Windows.Storage.Pickers

Android JumpList sample

File pickers allow the user to pick a folder or a file on the local file system so that the application can work with it. The following table shows which file picker experiences are available across Uno Platform targets. For detailed information see the next sections.

Legend

  • ✔️ Supported
  • 💬 Partially supported (see below for more details)
  • ❌ Not supported
Picker UWP WebAssembly Android iOS macOS WPF GTK
FileOpenPicker ✔️ ✔️ (1) ✔️ ✔️ ✔️ ✔️ ✔️
FileSavePicker ✔️ ✔️ (1) ✔️ ✔️ ✔️ ✔️ ✔️
FolderPicker ✔️ ✔️ ✔️ 💬 (2) ✔️ ✔️

(1) - Multiple implementations supported - see WebAssembly section below (2) - See iOS section below

On some platforms, you can further customize the file picking experience by utilizing additional properties:

Feature UWP WebAssembly Android iOS macOS WPF GTK
SuggestedFileName ✔️ ✔️ ✔️ ✔️ ✔️
SuggestedStartLocation ✔️ ✔️ (1) ✔️ ✔️ ✔️
SettingsIdentifier ✔️ ✔️ (1) ✔️

(1) - Only for the native file pickers - see WebAssembly section below

On platforms where the additional features are not supported yet, setting them will not have any effect.

Examples

FolderPicker

var folderPicker = new FolderPicker();
folderPicker.SuggestedStartLocation = PickerLocationId.ComputerFolder;
folderPicker.FileTypeFilter.Add("*");
StorageFolder pickedFolder = await folderPicker.PickSingleFolderAsync();
if (pickedFolder != null)
{
    // Folder was picked you can now use it
    var files = await pickedFolder.GetFilesAsync();
}
else
{
    // No folder was picked or the dialog was cancelled.
}

Notes: While the SuggestedStartLocation has currently no effect in Uno Platform targets, and FileTypeFilter has no effect for FolderPicker, they both must be set, otherwise the dialog crashes on UWP.

FileOpenPicker

Picking a single file

var fileOpenPicker = new FileOpenPicker();
fileOpenPicker.SuggestedStartLocation = PickerLocationId.ComputerFolder;
fileOpenPicker.FileTypeFilter.Add(".txt");
fileOpenPicker.FileTypeFilter.Add(".csv");
StorageFile pickedFile = await fileOpenPicker.PickSingleFileAsync();
if (pickedFile != null)
{
    // File was picked, you can now use it
    var text = await FileIO.ReadTextAsync(pickedFile);
}
else
{
    // No file was picked or the dialog was cancelled.
}

Notes: While the SuggestedStartLocation has currently no effect in Uno Platform targets, it must be set, otherwise the dialog crashes on UWP. FileTypes must include at least one item. You can add extensions in the format .extension, with the exception of * (asterisk) which allows picking any type of file.

Picking multiple files

var fileOpenPicker = new FileOpenPicker();
fileOpenPicker.SuggestedStartLocation = PickerLocationId.ComputerFolder;
fileOpenPicker.FileTypeFilter.Add(".jpg");
fileOpenPicker.FileTypeFilter.Add(".png");
var pickedFiles = await fileOpenPicker.PickMultipleFilesAsync();
if (pickedFiles.Count > 0)
{
    // At least one file was picked, you can use them
    foreach (var file in pickedFiles)
    {
        global::System.Diagnostics.Debug(file.Name);   
    }
}
else
{
    // No file was picked or the dialog was cancelled.
}

Notes: While the SuggestedStartLocation has currently no effect in Uno Platform targets, it must be set, otherwise the dialog crashes on UWP. FileTypes must include at least one item. You can add extensions in the format .extension, with the exception of * (asterisk) which allows picking any type of file.

FileSavePicker

var fileSavePicker = new FileSavePicker();
fileSavePicker.SuggestedStartLocation = PickerLocationId.ComputerFolder;
fileSavePicker.SuggestedFileName = "myfile.txt";
fileSavePicker.FileTypeChoices.Add("Plain Text", new List<string>() { ".txt", ".text" });
StorageFile saveFile = await fileSavePicker.PickSaveFileAsync();
if (saveFile != null)
{
    // Save file was picked, you can now write in it
    await FileIO.WriteTextAsync(saveFile, "Hello, world!");
}
else
{
    // No file was picked or the dialog was cancelled.
}

Notes: While the SuggestedStartLocation has no effect, it must be set for UWP. You must declare at least one item for FileTypeChoices. Each has a description and one or more extensions.

Picker configuration

File pickers have various configuration options that customize the experience (see the UWP documentation for full list of properties). Not all options are supported on all target platforms, in which case these are ignored.

To set which file type extensions you want to allow, use the FileTypeFilter property on FileOpenPicker and FolderPicker, and the FileTypeChoices property on FileSavePicker. Extensions must be in the format ".xyz" (starting with a dot). For FileOpenPicker and FolderPicker you can also include "*" (star) entry, which represents the fact that any file extension is allowed.

Some systems use MIME types to specify the file type. Uno includes a list of common predefined mappings (see list in MDN Docs). If a MIME type you require is missing, you can provide it by adding it to the Uno.WinRTFeatureConfiguration.FileTypes.FileTypeToMimeMapping dictionary:

Uno.WinRTFeatureConfiguration.FileTypes.FileTypeToMimeMapping.Add(".myextension", "some/mimetype");

For iOS and macOS, UTType is utilized for the same purpose. Here you can provide a custom mapping using Uno.WinRTFeatureConfiguration.FileTypes.FileTypeToUTTypeMapping dictionary:

Uno.WinRTFeatureConfiguration.FileTypes.FileTypeToUTTypeMapping.Add(".myextension", "my.custom.UTType");

Custom Uniform Type Identifiers must be declared in the info.plist of your iOS and macOS application. See a full example of this in Apple documentation.

