[API Proposal]: MakeWeakTypeReference as trimming indicator

[hez2010]

Background and motivation

While we were working with trimming, we found that we often want to express "code only needed if a type is preserved", but currently we do not have such thing in C#.

For example, in XAML frameworks, we use URLs to locate resources. With source generator, the code of the resource locator which is generated by a source generator may consist with a method with plenty of if statements, for example:

// auto-generated by a source generator
object? LoadResource(string path)
{
    if (path == "ms-appx://MyAssembly/My/Fancy/XamlControl1Style") return XamlLoader.Load<XamlControl1Style>();
    if (path == "ms-appx://MyAssembly/Another/XamlControl2Style") return XamlLoader.Load<XamlControl2Style>();
    return null;
}

With this source generator approach, we can achieve trimming compatibility. However, this would result in all types referenced in LoadResource being preserved even they are not being actually used. In this case, we only want XamlControl1Style to be preserved if XamlControl1 is referenced in other pieces of code.

API Proposal

namespace System.Diagnostics.CodeAnalysis;

public class ControlFlow
{
    public static bool MakeWeakTypeReference<T>() => true;
}

API Usage

object? LoadResource(string path)
{
    if (ControlFlow.MakeWeakTypeReference<XamlControl1>())
        if (path == "ms-appx://MyAssembly/My/Fancy/XamlControl1Style")
            return XamlLoader.Load<XamlControl1Style>();
    if (ControlFlow.MakeWeakTypeReference<XamlControl2>())
        if (path == "ms-appx://MyAssembly/Another/XamlControl2Style")
            return XamlLoader.Load<XamlControl2Style>();
    return null;
}

It behaves as a no-op which always returns true, but if there are no other type references to XamlControl1, the whole block of the if statement can be safely trimmed, which produces code after trimming like below:

object? LoadResource(string path)
{
    if (path == "ms-appx://MyAssembly/Another/XamlControl2Style")
        return XamlLoader.Load<XamlControl2Style>();
    return null;
}

Therefore, all types related to XamlControl1 and XamlControl1Type can also be trimmed away.

This should also support and and or so that we can express multiple types as a single prerequisite:

if (ControlFlow.MakeWeakTypeReference<Foo>() && ControlFlow.MakeWeakTypeReference<Bar>())
{
    // only preserved if both Foo and Bar are referenced
}

if (ControlFlow.MakeWeakTypeReference<Foo>() || ControlFlow.MakeWeakTypeReference<Bar>())
{
    // only preserved if either Foo or Bar is referenced
}

// Bogus
if (!ControlFlow.MakeWeakTypeReference<Foo>())
{
    // no-op, can never reach because `!ControlFlow.MakeWeakTypeReference<Foo>()` is always false, maybe a warning should be emitted by the compiler or illink?
}

Alternative Designs

No response

Risks

No response

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Tagging subscribers to 'linkable-framework': @eerhardt, @vitek-karas, @LakshanF, @sbomer, @joperezr
See info in area-owners.md if you want to be subscribed.

Issue Details

---

Background and motivation

While we were working with trimming, we found that we often want to express "code only needed if a type is preserved", but currently we do not have such thing in C#.

For example, in XAML frameworks, we use URLs to locate resources. With source generator, the code of the resource locator which is generated by a source generator may consist with a method with plenty of if statements, for example:

// auto-generated by a source generator
object? LoadResource(string path)
{
    if (path == "ms-appx://MyAssembly/My/Fancy/XamlControl1Style") return XamlLoader.Load<XamlControl1Style>();
    if (path == "ms-appx://MyAssembly/Another/XamlControl2Style") return XamlLoader.Load<XamlControl2Style>();
    return null;
}

With this source generator approach, we can achieve trimming compatibility. However, this would result in all types referenced in LoadResource being preserved even they are not being actually used. In this case, we only want XamlControl1Style to be preserved if XamlControl1 is referenced in other pieces of code.

API Proposal

namespace System.Diagnostics.CodeAnalysis;

public class ControlFlow
{
    public static bool MakeWeakTypeReference<T>() => true;
}

API Usage

object? LoadResource(string path)
{
    if (ControlFlow.MakeWeakTypeReference<XamlControl1>())
        if (path == "ms-appx://MyAssembly/My/Fancy/XamlControl1Style")
            return XamlLoader.Load<XamlControl1Style>();
    if (ControlFlow.MakeWeakTypeReference<XamlControl2>())
        if (path == "ms-appx://MyAssembly/Another/XamlControl2Style")
            return XamlLoader.Load<XamlControl2Style>();
    return null;
}

It behaves as a no-op which always returns true, but if there are no other type references to XamlControl1, the whole block of the if statement can be safely trimmed, which produces code after trimming like below:

object? LoadResource(string path)
{
    if (ControlFlow.MakeWeakTypeReference<XamlControl2>())
        if (path == "ms-appx://MyAssembly/Another/XamlControl2Style")
            return XamlLoader.Load<XamlControl2Style>();
    return null;
}

Therefore, all types related to XamlControl1 and XamlControl1Type can also be trimmed away.

