Cannot Marshal Non-Blittable types such as Bool through a CustomMarshaller using LibraryImport

[Blizzardo1]

So with much frustration, I'm working on SdlSharp3 in my Repo. With struct Event, I had some bool types. Without telling me the details of the issue at hand, VisualStudio demanded I use a custom marshaller. That's until I used a byte as a bool and VisualStudio happily said, "Ok, this will do".

My question is, why is bool a non-blittable type, and how could I use LibraryImport with my non-trivial structs that contain booleans and strings?

[Blizzardo1]

This is the referenced file in question: https://github.com/Blizzardo1/SharpSDL3/blob/main/SDL3/OwnedEventMarshaller.cs

Snippet:

public static Event ConvertToManaged(nint unmanaged) {
    Event managed = new();
        
    if (unmanaged == nint.Zero) {
        return managed;
    }


    uint type = (uint)unmanaged.ToInt32();
    EventType eventType = (EventType)type;

    managed.Type = eventType;
    return managed;
}

Essentially one of the structs contains a boolean: https://github.com/Blizzardo1/SharpSDL3/blob/main/SDL3/Structs/MouseButtonEvent.cs

[StructLayout(LayoutKind.Sequential)]
public struct MouseButtonEvent
{
	public EventType Type;
	public uint Reserved;
	public ulong Timestamp;
	public uint WindowId;
	public uint Which;
	public byte Button;
	public bool Down;
	public byte Clicks;
	public byte Padding;
	public float X;
	public float Y;
}

This caused VisualStudio to complain endlessly without reason as to why. Perhaps add to the error that marshalling does or does not support certain types.

Project Code:

        while (_game.IsRunning) {
            _fps.Start();
            _ = Sdl.PollEvent(out Event e); // Calling this caused an AccessViolationException after fixing ExecutionEngineException

            _game.Update(e);
            _game.Draw();
            uint delta = (uint)_fps.GetTicks();
            if (delta < 1000 / FramesPerSecond) {
                Sdl.Delay((1000 / FramesPerSecond) - delta);
            }
            CurrentFPS = delta;
        }

[Blizzardo1]

This issue is also happening with struct Ttf.Font.

using SharpSDL3.Structs;
using System.Runtime.InteropServices;

namespace SharpSDL3.TTF;

[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode, Size = 384)]
public unsafe struct Font {
    [MarshalAs(UnmanagedType.LPUTF8Str)]
    public string Name;
    public nint Face;
    public long FaceIndex;
    public nint Props;
    public readonly uint Generation => (uint)Ttf.GetFontGeneration(this);
    public nint Text;
    public readonly float PtSize => Ttf.GetFontSize(this);
    public int Hdpi;
    public int Vdpi;
    public readonly int Height => Ttf.GetFontHeight(this);
    public readonly int Ascent => Ttf.GetFontAscent(this);
    public readonly int Descent => Ttf.GetFontDescent(this);
    public readonly int Lineskip => Ttf.GetFontLineSkip(this);
    public readonly FontStyle Style => Ttf.GetFontStyle(this);
    public readonly FontWeight Weight => Ttf.GetFontWeight(this);
    public int Outline;
    public nint Stroker;
    public readonly SdlBool EnableKerning {
        get => Ttf.GetFontKerning(this);
        set => Ttf.SetFontKerning(this, value);
    }
#if !TTF_USE_HARFBUZZ
    public SdlBool UseKerning;
#endif
    public int GlyphOverhang;
    public int LineThickness;
    public int UnderlineTopRow;
    public int StrikethroughTopRow;
    public nint Glyphs;
    public nint GlyphIndices;
    public nint Src;
    public long SrcOffset;
    public SdlBool CloseIo;
    public nint Args;
    public int NextCachedPositions;
    public fixed byte CachedPositions[8 * 32]; // Replace sizeof(CachedGlyphPositions) with its known size (32 bytes in this case)
    public nint Positions;
    public int FtLoadTarget;
    public int RenderSubpixel;
#if TTF_USE_HARFBUZZ
       public nint HbFont;
       public nint HbLanguage;
#endif
    public uint Script;
    public int Direction;
    public SdlBool RenderSdf;
    public int HorizontalAlign;
    public nint Fallbacks;
    public nint FallbackFor;