WebAssembly

There are two implementations of file pickers available in WebAssembly - File System Access API pickers and download/upload pickers.

File System Access API pickers

The most powerful picker implementation on WebAssembly uses the File System Access API. This is not yet widely implemented across all browsers. See the following support tables for each picker:

FolderPicker is only supported for this type of pickers.

File System Access API pickers allow direct access to the picked files and folders. This means that any modifications the user does to the files are persisted on the target file system.

However, writing to the target file system is limited, so when a write-stream is opened for a file, Uno Platform creates a copy of the file in temporary storage and your changes are applied to this temporary file instead. When your file stream is then flushed, closed, or disposed of, the changes are written to the source file and the temporary file is discarded.

Download/upload pickers

In case the File System Access API is not available in the browser, Uno Platform also offers a fallback to "download" and "upload" experiences.

For the upload picker, the browser triggers a file picker dialog and Uno Platform then copies the selected files into temporary storage of the app. The StorageFile instance you receive is private for your application and the changes are not reflected in the original file. To save the changes, you need to trigger the "download picker".

For the download picker, the experience requires the use of CachedFileManager. Triggering PickSaveFileAsync does not actually show the download picker to the user. Instead, only a temporary file is created to allow you to write any content. Afterwards, calling CompleteUpdatesAsync opens the download dialog which allows the user to save the file.

The CachedFileManager class works transparently with both the Download picker and the File System Access API, which means you can write a single snippet of code that handles both scenarios correctly:

var savePicker = new FileSavePicker():
savePicker.SuggestedStartLocation = PickerLocationId.DocumentsLibrary;
savePicker.FileTypeChoices.Add("Text file", new List<string>() { ".txt" });
savePicker.SuggestedFileName = "New Document";

// For download picker, no dialog is actually triggered here
// and a temporary file is returned immediately.
// For Fil System Access API, this triggers the picker to allow
// user to select a local file.
var file = await savePicker.PickSaveFileAsync();

CachedFileManager.DeferUpdates(file);

// Write some content to the file
await FileIO.WriteTextAsync(file, "Hello, world!");

// For download picker this starts the download process of the browser
// and triggers the save dialog for the user.
// For File System Access API, this is a no-op and immediately
// completes.
await CachedFileManager.CompleteUpdatesAsync(file);

Checking the source of opened file

To know how the file needs to be handled, you need to check the type of storage provider it comes from. To do this, access the Provider property of the file:

if (file.Provider.Id == "jsfileaccessapi")
{
    // File was picked using File System Access pickers.
}

if (file.Provider.Id == "computer")
{
    // File is a temporary file created using Upload picker.
}

The local files have provider ID of computer, which matches the UWP behavior. jsfileaccessapi is used for files coming from the File System Access API.

Choosing supported type of pickers

By default, Uno Platform attempts to use File System Access API and falls back to download/upload pickers if not available. To control this behavior, you can use WinRTFeatureConfiguration:

#if __WASM__
Uno.WinRTFeatureConfiguration.Storage.Pickers.WasmConfiguration = 
    WasmPickerConfiguration.FileSystemAccessApiWithFallback;
#endif

The allowed values for the configuration are:

  • FileSystemAccessApiWithFallback - defaults to File System Access API, but falls back to download/upload pickers if not available
  • FileSystemAccessApi - uses File System Access API only. If not avaialable, pickers will throw NotSupportedException
  • DownloadUpload - uses download/upload pickers only.

Android

Files picked from file pickers on Android are provided by the Storage Access Framework API. Due to its limitations, it is not possible to write to existing file in-place. Instead, Uno Platform creates a copy of the file in temporary storage and your changes are applied to this temporary file instead. When your file stream is then flushed, closed, or disposed of, the changes are written to the source file and the temporary file is discarded.

iOS

iOS does not offer a built-in FileSavePicker experience. Luckily it is possible to implement this functionality for example using a combination of a FolderPicker and ContentDialog.

To provide your own custom implementation, create a class that implements the IFileSavePickerExtension which is only available on iOS. This class must have a public constructor with a object parameter. This will actually be an instance of FileSavePicker when invoked later. Then implement the PickSaveFileAsync method:

#if __IOS__
namespace Custom.Pickers
{
    public class CustomFileSavePickerExtension : IFileSavePickerExtension
    {
        private readonly FileSavePicker _fileSavePicker;
    
        public CustomFileSavePickerExtension(object owner)
        {
            _fileSavePicker = (FileSavePicker)picker;
        }
        
        public async Task<StorageFile> PickSaveFileAsync(CancellationToken token)
        {
            // ... your own implementation
        }
    }
}
#endif

When done, register this extension in App.xaml.cs:

public App()
{
#if __IOS__
    ApiExtensibility.Register(
        typeof(Uno.Ehttps://twitter.com/thebookisclosed/status/1375006215189753860?s=19xtensions.Storage.Pickers.IFileSavePickerExtension), 
        picker => new CustomFileSavePickerExtension(picker));
#endif
}

As this is quite complex, you can find a working implementation of a folder-based save file picker in Uno.Samples repository. You can modify and adjust this implementation as you see fit for your specific use case.