This should also support and and or so that we can express multiple types as a single prerequisite:

if (ControlFlow.MakeWeakTypeReference<Foo>() && ControlFlow.MakeWeakTypeReference<Bar>())
{
    // only preserved if both Foo and Bar are referenced
}

if (ControlFlow.MakeWeakTypeReference<Foo>() || ControlFlow.MakeWeakTypeReference<Bar>())
{
    // only preserved if either Foo or Bar is referenced
}

// Bogus
if (!ControlFlow.MakeWeakTypeReference<Foo>())
{
    // no-op, can never reach because `!ControlFlow.MakeWeakTypeReference<Foo>()` is always false, maybe a warning should be emitted by the compiler or illink?
}

Alternative Designs

No response

Risks

No response

Author:	hez2010
Assignees:	-
Labels:	api-suggestion, untriaged, linkable-framework
Milestone:	-

[teo-tsirpanis]

Interesting idea; the name does not sound right to me though.

[hez2010]

Interesting idea; the name does not sound right to me though.

Yes. I did think about the name for a while but didn't conclude a fancy name :)

[MichalPetryka]

Maybe IsTypePresent<T>?

[teo-tsirpanis]

That's better, and with that name it fits more on Type.

[eerhardt]

See the following code, which we use in System.Text.Json to do this exact scenario:

runtime/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Converters/Collection/IEnumerableConverterFactoryHelpers.cs

Lines 120 to 125 in e71a958

	// This method takes an unannotated string which makes linker reflection analysis lose track of the type we are
	// looking for. This indirection allows the removal of the type if it is not used in the calling application.
	[UnconditionalSuppressMessage("ReflectionAnalysis", "IL2057:TypeGetType",
	Justification = "This method exists to allow for 'weak references' to the Stack and Queue types. If those types are used in the app, " +
	"they will be preserved by the app and Type.GetType will return them. If those types are not used in the app, we don't want to preserve them here.")]
	private static Type? GetTypeIfExists(string name) => Type.GetType(name, false);

Tagging subscribers to this area: @vitek-karas, @agocke, @VSadov
See info in area-owners.md if you want to be subscribed.

Issue Details

---

Background and motivation

While we were working with trimming, we found that we often want to express "code only needed if a type is preserved", but currently we do not have such thing in C#.

For example, in XAML frameworks, we use URLs to locate resources. With source generator, the code of the resource locator which is generated by a source generator may consist with a method with plenty of if statements, for example:

// auto-generated by a source generator
object? LoadResource(string path)
{
    if (path == "ms-appx://MyAssembly/My/Fancy/XamlControl1Style") return XamlLoader.Load<XamlControl1Style>();
    if (path == "ms-appx://MyAssembly/Another/XamlControl2Style") return XamlLoader.Load<XamlControl2Style>();
    return null;
}

With this source generator approach, we can achieve trimming compatibility. However, this would result in all types referenced in LoadResource being preserved even they are not being actually used. In this case, we only want XamlControl1Style to be preserved if XamlControl1 is referenced in other pieces of code.

API Proposal

namespace System.Diagnostics.CodeAnalysis;

public class ControlFlow
{
    public static bool MakeWeakTypeReference<T>() => true;
}

API Usage

object? LoadResource(string path)
{
    if (ControlFlow.MakeWeakTypeReference<XamlControl1>())
        if (path == "ms-appx://MyAssembly/My/Fancy/XamlControl1Style")
            return XamlLoader.Load<XamlControl1Style>();
    if (ControlFlow.MakeWeakTypeReference<XamlControl2>())
        if (path == "ms-appx://MyAssembly/Another/XamlControl2Style")
            return XamlLoader.Load<XamlControl2Style>();
    return null;
}

It behaves as a no-op which always returns true, but if there are no other type references to XamlControl1, the whole block of the if statement can be safely trimmed, which produces code after trimming like below:

object? LoadResource(string path)
{
    if (path == "ms-appx://MyAssembly/Another/XamlControl2Style")
        return XamlLoader.Load<XamlControl2Style>();
    return null;
}

Therefore, all types related to XamlControl1 and XamlControl1Type can also be trimmed away.

This should also support and and or so that we can express multiple types as a single prerequisite:

if (ControlFlow.MakeWeakTypeReference<Foo>() && ControlFlow.MakeWeakTypeReference<Bar>())
{
    // only preserved if both Foo and Bar are referenced
}

if (ControlFlow.MakeWeakTypeReference<Foo>() || ControlFlow.MakeWeakTypeReference<Bar>())
{
    // only preserved if either Foo or Bar is referenced
}

// Bogus
if (!ControlFlow.MakeWeakTypeReference<Foo>())
{
    // no-op, can never reach because `!ControlFlow.MakeWeakTypeReference<Foo>()` is always false, maybe a warning should be emitted by the compiler or illink?
}

Alternative Designs

No response

Risks

No response

Author:	hez2010
Assignees:	-
Labels:	api-suggestion, area-AssemblyLoader-coreclr, untriaged, linkable-framework
Milestone:	-

[jkoritzinsky]

@eerhardt that's really clever and I think a good example for where an API like this could be useful. I think being able to have a well-known API that would enable this to work while using generics or strongly-typed System.Type objects instead of having to round-trip through type name parsing would be quite useful and likely more maintainable than the existing mechanism you shared.

[MichalPetryka]

@eerhardt that's really clever and I think a good example for where an API like this could be useful. I think being able to have a well-known API that would enable this to work while using generics or strongly-typed System.Type objects instead of having to round-trip through type name parsing would be quite useful and likely more maintainable than the existing mechanism you shared.

Additionally, it'd be more AOT safe.

[teo-tsirpanis]

namespace System;

public abstract partial class Type
{
    public static Type? GetTypeIfExists<T>() => typeof(T); // would be replaced with null by the trimmer if the type does not exist.
}