    internal nint Handle;

    public readonly Size GetTextSize(string text) => Ttf.MeasureString(this, text, 0);
    public readonly nint RenderTextSolid(string text, Color foregroundColor) => Ttf.RenderTextSolid(this, text, (ulong)text.Length, foregroundColor);
    public readonly nint RenderGlyphSolid(char c, Color foregroundColor) => Ttf.RenderGlyphSolid(this, c, foregroundColor);
    public readonly nint RenderTextShaded(string text, Color foregroundColor, Color bg) => Ttf.RenderTextShaded(this, text, (ulong)text.Length, foregroundColor, bg);
    public readonly nint RenderGlyphShaded(char c, Color foregroundColor, Color bg) => Ttf.RenderGlyphShaded(this, c, foregroundColor, bg);
    public readonly nint RenderTextBlended(string text, Color foregroundColor) => Ttf.RenderTextBlended(this, text, foregroundColor);
    public readonly nint RenderGlyphBlended(char c, Color foregroundColor) => Ttf.RenderGlyphBlended(this, c, foregroundColor);
    public readonly nint RenderTextBlendedWrapped(string text, Color foregroundColor, int wrapped) => Ttf.RenderTextBlendedWrapped(this, text, foregroundColor, wrapped);

    public readonly SdlBool GetFontKerningSize() => Ttf.GetFontKerning(this);

    public readonly void Close() => Ttf.CloseFont(this);
}

This struct requires a custom marshaller which I'm trying to avoid; however, using the FontMarshaller did load the fonts, except Font.Name seems to create an ExecutionEngineException when I tried closing the font as I was logging the closure. My thoughts are that string is the culprit and either I need an alternative or some sort of fix to handle marshalliing by the compiler and not a custom marshaller.

[BreyerW]

did you disable runtime marshalling? bool is NOT blittable by default as you saw (this is legacy decision that cannot be changed easily unfortunetly) but u can make it blittable by disabling runtime marshalling (in which case default marshalling is done by source generators if i remember correctly)

[BreyerW]

@Blizzardo1 i believe this is what u need to make it work: https://learn.microsoft.com/en-us/dotnet/standard/native-interop/custom-marshalling-source-generation

[Blizzardo1]

Right, and when I do that, Fonts don't work for example. They fail to load and retain data, even though I have a handle that takes care most of the struct. The only problem I get with the OwnedFontMarshaller class is font.Name. It fast fails with an ExecutionEngineException.

[BreyerW]

@Blizzardo1 hm thats weird but i also found this tutorial that mentions there should be several built in marshallers for string and shows how to do custom for utf-32 string, what is your native side for string? Maybe wchar_t*? And the encoding?

Edit im assuming that pesky font.Name is also a string hence the questions

[Blizzardo1]

The original struct's Name is a char*. So it technically is an ASCII representation; however, it's supposed to be a UTF8-String.
Do note that StringType resolves to UnmanagedType.LPUTF8Str

Font.cs

using SharpSDL3.Structs;
using System.Runtime.InteropServices;

namespace SharpSDL3.TTF;

[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode, Size = 384)]
public unsafe struct Font {
    [MarshalAs(Sdl.StringType)]
    public string Name;
    public nint Face;
    public long FaceIndex;
    public nint Props;
    public readonly uint Generation => (uint)Ttf.GetFontGeneration(this);
    public nint Text;
    public readonly float PtSize => Ttf.GetFontSize(this);
    public int Hdpi;
    public int Vdpi;
    public readonly int Height => Ttf.GetFontHeight(this);
    public readonly int Ascent => Ttf.GetFontAscent(this);
    public readonly int Descent => Ttf.GetFontDescent(this);
    public readonly int Lineskip => Ttf.GetFontLineSkip(this);
    public readonly FontStyle Style => Ttf.GetFontStyle(this);
    public readonly FontWeight Weight => Ttf.GetFontWeight(this);
    public int Outline;
    public nint Stroker;
    public readonly SdlBool EnableKerning {
        get => Ttf.GetFontKerning(this);
        set => Ttf.SetFontKerning(this, value);
    }
#if !TTF_USE_HARFBUZZ
    public SdlBool UseKerning;
#endif
    public int GlyphOverhang;
    public int LineThickness;
    public int UnderlineTopRow;
    public int StrikethroughTopRow;
    public nint Glyphs;
    public nint GlyphIndices;
    public nint Src;
    public long SrcOffset;
    public SdlBool CloseIo;
    public nint Args;
    public int NextCachedPositions;
    public fixed byte CachedPositions[8 * 32]; // Replace sizeof(CachedGlyphPositions) with its known size (32 bytes in this case)
    public nint Positions;
    public int FtLoadTarget;
    public int RenderSubpixel;
#if TTF_USE_HARFBUZZ
       public nint HbFont;
       public nint HbLanguage;
#endif
    public uint Script;
    public int Direction;
    public SdlBool RenderSdf;
    public int HorizontalAlign;
    public nint Fallbacks;
    public nint FallbackFor;

