WindowsCOMFileBrowser.cs

using System;
using System.Runtime.InteropServices;
using UnityEngine;

// Terrifying!

namespace NativeFileBrowser
{
    public static class WindowsCOMFileBrowser
    {
        public static string[] Pick(FileBrowserSettings settings)
        {
            // Default flags from https://learn.microsoft.com/en-us/windows/win32/shell/common-file-dialog
            var fosOptions = FOS_PATHMUSTEXIST | FOS_FILEMUSTEXIST | FOS_NOCHANGEDIR;
            if (settings.DialogKind == DialogKind.Folder)
                fosOptions |= FOS_PICKFOLDERS;
            if (settings.MultiSelectKind == MultiSelectKind.Multiple)
                fosOptions |= FOS_ALLOWMULTISELECT;
            var dialog = new FileOpenDialogRCW();
            var idialog = (IFileOpenDialog)dialog;
            idialog.SetOptions(fosOptions);
            if (settings.Title != null)
                idialog.SetTitle(settings.Title);
            if (settings.InitialFolder != null)
            {
                var iidShellItem2 = new Guid("43826d1e-e718-42ee-bc55-a1e261c37bfe");
                object unk;
                var hr = SHCreateItemFromParsingName(settings.InitialFolder, IntPtr.Zero, ref iidShellItem2, out unk);
                // most common reason to fail is probably ERROR_FILE_NOT_FOUND=2 or ERROR_PATH_NOT_FOUND=3
                if (hr != 0)
                    Debug.LogException(Marshal.GetExceptionForHR(hr));
                else if (unk != null)
                    idialog.SetFolder((IShellItem)unk);
            }
            if (settings.FileExtensionFilters != null)
            {
                var filterSpecs = new COMDLG_FILTERSPEC[settings.FileExtensionFilters.Length];
                for (var i = 0; i < filterSpecs.Length; i++)
                {
                    // mime type is ignored
                    var setting = settings.FileExtensionFilters[i];
                    var filterSpec = string.Join(';', setting.Patterns ?? Array.Empty<string>());
                    filterSpecs[i] = new COMDLG_FILTERSPEC(setting.Name ?? "", filterSpec);
                }
                idialog.SetFileTypes((uint)filterSpecs.Length, filterSpecs);
            }
            var showResult = idialog.Show(IntPtr.Zero);
            string[] result;
            if (showResult == ERROR_CANCELLED)
                result = Array.Empty<string>();
            else
            {
                Marshal.ThrowExceptionForHR(showResult);
                var shellItems = idialog.GetResults();
                result = new string[shellItems.GetCount()];
                for (var i = 0u; i < result.Length; i++)
                {
                    var shellItem = shellItems.GetItemAt(i);
                    // wpf uses SIGDN_DESKTOPABSOLUTEPARSING, not SIGDN_FILESYSPATH
                    // DesktopAbsoluteParsing still returns a full rooted C:\blahblah\file.txt path
                    // https://github.com/dotnet/wpf/blob/fe3d2c72ea8a7acf4371592ba41d7f7b49e141b7/src/Microsoft.DotNet.Wpf/src/PresentationFramework/MS/Internal/AppModel/ShellProvider.cs#L939
                    result[i] = shellItem.GetDisplayName(SIGDN_DESKTOPABSOLUTEPARSING);
                    Marshal.ReleaseComObject(shellItem);
                }
                Marshal.ReleaseComObject(shellItems);
            }

            Marshal.ReleaseComObject(dialog);
            return result;
        }

        private const int ERROR_CANCELLED = unchecked((int)0x800704C7);
        // https://learn.microsoft.com/en-us/windows/win32/api/shobjidl_core/ne-shobjidl_core-sigdn
        private const int SIGDN_DESKTOPABSOLUTEPARSING = unchecked((int)0x80028000);
        // https://learn.microsoft.com/en-us/windows/win32/api/shobjidl_core/ne-shobjidl_core-_fileopendialogoptions
        private const int FOS_NOCHANGEDIR = 0x8;
        private const int FOS_PICKFOLDERS = 0x20;
        private const int FOS_ALLOWMULTISELECT = 0x200;
        private const int FOS_PATHMUSTEXIST = 0x800;
        private const int FOS_FILEMUSTEXIST = 0x1000;

        [DllImport("shell32.dll")]
        internal static extern int SHCreateItemFromParsingName([MarshalAs(UnmanagedType.LPWStr)] string pszPath, IntPtr pbc, [In] ref Guid riid, [MarshalAs(UnmanagedType.Interface)] out object ppv);

