C#•16mo ago

✅ Async PushButton Functions

What is the proper way to handle FileIO asynchronously? I have a Utility class that performs various functions, one in particular:

public static async Task<string> PickPhoto()
{
    return await Task.Run(() =>
    {
        Task<FileResult> task = MediaPicker.Default.PickPhotoAsync();
        if (task != null)
        {
            FileResult result = task.GetAwaiter().GetResult();
            return result.FullPath;
        }

        return string.Empty;
    });
}


protected async void OnPickPhoto(object sender, EventArgs args)
{
    await MainThread.InvokeOnMainThreadAsync(() =>
    {
        if (MediaPicker.Default.IsCaptureSupported)
        {
            Task<string> task = await Utils.PickPhoto();
            if (task != null)
            {
                string filename = task.GetAwaiter().GetResult();
                imageFilename.Source = filename;
            }
        }
    });
}

public static async Task<string> PickPhoto()
{
    return await Task.Run(() =>
    {
        Task<FileResult> task = MediaPicker.Default.PickPhotoAsync();
        if (task != null)
        {
            FileResult result = task.GetAwaiter().GetResult();
            return result.FullPath;
        }

        return string.Empty;
    });
}


protected async void OnPickPhoto(object sender, EventArgs args)
{
    await MainThread.InvokeOnMainThreadAsync(() =>
    {
        if (MediaPicker.Default.IsCaptureSupported)
        {
            Task<string> task = await Utils.PickPhoto();
            if (task != null)
            {
                string filename = task.GetAwaiter().GetResult();
                imageFilename.Source = filename;
            }
        }
    });
}

The compiler complains at the await Utils.PickPhoto(); If I remove it, and satisfy the complier errors no result is returned. Is my setup correct? What am I doing wrong? Thank you in advance.

9 Replies

Kouhai•16mo ago

You should have

public static async Task<string> PickPhoto() {
  FileResult result = await MediaPicker.Default.PickPhotoAsync();
  return result.FullPath;
}

public static async Task<string> PickPhoto() {
  FileResult result = await MediaPicker.Default.PickPhotoAsync();
  return result.FullPath;
}

This assumes result is never null Also

Task<string> task = await Utils.PickPhoto();
if (task != null)
{
    string filename = task.GetAwaiter().GetResult();
    imageFilename.Source = filename;
}

Task<string> task = await Utils.PickPhoto();
if (task != null)
{
    string filename = task.GetAwaiter().GetResult();
    imageFilename.Source = filename;
}

Is not right, when you await a PickPhoto the return type is not Task<string> but just string

r2d25551OP•16mo ago

Let me give it a try. And I will test for null and return string.Empty if so.

Kouhai•16mo ago

You should not GetAwaiter().GetResult() do that at all

r2d25551OP•16mo ago

That worked rather well 🙂

Kouhai•16mo ago

Awesome! The general rule of thumb, when you see an async method (a method that returns Task) and you need it's result use await it instead of task.GetAwaiter().GetResult();

r2d25551OP•16mo ago

To confirm what I have now is:

public static async Task<string> PickPhoto()
{
    FileResult task = await MediaPicker.Default.PickPhotoAsync();
    if (task != null)
        return task.FullPath;
    return string.Empty;
}

protected async void OnPickPhoto(object sender, EventArgs args)
{
    string filename = await Utils.PickPhoto();
    imageFilename.Source = filename;
}

public static async Task<string> PickPhoto()
{
    FileResult task = await MediaPicker.Default.PickPhotoAsync();
    if (task != null)
        return task.FullPath;
    return string.Empty;
}

protected async void OnPickPhoto(object sender, EventArgs args)
{
    string filename = await Utils.PickPhoto();
    imageFilename.Source = filename;
}

What about the UI thread and controls being modified. I do not need any protection?

await MainThread.InvokeOnMainThreadAsync()

await MainThread.InvokeOnMainThreadAsync()

Kouhai•16mo ago

You don't need to, when you await the method, it continues on the calling thread which would be the UI thread

r2d25551OP•16mo ago

Excellent. Thank you so much again.

Kouhai•16mo ago

No problem!

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✅ Async PushButton Functions