Simpler FilePicker

gistfile1.txt

using System;
using System.Threading.Tasks;
using Windows.ApplicationModel.Activation;
using Windows.ApplicationModel.Core;
using Windows.Storage;
using Windows.Storage.Pickers;
using Windows.Storage.Streams;

namespace Pinnacle.Pickers
{
    public class FilePicker
    {
        private TaskCompletionSource<IRandomAccessStream> _imageCompletionSource;
        private IEnumerable<string> _fileTypes;

        public FilePicker(IEnumerable<string> fileTypes)
        {
            _fileTypes = fileTypes;
        }
        
#if WINDOWS_PHONE_APP
        public Task<IRandomAccessStream> PickAsync()
#else
        public async Task<IRandomAccessStream> PickAsync()
#endif
        {
            FileOpenPicker openPicker = new FileOpenPicker();
            openPicker.ViewMode = PickerViewMode.Thumbnail;
            openPicker.SuggestedStartLocation = PickerLocationId.PicturesLibrary;
            foreach(var type in _fileTypes)
            {
                openPicker.FileTypeFilter.Add(type);
            }
            
#if WINDOWS_PHONE_APP
            _imageCompletionSource = new TaskCompletionSource<IRandomAccessStream>();
            CoreApplication.GetCurrentView().Activated += OnViewActivated;
            openPicker.PickSingleFileAndContinue();
            return _imageCompletionSource.Task;
#else
            StorageFile storageFile = await openPicker.PickSingleFileAsync();
            return await storageFile.OpenAsync(FileAccessMode.Read);
#endif
        }

        private async void OnViewActivated(CoreApplicationView sender, IActivatedEventArgs args)
        {
            sender.Activated -= OnViewActivated;
            FileOpenPickerContinuationEventArgs pickerArgs = args as FileOpenPickerContinuationEventArgs;

            IRandomAccessStream stream = null;
            if (pickerArgs != null)
            {
                if (pickerArgs.Files.Count > 0)
                {
                    StorageFile storageFile = pickerArgs.Files[0];
                    stream = await storageFile.OpenAsync(FileAccessMode.Read);
                }
            }
            _imageCompletionSource.SetResult(stream);
        }
    }
}