Marshal filename to UTF-8

[Ema Ema]

AssimpContext.ImportFile can not load files or materials which includes a multi-byte string.

Please specify UnmanagedType.LPUTF8Str instead of UnmanagedType.LP8Str for I/O functions to DllImport (UnmanagedType Enum).

It works fine by way of trial, but I'm not sure that is correct way to path UTF-8 string to DLL method (Roslyn reports CA2101).

    [DllImport("assimp.dll", CallingConvention = CallingConvention.Cdecl, EntryPoint = "aiImportFileExWithProperties")]
    private static extern IntPtr AiImportFileExWithProperties([In, MarshalAs(UnmanagedType.LPUTF8Str)] string file, uint flag, IntPtr fileIO, IntPtr propStore);

I will create a simple project for test in this weekend.

regards.

[Ema Ema]

I create an example project on github.

Currently I needs AssimpContext.ImportFile so that AssimpContext_ImportFile_MarshalAs_LPUTF8Str.cs includes just a cut out code.

I guess a better all-embracing way to convert UTF-8 string in Assimp to Unicode string in .NET. I don’t know how it is.

[Nicholas Woodfield]

That unmanaged type isn’t supported in the .net standard the library targets, a fix would have to marshal it manually before passing it to the native layer. I see there’s been some advancement on supporting UTF8 in the native library since I last looked (years lol), so there probably will be other code points (e.g. string names in returned data) where there may be failures too.

[Ema Ema]

Thank you for check.

Assimp library seems to use UTF-8 as an internal codepoint, so that byte[] filename (or AiString) might be an alternative API way (library user has a responsibility to ensure UTF-8 encoding from outside).

I know that is not easy to use and is not desirable changes. `.NET 7` still be not settled on how to handle UTF-8, so that I cannot believe the way is suitable one (although it seems likely that it will be a raw byte sequence).

Fortunately I can use patched own class to use currently.
I will do so for the time being.

[Jan Selchow]

.Net7 did actually introduce a new way of Interop that seems to handle UTF8 Marshaling for you.

Instead of [DllImport("mydll.dll")] you do [LibraryImport("mydll.dll", StringMarshalling = StringMarshalling.Utf8)] fixed all my troubles with umlauts in filenames i had with a different c++ lib.

Documentation: https://learn.microsoft.com/en-us/dotnet/standard/native-interop/pinvoke-source-generation.
.NetConf2022 Talk about it : https://youtu.be/ucNcRtnifXY

[Nicholas Woodfield]

Cool, looks like it can also replace a lot of the plumbing for how I marshal the assimp types too.

[Ema Ema]

Thanks, it looks really nice. I still not be ready to .NET7, but it will be resolve Marshaling.

[Jan Selchow]

@Nicholas Woodfield Would you consider using it? I know it would probably mean maintaining code for two different target runtimes but not having to worry about the UTF-8 marshalling out of the box would be great.

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