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public class FilePicked : EventArgs
{
    public string AbsolutePath { get; set; }
}

[Activity]
public class PickActivity: Activity
{
    static event EventHandler<FilePicked> OnFilePicked;

    protected override void OnCreate(Bundle savedInstanceState)
    {
        base.OnCreate(savedInstanceState);

        var intent = new Intent();
        intent.SetType("image/*");
        intent.SetAction(Intent.ActionPick);
        StartActivityForResult(intent, 0);
    }

    protected override void OnActivityResult(int requestCode, Result resultCode, Intent data)
    {
        base.OnActivityResult(requestCode, resultCode, data);

        if (requestCode == 0 && resultCode == Result.Ok)
        {
            if (OnFilePicked != null)
            {
                OnFilePicked(this, new FilePicked { AbsolutePath = data.Data.Path });
            }
        }
        
        Finish(); 
    }
}

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public class PickFileService: IPickFileService
{
    TaskCompletionSource<string> tcs;

    public Task<string> PickFile()
    {
        var uniqueId = Guid.NewGuid();
        var next = new TaskCompletionSource<string>(uniqueId); 

        // Interlocked.CompareExchange(ref object location1, object value, object comparand)
        // Compare location1 with comparand.
        // If equal replace location1 by value.
        // Returns the original value of location1.
        // ---
        // In this context, tcs is compared to null, if equal tcs is replaced by next,
        // and original tcs is returned.
        // We then compare original tcs with null, if not null it means that a task was 
        // already started.
        if (Interlocked.CompareExchange(ref tcs, next, null) != null)
        {
            return Task.FromResult<string>(null);
        }

        EventHandler<FilePicked> handler = null;

        handler = (sender, e) => {
            
            // Interlocaked.Exchange(ref object location1, object value)
            // Sets an object to a specified value and returns a reference to the original object.
            // ---
            // In this context, sets tcs to null and returns it.
            var task = Interlocked.Exchange(ref tcs, null);

            PickActivity.OnFilePicked -= handler;

            if (!String.IsNullOrWhiteSpace(e.AbsolutePath))
            {
                task.SetResult(e.AbsolutePath);
            }
            else
            {
                task.SetCanceled();
            }
        };

        PickActivity.OnFilePicked += handler;
        var pickIntent = new Intent(Forms.Context, typeof(PickActivity));
        pickIntent.SetFlags(ActivityFlags.NewTask);
        Forms.Context.StartActivity(pickIntent);

        return tcs.Task;
    }
}

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var buttonPickFile = new Button { Text = "Pick file" };

buttonPickFile.Clicked += async (sender, e) =>
{
    var file = await DependencyService.Get<IPickFileService>().PickFile();
    await page.DisplayAlert("File picked", file, "OK");
};