FileBrowser.cs

using System;
using UnityEngine;

namespace NativeFileBrowser
{
    // any/all settings are allowed to be null
    public struct FileBrowserSettings
    {
        public DialogKind DialogKind;

        public MultiSelectKind MultiSelectKind;

        // Title on MacOS sets NSOpenPanel.message, not title (because title is no longer displayed)
        public string Title;

        // Both Windows and Linux (GTK3) documentation strongly discourages setting InitialFolder,
        // because the previous folder the user selected is remembered. Oh well. It's here if you want it.
        public string InitialFolder;

        public FileExtensionFilter[] FileExtensionFilters;
    }

    // any/all settings are allowed to be null
    public struct FileExtensionFilter
    {
        // Name is ignored on MacOS
        // Windows automatically displays Patterns after Name (e.g. "text file (*.txt;*.md)"), but Linux (GTK3) does not,
        // so this library automatically appends $" ({string.Join(", ", Patterns)})" to Name on Linux (GTK3).
        public string Name;

        // Patterns must begin with "*." on MacOS (the MacOS API expects a string like "txt"
        // to be provided here, so this library strips a "*." prefix off this field)
        public string[] Patterns;

        // MimeTypes is ignored on Windows and MacOS
        public string[] MimeTypes;

        public FileExtensionFilter(string name, string[] patterns, string[] mimeTypes)
        {
            Name = name;
            Patterns = patterns;
            MimeTypes = mimeTypes;
        }
    }

    public enum DialogKind
    {
        File,
        Folder,
    }

    public enum MultiSelectKind
    {
        Single,
        Multiple,
    }

    public static class FileBrowser
    {
        // returns an empty array on cancel
        public static string[] Pick(FileBrowserSettings settings)
        {
            switch (Application.platform)
            {
                case RuntimePlatform.WindowsEditor or RuntimePlatform.WindowsPlayer:
                    return WindowsCOMFileBrowser.Pick(settings);
                case RuntimePlatform.OSXEditor or RuntimePlatform.OSXPlayer:
                    return MacOpenPanel.Pick(settings);
                case RuntimePlatform.LinuxEditor or RuntimePlatform.LinuxPlayer:
                    return Gtk3FileDialog.Pick(settings);
                default:
                    return Array.Empty<string>();
            }
        }
    }
}