C# wrapper for the SDL3 file dialog API

SDLFileDialogs.cs

using System;
using System.Runtime.InteropServices;
using System.Text;
using SDL3;

namespace Segadora.UI;

public static class SDLFileDialogs
{
	private unsafe class DialogData : IDisposable
	{
		public DialogFileCallback Callback { get; }

		/*
		* Memory block containing the filters and the strings used by the filters
		*
		* SDL requires validity of the filter pointer and the strings until the callback is triggered
		* which is a very annoying lifetime requirement to deal with in C#, so I'm just wrangling
		* a big old block of unmanaged memory by hand for it.
		*
		* There's probably a better way to do this. Too bad!
		*/
		private byte* buffer;

		public DialogData(DialogFileCallback callback)
		{
			Callback = callback;
		}

		public Span<SDL.SDL_DialogFileFilter> AllocFilters(DialogFileFilter[] filters)
		{
			if (filters == null)
			{
				return Span<SDL.SDL_DialogFileFilter>.Empty;
			}

			long bufferSize = filters.Length * sizeof(SDL.SDL_DialogFileFilter);

			foreach (DialogFileFilter filter in filters)
			{
				bufferSize += Encoding.UTF8.GetByteCount(filter.name) + 1;
				bufferSize += Encoding.UTF8.GetByteCount(filter.pattern) + 1;
			}

			buffer = (byte*) NativeMemory.Alloc((nuint) bufferSize);

			Span<SDL.SDL_DialogFileFilter> filterSpan = new Span<SDL.SDL_DialogFileFilter>(buffer, filters.Length);
			long bufferOffset = filters.Length * sizeof(SDL.SDL_DialogFileFilter);

			for (int i = 0; i < filters.Length; i++)
			{
				byte* name = buffer + bufferOffset;
				long nameLength = Encoding.UTF8.GetByteCount(filters[i].name) + 1;

				bufferOffset += nameLength;

				byte* pattern = buffer + bufferOffset;
				long patternLength = Encoding.UTF8.GetByteCount(filters[i].pattern) + 1;

				bufferOffset += patternLength;

				Encoding.UTF8.GetBytes(filters[i].name, new Span<byte>(name, (int) nameLength - 1));
				new Span<byte>(name, (int) nameLength)[(int) nameLength - 1] = 0;

				Encoding.UTF8.GetBytes(filters[i].pattern, new Span<byte>(pattern, (int) patternLength - 1));
				new Span<byte>(pattern, (int) patternLength)[(int) patternLength - 1] = 0;

				filterSpan[i] = new SDL.SDL_DialogFileFilter
				{
					name = name,
					pattern = pattern,
				};
			}

			return filterSpan;
		}

		protected virtual void Dispose(bool disposing)
		{
			if (disposing)
			{
			}

			if (buffer != null)
			{
				NativeMemory.Free(buffer);
			}
		}

		~DialogData()
		{
			Dispose(false);
		}

		public void Dispose()
		{
			Dispose(true);
			GC.SuppressFinalize(this);
		}
	}

	public delegate void DialogFileCallback(string[] files, int filter);

	public record struct DialogFileFilter(string name, string pattern);

	public static void ShowOpenFileDialog(
		DialogFileCallback callback,
		DialogFileFilter[] filters = null,
		bool allowMany = false
	)
	{
		DialogData data = new DialogData(callback);
		Span<SDL.SDL_DialogFileFilter> filterSpan = data.AllocFilters(filters);

		GCHandle gch = GCHandle.Alloc(data);

		SDL.SDL_ShowOpenFileDialog(
			SDLFileCallback,
			GCHandle.ToIntPtr(gch),
			IntPtr.Zero,
			filterSpan,
			filterSpan.Length,
			null, // doesn't work on Windows, todo: figure out why
			allowMany
		);
	}

	public static void ShowSaveFileDialog(
		DialogFileCallback callback,
		DialogFileFilter[] filters = null
	)
	{
		DialogData data = new DialogData(callback);
		Span<SDL.SDL_DialogFileFilter> filterSpan = data.AllocFilters(filters);

		GCHandle gch = GCHandle.Alloc(data);

		SDL.SDL_ShowSaveFileDialog(
			SDLFileCallback,
			GCHandle.ToIntPtr(gch),
			IntPtr.Zero,
			filterSpan,
			filterSpan.Length,
			null
		);
	}

	public static void ShowOpenFolderDialog(
		DialogFileCallback callback,
		bool allowMany = false
	)
	{
		DialogData data = new DialogData(callback);

		GCHandle gch = GCHandle.Alloc(data);

		SDL.SDL_ShowOpenFolderDialog(
			SDLFileCallback,
			GCHandle.ToIntPtr(gch),
			IntPtr.Zero,
			null,
			allowMany
		);
	}

	private static void SDLFileCallback(IntPtr userdata, IntPtr filelist, int filter)
	{
		GCHandle gch = GCHandle.FromIntPtr(userdata);
		DialogData data = (DialogData) gch.Target;
		gch.Free();

		if (filelist == IntPtr.Zero)
		{
			throw new Exception(SDL.SDL_GetError());
		}

		// What follows is some stupidity.
		// todo: Rewrite everything

		int fileCount = 0;
		IntPtr filePathPtr = filelist;

		while (Marshal.ReadIntPtr(filePathPtr) != IntPtr.Zero)
		{
			fileCount++;
			filePathPtr += Marshal.SizeOf<IntPtr>();
		}

		if (fileCount == 0)
		{
			data?.Callback(null, filter);
			data?.Dispose();

			return;
		}

		string[] filePaths = new string[fileCount];
		filePathPtr = filelist;

		for (int i = 0; i < fileCount; i++)
		{
			filePaths[i] = Marshal.PtrToStringUTF8(Marshal.ReadIntPtr(filePathPtr));
			filePathPtr += Marshal.SizeOf<IntPtr>();
		}

		data?.Callback(filePaths, filter);
		data?.Dispose();
	}
}