.InitializeComponent ()... failed.
1>    D:\Dev\Xamarin\XamBugGenerics\XamBugGenerics\XamBugGenerics\MainPage.xaml : error : Object reference not set to an instance of an object.
1>       at Xamarin.Forms.Build.Tasks.TypeReferenceExtensions.ResolveGenericParameters(TypeReference self, TypeReference declaringTypeReference)
1>       at Xamarin.Forms.Build.Tasks.TypeReferenceExtensions.InheritsFromOrImplements(TypeReference typeRef, TypeReference baseClass)
1>       at Xamarin.Forms.Build.Tasks.SetNamescopesAndRegisterNamesVisitor.Visit(ElementNode node, INode parentNode)
1>       at Xamarin.Forms.Xaml.ElementNode.Accept(IXamlNodeVisitor visitor, INode parentNode)
1>       at Xamarin.Forms.Xaml.ElementNode.Accept(IXamlNodeVisitor visitor, INode parentNode)
1>       at Xamarin.Forms.Xaml.RootNode.Accept(IXamlNodeVisitor visitor, INode parentNode)
1>       at Xamarin.Forms.Build.Tasks.XamlCTask.TryCoreCompile(MethodDefinition initComp, MethodDefinition initCompRuntime, ILRootNode rootnode, String resourceId, Exception& exception)
1>      Changing the module MVID...done.
1>
1>    Writing the assembly... done.
1>  Done executing task "XamlCTask" -- FAILED.
1>Done building target "XamlC" in project "XamBugGenerics.csproj" -- FAILED.

Because of this bug I had to deactivate XAML compilation on every page where I'm validating user input.

Basic Information

• Version with issue: Xamarin.Forms 2.5.0.121934
• IDE: Visual Studio Enterprise 2017 v15.5.2
• Platform Target Frameworks: not relevant. The bug comes from either a .NET standard library or PCL project.
• Nuget Packages: NETStandard.Library 2.0.0 and Xamarin.Forms v2.5.0.121934

Screenshots

Xamarin Forms project configuration

Xamarin Forms project structure

Reproduction Link

XamBugGenericsCode.zip

[kingces95]

Reproduction exhibits stated actual behavior.

[Aquastorm]

@StephaneDelcroix thank you for fixing this issue 👍

[thomasgalliker]

Quick question: By when do we expect to have this fix in a stable Xamarin.Forms release? (Thanks for fixing it, @StephaneDelcroix & co)

[Aquastorm]

XamlC works as expected now, but building the solution for iOS while the code optimization is enabled (i.e. when using the LLVM optimizing compiler which is typically used when releasing apps to the App Store) doesn't work.

The problem comes from the Mono Ahead of Time compiler, which is unable to compile the method InitializeComponent() on pages using the EntryValidationBehavior and where the compilation is enabled. See points 4 and 5 in my issue description above.
Note: skipping the compilation with [XamlCompilation(XamlCompilationOptions.Skip)] makes the build working.

Mono Ahead of Time compiler outputs something like the following:

Unable to compile method 'void XamBugGenerics.MainPage :InitializeComponent ()' due to: 'Method with open type while not compiling gshared assembly:XamBugGenerics type: member:(null) signature:'.

Consequently the Mono CIL Stripper fails with the following message:

MTOUCH : error : System.ArgumentOutOfRangeException: Index was out of range. Must be non-negative and less than the size of the collection.
Parameter name: index

Note: everything works fine for Android, including code optimizations done by ProGuard.

[StephaneDelcroix]

@Aquastorm: I Tried to reproduce your issue in the attached project:

G1497.zip

but it compiles and executes fine, in Release mode, with LLVM enabled, using XF 3.2

Could you please try to reproduce the behavior you were reporting in a new project, and if it still fails, open a new issue ?

Thanks a lot.

/cc @spouliot @bholmes

[Aquastorm]

@StephaneDelcroix thank you for checking this issue again.

I'm quite busy with another task at the moment, but as soon as I get some "free" time I will provide more information/open a new issue (with the repro steps).