Maui's usage of Reflection to find op_Implicit operators is not trim-compatible

[eerhardt]

Description

We are getting ILLink trim warnings in the following code:

maui/src/Controls/src/Core/Xaml/TypeConversionExtensions.cs

Lines 254 to 271 in cf6896e

	internal static MethodInfo GetImplicitConversionOperator(this Type onType, Type fromType, Type toType)
	{
	#if NETSTANDARD1_0
	var mi = onType.GetRuntimeMethod("op_Implicit", new[] { fromType });
	if (mi == null) return null;
	if (!mi.IsSpecialName) return null;
	if (!mi.IsPublic) return null;
	if (!mi.IsStatic) return null;
	if (!toType.IsAssignableFrom(mi.ReturnType)) return null;
	return mi;
	#else
	var bindingAttr = BindingFlags.Public | BindingFlags.Static | BindingFlags.FlattenHierarchy;
	IEnumerable<MethodInfo> mis = null;
	try
	{
	mis = new[] { onType.GetMethod("op_Implicit", bindingAttr, null, new[] { fromType }, null) };
	}

Here are the warnings:

C:\git\maui\src\Controls\src\Core\Xaml\TypeConversionExtensions.cs(270,5): Trim analysis warning IL2070: Microsoft.Maui.Controls.Xaml.TypeConversionExtensions.GetImplicitConversionOperator(Type,Type,Type): 'this' argument does not satisfy 'DynamicallyAccessedMemberTypes.PublicMethods' in call to 'System.Type.GetMethod(String,BindingFlags,Binder,Type[],ParameterModifier[])'. The parameter 'onType' of method 'Microsoft.Maui.Controls.Xaml.TypeConversionExtensions.GetImplicitConversionOperator(Type,Type,Type)' does not have matching annotations. The source value must declare at least the same requirements as those declared on the target location it is assigned to. [C:\git\WeatherTwentyOne\src\WeatherTwentyOne\WeatherTwentyOne.csproj]
C:\git\maui\src\Controls\src\Core\Xaml\TypeConversionExtensions.cs(275,24): Trim analysis warning IL2070: Microsoft.Maui.Controls.Xaml.TypeConversionExtensions.GetImplicitConversionOperator(Type,Type,Type): 'this' argument does not satisfy 'DynamicallyAccessedMemberTypes.PublicMethods' in call to 'System.Type.GetMethods(BindingFlags)'. The parameter 'onType' of method 'Microsoft.Maui.Controls.Xaml.TypeConversionExtensions.GetImplicitConversionOperator(Type,Type,Type)' does not have matching annotations. The source value must declare at least the same requirements as those declared on the target location it is assigned to. [C:\git\WeatherTwentyOne\src\WeatherTwentyOne\WeatherTwentyOne.csproj]

This issue is that if the trimmer doesn't see these op_Implicit operators being used in IL, it will trim them. This will cause the above code to behave differently in a trimmed application, since the operator is removed. This code won't find it, and it won't be used to do the conversions that it is supposed to do.

Options

I can only think of 2 high-level options to fix this:

1. Remove this logic from Maui and only allow TypeConverters to be used to convert values. This means anything that relies on these op_Implicit operators from working would need to be fixed. But at least the behavior would be consistent with or without trimming.
2. Teach the trimmer (somehow, either intrinsically or a custom step) to preserve these op_Implicit operators. We did something similar for System.Linq.Expressions in Preserving operator methods necessary for System.Linq.Expressions linker#1821.

cc @mattleibow @PureWeen @StephaneDelcroix @vitek-karas

Steps to Reproduce

(These steps are fuzzy, let me know if you need more detailed steps)

1. Implement a BindableProperty that uses an op_Implicit operator to do a conversion from a string.
2. Use that BindableProperty in an app.
3. Trim the app

Notice that the conversion through the op_Implicit operator doesn't happen in the trimmed app because it was trimmed away.

Version with bug

Preview 13 (current)

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

[vitek-karas]

Ideally we would pursue 1. if possible - remove this code from Maui.

If that doesn't work, we could extend the fix we did for Linq.Expression to also handle MAUI. It's kind of a hack, but I don't really see any other option. I think it would be good to measure the impact of this - what it would mean to include all operators everywhere in MAUI. Technically we could do a targeted fix and only include implicit operators, but that would mean slightly different (and yet another) hacky behavior compared to Linq.Expressions in the linker - would love to avoid that.

[mattleibow]

Is this an improvement? #5073

Should we do the whole thing differently? What happens if we have a non - generic base or something? We cast anyways, so we can maybe get the value and then cast. It will throw us out was wrong.

[StephaneDelcroix]

the reason op_implicit is there isn't to convert from string (we have TypeConverters for that), but for the other cases, like when you provide a Color to a Brush. Iirc it was added for Shell support, so participants in the Shell can be promoted to their parent (you declare a ShellContent when it expects a ShellSection).

As with everything else XAML and Binding, we don't know what we'll be used til runtime unless we enforce XamlC everywhere. That's something I'd want, but won't happen right away

[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.

[dotnet-policy-service]

Hi @eerhardt. We have added the "s/try-latest-version" label to this issue, which indicates that we'd like you to try and reproduce this issue on the latest available public version. This can happen because we think that this issue was fixed in a version that has just been released, or the information provided by you indicates that you might be working with an older version.

You can install the latest version by installing the latest Visual Studio (Preview) with the .NET MAUI workload installed. If the issue still persists, please let us know with any additional details and ideally a reproduction project provided through a GitHub repository.

This issue will be closed automatically in 7 days if we do not hear back from you by then - please feel free to re-open it if you come back to this issue after that time.