Async operations in one function under c++

Question

Hello,

can somebody help me to translate the following c# code to c++?

The problem is to merge the two async functions to a synchron function and return the result as a async result.

        private async void Button_Click_2(object sender, RoutedEventArgs e)
        {
            System.Diagnostics.Debug.WriteLine("before BeadBytesFromFile");
            byte[] b = await ReadBytesFromFile();
            System.Diagnostics.Debug.WriteLine("after BeadBytesFromFile");

        }

        private async static Task<byte[]> ReadBytesFromFile()
        {
            System.Diagnostics.Debug.WriteLine("start BeadBytesFromFile");
            var stream = await KnownFolders.MusicLibrary.OpenStreamForReadAsync("testfile.bin");
            byte[] b = new byte[stream.Length];
            int readedBytes = await stream.ReadAsync(b, 0, (int)stream.Length);
            System.Diagnostics.Debug.WriteLine("end BeadBytesFromFile");
            return b;
        }

---

u

Answer 1

Hi,

Please try these codes

	create_task(KnownFolders::MusicLibrary->GetFileAsync("testfile.bin")).then([this](task<StorageFile^> t)
     {
         try {
             StorageFile^ file = t.get();
             create_task(FileIO::ReadBufferAsync(file)).then([this, file](task<IBuffer^> tt)
             {
                 try
                 {
                     IBuffer^ buffer = tt.get();
					 
                     DataReader^ dataReader = DataReader::FromBuffer(buffer);
					
					 Array<unsigned char>^ value= ref new Array<unsigned char>(buffer->Length);
                     dataReader->ReadBytes(value);
				 }
                 catch(COMException^ ex)
                 {
                   
                 }
             });
         } catch(COMException^ ex) {
         }
	});

Best regards,
Jesse

---

Jesse Jiang [MSFT]
MSDN Community Support | Feedback to us

Answer 2

Hello,

thank you for your answer.

I did not understand how i can handle the return value. In my example b (byte[]) and in your c++ code the buffer.

And a second question:

Is there a similar possibility how System.Diagnostics.Debug.WriteLine under c++?

thanks

---

u

Answer 3

You can use a loop to convert char array to byte[]
And the OutputDebugString API is the same function with System.Diagnostics.Debug.WriteLine

                     IBuffer^ buffer = tt.get();
					 
                     DataReader^ dataReader = DataReader::FromBuffer(buffer);	
					 Array<unsigned char>^ value= ref new Array<unsigned char>(buffer->Length);
                     dataReader->ReadBytes(value);
					 byte bytes[255];
					 for(int i=0;i<value->Length;i++)
					 {
						 bytes[i]=value->get(i);
					 }
					 OutputDebugString(L"Read buffer completed");

Best regards,
Jesse

---

Jesse Jiang [MSFT]
MSDN Community Support | Feedback to us