Problems with the folder picker - what am I doing wrong?

[c64audio]

I've got a problem with the OpenFolderPickerAsync command on Windows and Mac (using 11 preview 8, behaved the same on 11 preview 1).

Trying to do what should be a simple thing: throw up a folder picker in the middle of some code.

The code is below. I've tried invoking the picker in two ways: once in the normal thread, once in the UI thread.

On Windows, using the normal method works as expected (yay), but when you use the UI thread, you never see a picker.

On Mac, the normal thread crashes with a nasty NSException, and if you use the UI thread, you never see a picker (changing the dispatcherpriority doesn't seem to help).

        public string SelectFolder(string title)
        {
            Task<string> task = Task.Run(() => SelectFolderAsync(title));
            return task.Result;
        }

        public async Task<string> SelectFolderAsync(string title)
        {
            FolderPickerOpenOptions opts = new()
            {
                Title = title,
                AllowMultiple = false
            };

// ui thread - this doesn't show a picker on Windows nor Mac
            IReadOnlyList<IStorageFolder> folderPaths = await Dispatcher.UIThread.InvokeAsync(() => this.StorageProvider.OpenFolderPickerAsync(opts), DispatcherPriority.Input);

// normal thread - works on Windows, crashes on Mac 10.15
            //IReadOnlyList<IStorageFolder> folderPaths = await this.StorageProvider.OpenFolderPickerAsync(opts);

            if (folderPaths.Count > 0)
            {
                return folderPaths[0].Name;
            }
            return string.Empty;
        }

Here's what the Mac crash looks like when you're not using the UI thread:

??? 0x0000000124867cd3 0x0 + 4907760851
19 ??? 0x0000000121d94b9b 0x0 + 4862856091
20 ??? 0x0000000121d97f92 0x0 + 4862869394
21 ??? 0x000000012486720d 0x0 + 4907758093
22 ??? 0x0000000121d19651 0x0 + 4862350929
23 ??? 0x0000000121b7d716 0x0 + 4860663574
24 ??? 0x0000000121b63ee6 0x0 + 4860559078
25 ??? 0x0000000121b78908 0x0 + 4860643592
26 ??? 0x0000000121b78818 0x0 + 4860643352
27 ??? 0x0000000121b787b7 0x0 + 4860643255
28 ??? 0x0000000121b741dc 0x0 + 4860625372
29 ??? 0x0000000121b5880e 0x0 + 4860512270
30 libcoreclr.dylib 0x000000010b79b5d9 CallDescrWorkerInternal + 124
31 libcoreclr.dylib 0x000000010b5e730f _Z18DispatchCallSimplePmjmj + 223
32 libcoreclr.dylib 0x000000010b5fe488 _ZN12ThreadNative20KickOffThread_WorkerEPv + 136
33 libcoreclr.dylib 0x000000010b5af808 _ZL31ManagedThreadBase_DispatchOuterP22ManagedThreadCallState + 296
34 libcoreclr.dylib 0x000000010b5afd70 ZN17ManagedThreadBase7KickOffEPFvPvES0 + 32
35 libcoreclr.dylib 0x000000010b5fe55a _ZN12ThreadNative13KickOffThreadEPv + 170
36 libcoreclr.dylib 0x000000010b4b13f7 _ZN7CorUnix10CPalThread11ThreadEntryEPv + 407
37 libsystem_pthread.dylib 0x00007fff6f4b7109 _pthread_start + 148
38 libsystem_pthread.dylib 0x00007fff6f4b2b8b thread_start + 15
)
libc++abi.dylib: terminating with uncaught exception of type NSException

[maxkatz6]

what am I doing wrong?

This:

        public string SelectFolder(string title)
        {
            Task<string> task = Task.Run(() => SelectFolderAsync(title));
            return task.Result;
        }

You should not do sync over async in GUI applications.

// ui thread - this doesn't show a picker on Windows nor Mac

Because you blocked UI thread by SelectFolder call above.

// normal thread - works on Windows, crashes on Mac 10.15

Because macOS requires file picker to run on UI thread.

In 11.0 there are some hacks to make it work, but these only help with macos, but not mobile/wasm.

Make your code properly async. And it will work as designed - asynchronously and on UI thread.

[c64audio]

I accept that this is the answer (thanks), but I'm going to need the hack. Let me explain why.

For context, this is an install script, which is triggered from a form button.

The script obviously does a number of things in a strict order, and doesn't require the calling form/UI to be active or responsive during that process. It also doesn't have to run on IOS or Android.

So, this install script (unfortunately) requires the user to be asked for a folder path an arbitrary number of times: this means I can't gather this information into a form field before the install button is pressed. In this particular use case, a modal folder dialog box would have been an ideal solution. But the Gods seem to have decided that my use case is morally corrupt or something.

So, if there's a hack here for MacOSX to make this process sane, I'll happily take it: re-factoring entirely satisfactory working code into an asynchronous mess just so I can get this one item of data from the user seems to be the tail wagging the dog.

Chris

[c64audio]

Ho hum, well, I solved the problem with the minimal amount of faffing by separating out a bit of logic and putting that on a worker thread so I could dispatch the folder request back to the UI thread when necessary.