VisualTypeConverter loops over all assemblies looking for IVisual types

[eerhardt]

Description

We are getting trimmer warnings here:

C:\git\maui\src\Controls\src\Core\Visuals\VisualTypeConverter.cs(75,26): Trim analysis warning IL2026: Microsoft.Maui.Controls.VisualTypeConverter.Register(Assembly,Dictionary<String,IVisual>): Using member 'System.Reflection.Assembly.GetExportedTypes()' which has 'RequiresUnreferencedCodeAttribute' can break functionality when trimming application code. Types might be removed. [C:\DotNetTest\MauiTest\MauiTest.csproj]
C:\git\maui\src\Controls\src\Core\Visuals\VisualTypeConverter.cs(77,7): Trim analysis warning IL2062: Microsoft.Maui.Controls.VisualTypeConverter.Register(Assembly,Dictionary<String,IVisual>): Value passed to parameter 'visual' of method 'Microsoft.Maui.Controls.VisualTypeConverter.Register(Type,Dictionary<String,IVisual>)' can not be statically determined and may not meet 'DynamicallyAccessedMembersAttribute' requirements. [C:\DotNetTest\MauiTest\MauiTest.csproj]

The issue is that the VisualTypeConverter looks at all the assemblies in the process looking for types that implement IVisual:

maui/src/Controls/src/Core/Visuals/VisualTypeConverter.cs

Lines 27 to 38 in 5d8b798

	void InitMappings()
	{
	var mappings = new Dictionary<string, IVisual>(StringComparer.OrdinalIgnoreCase);
	Assembly[] assemblies = AppDomain.CurrentDomain.GetAssemblies();
	// Check for IVisual Types
	foreach (var assembly in assemblies)
	Register(assembly, mappings);
	if (Internals.Registrar.ExtraAssemblies != null)
	foreach (var assembly in Internals.Registrar.ExtraAssemblies)
	Register(assembly, mappings);

maui/src/Controls/src/Core/Visuals/VisualTypeConverter.cs

Lines 68 to 77 in 5d8b798

	static void Register(Assembly assembly, Dictionary<string, IVisual> mappings)
	{
	if (assembly.IsDynamic)
	return;
	try
	{
	foreach (var type in assembly.GetExportedTypes())
	if (typeof(IVisual).IsAssignableFrom(type) && type != typeof(IVisual))
	Register(type, mappings);

This is not trim-safe, since Types that are not referenced by IL can be trimmed by the trimmer. But in the converter code, it takes a string object and then tries looking up in the "registered mapping" for the IVisual by the string name. This can break trimmed apps because the Type being referenced by that string may have been trimmed.

In talking with @Redth and @PureWeen this approach is out-of-date, and we should remove the assembly scanning. Instead, we should add a "trim-compatible" method for doing this registration. For example:

1. Using an assembly-level attribute (which we already have: lower in that InitMapping method).
2. Adding a new builder.RegisterVisual<TVisual>() method that people can add to their CreateMauiApp() method. This will register the Visual type statically, and the Types that aren't referenced can be safely trimmed.

Steps to Reproduce

1. dotnet new maui
2. dotnet restore
3. dotnet build -f net6.0-android -r android-arm64 -t:Run -c Release --no-restore -p:SuppressTrimAnalysisWarnings=false -p:TrimmerSingleWarn=false /bl
4. Inspect the trimmer warnings for mentions of VisualTypeConverter.

Version with bug

Unknown/Other (please specify)

Last version that worked well

Unknown/Other

Affected platforms

iOS, Android

Affected platform versions

All

Did you find any workaround?

No response

Relevant log output

No response

[Redth]

@PureWeen is this done? can this be closed now? I see a PR linked that was closed but not merged.

[PureWeen]

@Redth not yet

The solution for this changed once we decided to move a bunch of stuff around between Controls and Core. A new PR still need to be put together but getting this fully working is dependent on unwinding the Registrar code and getting that moved into Compatibility

[ghost]

We've moved this issue to the Backlog milestone. This means that it is not going to be worked on for the coming release. We will reassess the backlog following the current release and consider this item at that time. To learn more about our issue management process and to have better expectation regarding different types of issues you can read our Triage Process.

[jinxinjuan]

Verified this issue with Visual Studio Enterprise 17.7.0 Preview 1.0. Can repro with provided repro steps in terminal.

[rmarinho]

Fixed by #20417