Why FileSavePicker is not replacing file?

Question

when I used FileSavePicker to select (or create) storage file, if I select existing file, it will prompt for replace. when I press yes, I expect the file to be replaced later but it doesn't.

// existing file is picked, I pressed yes when replace was requested.
file = await picker.PickSaveFileAsync();

so with above code, existing file is picked by FileSavePicker, I expected that file is now replaced but its not.

await file.OpenAsync(FileAccessMode.ReadWrite, StorageOpenOptions.AllowOnlyReaders)

This will only open file for write, it does not replace it, but I want it to be replaced. my issue is same as this question, but I'm not writing text file, I'm writing binary data.

How can I replace file with new one?

Ive tried following but it throws NRE, because GetParentAsync is returning null.

var file = await (await file.GetParentAsync()).CreateFileAsync(file.Name,
                        CreationCollisionOption.ReplaceExisting);

I don't want user to explicitly create a new file by deleting old one and creating a new one. What I want is that if user selects existing file and accepts replace request from FileSavePicker, the file should be replaced.

---

Ok, here is code that you can test on your machine.

var picker = new FileSavePicker
{
    SuggestedStartLocation = PickerLocationId.Desktop,
    DefaultFileExtension = ".bin",
    SuggestedFileName = "Binary file"
};
picker.FileTypeChoices.Add("Binary File", new List<string> {".bin"});

// in second run, choose existing file.
var file = await picker.PickSaveFileAsync();

using (var stream = await file.OpenAsync(FileAccessMode.ReadWrite, StorageOpenOptions.AllowOnlyReaders))
{
    using (var output = stream.GetOutputStreamAt(0))
    {
        using (var writer = new DataWriter(output))
        {
            writer.WriteBytes(Enumerable.Repeat<byte>(10, 10000).ToArray());
            await writer.StoreAsync();
            writer.DetachStream();
        }
        await output.FlushAsync();
    }
}

Run this two times, in first run, create file. in second run select existing file, and also change Repeat<byte>(10, 10000) to something smaller and different, like Repeat<byte>(80, 50) and check content of file.

Answer 1

You can always SetLength of your opened stream to 0 first - which will clear all the old content and set stream's position to 0. Afterwards you can just write to the stream your new content:

var picker = new FileSavePicker
{
    SuggestedStartLocation = PickerLocationId.Desktop,
    DefaultFileExtension = ".bin",
    SuggestedFileName = "Binary file"
};
picker.FileTypeChoices.Add("Binary File", new List<string> { ".bin" });
var file = await picker.PickSaveFileAsync();

using (var stream = await file.OpenStreamForWriteAsync())
{
    stream.SetLength(0);
    var bytes = Encoding.ASCII.GetBytes("Content");
    await stream.WriteAsync(bytes, 0, bytes.Length);
    await stream.FlushAsync();
}