(Unmanaged) Strings in callbacks are not considered as blittable by WasmAppBuilder

[aienabled]

Description

The strings are not considered as blittable when defined in Pinvoke callbacks.

runtime/src/tasks/WasmAppBuilder/PInvokeCollector.cs

Lines 170 to 176 in a5b75b8

	public static bool IsBlittable(Type type)
	{
	if (type.IsPrimitive || type.IsByRef || type.IsPointer || type.IsEnum)
	return true;
	else
	return false;
	}

runtime/src/tasks/WasmAppBuilder/PInvokeCollector.cs

Lines 141 to 142 in a5b75b8

	if (!IsBlittable(p.ParameterType))
	Error("Parameter types of pinvoke callback method '" + method + "' needs to be blittable.");

Reproduction Steps

private delegate void Callback_RegisterType(string typeName);
private static Callback_RegisterType _registerType = RegisterType;

[MonoPInvokeCallback(typeof(Callback_RegisterType))]
private static void RegisterType(string typeName)
{
}

Expected behavior

The above code should compile.

Actual behavior

WasmApp.Native.targets(290,5): Error : Parameter types of pinvoke callback method 'System.Void RegisterType(System.String)' needs to be blittable.

Regression?

No response

Known Workarounds

No response

Configuration

Tested on .NET 7.0.11. "WebAssembly Browser App" project type.

Other information

I'm working on porting a UI library to WASM and its C# SDK makes extensive use of strings in Pinvoke callbacks (dozens of methods). The C# SDK has no such error when building for other platforms (including WebGL in Unity).

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Issue Details

---

Description

The strings are not considered as blittable when defined in Pinvoke callbacks.

runtime/src/tasks/WasmAppBuilder/PInvokeCollector.cs

Lines 170 to 176 in a5b75b8

	public static bool IsBlittable(Type type)
	{
	if (type.IsPrimitive || type.IsByRef || type.IsPointer || type.IsEnum)
	return true;
	else
	return false;
	}

runtime/src/tasks/WasmAppBuilder/PInvokeCollector.cs

Lines 141 to 142 in a5b75b8

	if (!IsBlittable(p.ParameterType))
	Error("Parameter types of pinvoke callback method '" + method + "' needs to be blittable.");

Reproduction Steps

private delegate void Callback_RegisterType(string typeName);
private static Callback_RegisterType _registerType = RegisterType;

[MonoPInvokeCallback(typeof(Callback_RegisterType))]
private static void RegisterType(string typeName)
{
}

Expected behavior

The above code should compile.

Actual behavior

WasmApp.Native.targets(290,5): Error : Parameter types of pinvoke callback method 'System.Void RegisterType(System.String)' needs to be blittable.

Regression?

No response

Known Workarounds

No response

Configuration

Tested on .NET 7.0.11. "WebAssembly Browser App" project type.

Other information

I'm working on porting a UI library to WASM and its C# SDK makes extensive use of strings in Pinvoke callbacks (dozens of methods). The C# SDK has no such error when building for other platforms (including WebGL in Unity).

Author:	aienabled
Assignees:	-
Labels:	arch-wasm, untriaged, needs-area-label
Milestone:	-

[radical]

cc @maraf

[Lurler]

Any news about this? This is extremely important for us.

[kg]

Strings aren't blittable, see https://learn.microsoft.com/en-us/dotnet/framework/interop/blittable-and-non-blittable-types

I assume what you want is for strings to be accepted as arguments to pinvoke callbacks.

Are you unable to pass char pointers instead by pinning your strings? (I'm aware that doing this would be an annoying change to make, so I'm not discounting the value of adding support for strings as arguments)

[dp901]

This is an issue that has been blocking us for a while. Is there any chance it could be addressed sooner?

[aienabled]

@kg

Strings aren't blittable

Indeed, but there is no problem using the code as shown in the snippet above on other platforms. I suppose the strings are automatically converted to char pointers.

I assume what you want is for strings to be accepted as arguments to pinvoke callbacks.

Yes, exactly.

Are you unable to pass char pointers instead by pinning your strings? (I'm aware that doing this would be an annoying change to make, so I'm not discounting the value of adding support for strings as arguments)

That's a good idea though it would be quite cumbersome to maintain a fork considering there are dozens of methods where a string is passed in a callback. However, when I attempted to rewrite the code to use char pointers, I encountered another compilation problem (the delegate/callback pointer declared with [UnmanagedFunctionPointer(CallingConvention.Cdecl)] is also not blittable, again this is not an issue on other platforms; there is a good dozen of such cases in the library I've tried to fork). I suppose we will have to wait until .NET WASM becomes more mature.

[kg]

@kg

Strings aren't blittable

Indeed, but there is no problem using the code as shown in the snippet above on other platforms. I suppose the strings are automatically converted to char pointers.

Just to provide some more clarity: WASM has a different ABI from most other platforms (x64, ARM, etc) and is a platform with unique constraints. As a result we aren't able to use the regular p/invoke implementations from mono or CoreCLR as-is on this target. That's where warnings about blittable types and such are coming from - there is custom WASM-specific tooling that is designed to generate adapter code and tables that are used to make p/invoke work at all.

We're in the process of making improvements in this area, so it's possible you may see things start working by the time 9 previews come out. This specific feature (passing strings and getting them auto-converted to temporary char pointers) is not something we'd be able to backport to 8 for you - it's too big - but we're definitely aware of the need for it.

Function pointers are another known issue and we are hoping to fix that one in 9. If you cast the function pointer to IntPtr and pass it that way, it should work.