        [ComImport,
         ClassInterface(ClassInterfaceType.None),
         TypeLibType(TypeLibTypeFlags.FCanCreate),
         Guid("DC1C5A9C-E88A-4DDE-A5A1-60F82A20AEF7")]
        private class FileOpenDialogRCW
        {
        }

        // https://github.com/dotnet/wpf/blob/main/src/Microsoft.DotNet.Wpf/src/PresentationFramework/MS/Internal/AppModel/ShellProvider.cs
        [ComImport, InterfaceType(ComInterfaceType.InterfaceIsIUnknown), Guid("D57C7288-D4AD-4768-BE02-9D969532D960")]
        internal interface IFileOpenDialog
        {
            [PreserveSig]
            int Show(IntPtr parent);

            void SetFileTypes(uint cFileTypes, [In] COMDLG_FILTERSPEC[] rgFilterSpec);
            void SetFileTypeIndex(uint iFileType);
            uint GetFileTypeIndex();
            uint Advise([MarshalAs(UnmanagedType.Interface)] IntPtr pfde);
            void Unadvise(uint dwCookie);
            void SetOptions(int fos);
            int GetOptions();
            void SetDefaultFolder(IShellItem psi);
            void SetFolder(IShellItem psi);
            IShellItem GetFolder();
            IShellItem GetCurrentSelection();
            void SetFileName([MarshalAs(UnmanagedType.LPWStr)] string pszName);

            [return: MarshalAs(UnmanagedType.LPWStr)]
            void GetFileName();

            void SetTitle([MarshalAs(UnmanagedType.LPWStr)] string pszTitle);
            void SetOkButtonLabel([MarshalAs(UnmanagedType.LPWStr)] string pszText);
            void SetFileNameLabel([MarshalAs(UnmanagedType.LPWStr)] string pszLabel);
            IShellItem GetResult();
            void AddPlace(IShellItem psi, int fdcp);
            void SetDefaultExtension([MarshalAs(UnmanagedType.LPWStr)] string pszDefaultExtension);
            void Close([MarshalAs(UnmanagedType.Error)] int hr);
            void SetClientGuid([In] ref Guid guid);
            void ClearClientData();
            void SetFilter([MarshalAs(UnmanagedType.Interface)] object pFilter);

            IShellItemArray GetResults();

            IShellItemArray GetSelectedItems();
        }

        [ComImport, InterfaceType(ComInterfaceType.InterfaceIsIUnknown), Guid("43826D1E-E718-42EE-BC55-A1E261C37BFE")]
        internal interface IShellItem
        {
            [return: MarshalAs(UnmanagedType.Interface)]
            object BindToHandler(IntPtr pbc, [In] ref Guid bhid, [In] ref Guid riid);

            IShellItem GetParent();

            [return: MarshalAs(UnmanagedType.LPWStr)]
            string GetDisplayName(int sigdnName);

            uint GetAttributes(int sfgaoMask);
            int Compare(IShellItem psi, int hint);
        }

        [ComImport, InterfaceType(ComInterfaceType.InterfaceIsIUnknown), Guid("B63EA76D-1F85-456F-A19C-48159EFA858B"),]
        internal interface IShellItemArray
        {
            [return: MarshalAs(UnmanagedType.Interface)]
            object BindToHandler(IntPtr pbc, [In] ref Guid rbhid, [In] ref Guid riid);

            [return: MarshalAs(UnmanagedType.Interface)]
            object GetPropertyStore(int flags, [In] ref Guid riid);

            [return: MarshalAs(UnmanagedType.Interface)]
            object GetPropertyDescriptionList([In] ref PKEY keyType, [In] ref Guid riid);

            uint GetAttributes(int dwAttribFlags, uint sfgaoMask);

            uint GetCount();

            IShellItem GetItemAt(uint dwIndex);

            [return: MarshalAs(UnmanagedType.Interface)]
            object EnumItems();
        }

        [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
        internal readonly struct COMDLG_FILTERSPEC
        {
            [MarshalAs(UnmanagedType.LPWStr)]
            public readonly string pszName;
            [MarshalAs(UnmanagedType.LPWStr)]
            public readonly string pszSpec;

            public COMDLG_FILTERSPEC(string name, string spec)
            {
                pszName = name;
                pszSpec = spec;
            }
        }

        [StructLayout(LayoutKind.Sequential, Pack = 4)]
        internal struct PKEY
        {
            private readonly Guid _fmtid;
            private readonly uint _pid;
        }
    }
}