storagefile::ReadAsync exception in c++/cx?

Question

I have been trying to use c++/cx StorageFile::ReadAsync() to read a file in a store-apps, but it always return an invalid params exception no matter what

// "file" are returned from FileOpenPicker
IRandomAccessStream^ reader = create_task(file->OpenAsync(FileAccessMode::Read)).get();
if (reader->CanRead)
{
    BitmapImage^ b = ref new BitmapImage();
    const int count = 1000000;
    Streams::Buffer^ bb = ref new Streams::Buffer(count);
    create_task(reader->ReadAsync(bb, 1, Streams::InputStreamOptions::None)).get();
}

I have turn on all the manifest capabilities and added "file open picker" + "file type association" for Declarations. Any ideas ? thanks!

ps: most solutions I found is for C#, but the code structure are similar...

Answer 1

If this code is executing on the UI thread (or in any other Single Threaded Apartment, or STA), then the calls to .get() will throw if the tasks have not yet completed, because the call to .get() would block the thread. You must not block the UI thread or any other STA, and when compiling with C++/CX support enabled, the libraries enforce this.

If you turn on first chance exception handling in the debugger (Debug -> Exceptions..., check the C++ Exceptions check box), you should see that the first exception to be thrown is an invalid_operation exception, from the following line in <ppltasks.h>:

// In order to prevent Windows Runtime STA threads from blocking the UI, calling
// task.wait() task.get() is illegal if task has not been completed.
if (!_IsCompleted() && !_IsCanceled())
{
  throw invalid_operation("Illegal to wait on a task in a Windows Runtime STA");
}

The "invalid parameter" you are reporting is the fatal error that is caused when this exception reaches the ABI boundary: the debugger is notified that the application is about to terminate because this exception was unhandled.

You need to restructure your code to use continuations, using task::then, as described in the article Asynchronous Programming in C++ Using PPL