    internal nint Handle;

    public readonly Size GetTextSize(string text) => Ttf.MeasureString(this, text, 0);
    public readonly nint RenderTextSolid(string text, Color foregroundColor) => Ttf.RenderTextSolid(this, text, (ulong)text.Length, foregroundColor);
    public readonly nint RenderGlyphSolid(char c, Color foregroundColor) => Ttf.RenderGlyphSolid(this, c, foregroundColor);
    public readonly nint RenderTextShaded(string text, Color foregroundColor, Color bg) => Ttf.RenderTextShaded(this, text, (ulong)text.Length, foregroundColor, bg);
    public readonly nint RenderGlyphShaded(char c, Color foregroundColor, Color bg) => Ttf.RenderGlyphShaded(this, c, foregroundColor, bg);
    public readonly nint RenderTextBlended(string text, Color foregroundColor) => Ttf.RenderTextBlended(this, text, foregroundColor);
    public readonly nint RenderGlyphBlended(char c, Color foregroundColor) => Ttf.RenderGlyphBlended(this, c, foregroundColor);
    public readonly nint RenderTextBlendedWrapped(string text, Color foregroundColor, int wrapped) => Ttf.RenderTextBlendedWrapped(this, text, foregroundColor, wrapped);

    public readonly SdlBool GetFontKerningSize() => Ttf.GetFontKerning(this);

    public readonly void Close() => Ttf.CloseFont(this);
}

[BreyerW]

@Blizzardo1 Im no marshaler expert but look at this https://learn.microsoft.com/en-us/dotnet/api/system.runtime.interopservices.marshalling?view=net-9.0 and apply utf8 then ansi one maybe. If that doesnt help i give up you will need to ask actual experts whats wrong (and i think u will need to show ur custom marshaller if u used one)

[colejohnson66]

bool is not "blittable" because its size and representation depends on the language. Even in Win32, the term "boolean" has a few different meanings: BOOL is an int (1 for true, 0 for false), BOOLEAN is a byte (1 for true, 0 for false), VARIANT_BOOL (from that cursed language known as VB) is a short (negative 1 for true, 0 for false), etc.

A marshalling method must be used because dotnet doesn't know which kind you want (size and representation). Raymond Chen, as always, has a blog post about this.

It gets weirder when you consider an optimization dotnet (and other C compilers) do where any non-zero value is "truthy", but only "1" is true, except for when you compare against the literal, and the compiler optimizes it out?

https://sharplab.io/#v2:C4LgTgrgdgPgAgJgIwFgBQcAMACOSB0AStMAJYC2ApvgMID25ADqQDaVgDK7AbqQMaUAzgG506AEZ06LbAENsAXmwBVKINkAzagCFSwGrMHAAPOICewSgBpsk6QD4AFAgCUotHZnjF2YJErueACcjrJu2AD0EdgxsbF+EJTowaGKSgmU4VG+/slIIfIKSuJZ0bEasiyClACEAPx1QA==

using System;
using System.Runtime.CompilerServices;

bool a = Unsafe.BitCast<byte, bool>(2);
bool b = true;
Console.WriteLine(a); //         true
Console.WriteLine(a == true); // true
Console.WriteLine(a == b); //    false!??