LibraryImport fails with MarshalAsAttribute errors when return value or parameter types are from another assembly

[leandro-benedet-garcia]

Description

While making a binding using SDL3-CS as a reference project I found a very weird error:

/bug_project/bug.cs(21,35): error SYSLIB1052: The specified 'MarshalAsAttribute' configuration for the return value of method 'SDL_ShaderCross_Init' is not supported by source-generated P/Invokes. If the specified configuration is required, use a regular 'DllImport' instead. (https://learn.microsoft.com/dotnet/fundamentals/syslib-diagnostics/syslib1052)

The binding in question is:

[LibraryImport("SDL3_shadercross")]
[UnmanagedCallConv(CallConvs = [typeof(CallConvCdecl)])]
public static partial SDLBool SDL_ShaderCross_Init();

The declaration of SDLBool can be found here https://github.com/flibitijibibo/SDL3-CS/blob/42d1b438aabeabd68385cba3ed5c21e73e8cc877/SDL3/SDL3.Core.cs#L44-L75

Reproduction Steps

Minimum reproducible project:

binding_bug.zip

Expected behavior

I would expect the code to work with and without the project reference.

Or at least, be told that I cannot use bindings from a referenced project.

Actual behavior

A confusing error message.

Regression?

No response

Known Workarounds

If I instead remove the reference and add this to the project file:

<ItemGroup>
  <Compile Include="lib\SDL3-CS\SDL3\SDL3.Core.cs" />
</ItemGroup>

The code compiles fine.

Configuration

• Which version of .NET is the code running on?
  Dotnet version 9.0.102
• What OS and version, and what distro if applicable?
  Arch Linux kernel version 6.13.2-arch1-1
• What is the architecture (x64, x86, ARM, ARM64)?
  x86_64
• Do you know whether it is specific to that configuration?
  Unsure

Other information

No response

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[kg]

Additional context from discord troubleshooting:

• void X () worked
• primitive return types and parameter types i.e. int worked
• They were using ProjectReference to pull in types like SDLBool from another assembly that contained its own LibraryImports, not an assembly or package reference

[huoyaoyuan]

This is expected. See also #98616 . Type from other assembly are not considered as invariant in compile time.

[kg]

OK, sounds like this is a documentation/error quality issue then? Because none of the documentation I found explains this, and the error suggests it's related to use of MarshalAs when MarshalAs isn't in use

[leandro-benedet-garcia]

This is expected. See also #98616 . Type from other assembly are not considered as invariant in compile time.

I still would expect, that at least, the error tell me that I cannot do that.

I can understand that It is not supported.

OK, sounds like this is a documentation/error quality issue then? Because none of the documentation I found explains this, and the error suggests it's related to use of MarshalAs when MarshalAs isn't in use

Yeah, the link that it gives, where it could provide me with information in how to fix the issue, or why it happens, just repeats the error. And since the error is not directly related to the cause of the issue, looking up a search engine does not help at all.

[AaronRobinsonMSFT]

@jkoritzinsky or @jtschuster Creating a clearer error message here seems appropriate.

@huoyaoyuan If you're feeling ambitious, feel free to handle this :)