How to add content to an access file?

Question

Hi,

According to the Metro style app samples "ApplicationData sample" and "File access sample" I'v know how to create a file and how to write content to the file.

But I can't find how to add content to the end of a file  in samples as well as in library.

I need to add some contents to the end of a file,just like the usage in c++:

         ofstream file("example.bin", ios::out | ios::app);

How to do that in metro apps?

 

Here is the code what I referenced:

 

void MainPage::CreateFileButton_Click(Platform::Object^ sender, Windows::UI::Xaml::RoutedEventArgs^ e)
{
    StorageFolder^ documentsFolder = KnownFolders::DocumentsLibrary;
    auto fileOperation = documentsFolder->CreateFileAsync("sample.txt", CreationCollisionOption::ReplaceExisting);
    fileOperation->Completed = ref new AsyncOperationCompletedHandler<StorageFile^>([this](IAsyncOperation<StorageFile^>^ operation) 
    {
        if (operation->Status == AsyncStatus::Completed)
        {
            StorageFile^ fileNew = operation->GetResults();
            Scenario1Output_textblock->Text = "The file " + fileNew->FileName + " was created.";
        }
        else
        {
            Scenario1Output_textblock->Text = "There was an error creating sample.txt.";
        }
    });
    fileOperation->Start();
}

void MainPage::WriteButton_Click(Platform::Object^ sender, Windows::UI::Xaml::RoutedEventArgs^ e)
{
    // First attempt to get the file
    StorageFolder^ documentsFolder = KnownFolders::DocumentsLibrary;
    auto fileOperation = documentsFolder->GetFileAsync("sample.txt");
    MainPage^ page = this;
    fileOperation->Completed = ref new AsyncOperationCompletedHandler<StorageFile^>([page](IAsyncOperation<StorageFile^>^ operation) 
    {
        if (operation->Status == AsyncStatus::Completed)
        {
            // Get the returned file and attempt to open it for a read+write operation
            StorageFile^ storagefileSample = operation->GetResults();
            auto openOperation = storagefileSample->OpenAsync(FileAccessMode::ReadWrite);
            openOperation->Completed = ref new AsyncOperationCompletedHandler<IRandomAccessStream^>([page](IAsyncOperation<IRandomAccessStream^>^ operation)
            {
                if (operation->Status == AsyncStatus::Completed)
                {
                    // Create a DataReader to read the opened file's contents
                    IRandomAccessStream^ accessStream = operation->GetResults();
                    IOutputStream^ outputStream = accessStream->GetOutputStreamAt(0);
                    DataWriter^ dataWriter  = ref new DataWriter(outputStream);

                    // Write the string and store it in the writer
                    dataWriter->WriteString(page->Scenario2Textbox->Text);
                    auto writeOperation = dataWriter->StoreAsync();
                    writeOperation->Completed = ref new AsyncOperationCompletedHandler<unsigned int>([page, outputStream](IAsyncOperation<unsigned int>^ operation)
                    {
                        auto flushOperation = outputStream->FlushAsync();
                        flushOperation->Completed = ref new AsyncOperationCompletedHandler<bool>([page](IAsyncOperation<bool>^ operation)
                        {
                            page->Scenario2Output_textblock->Text = "\"" + page->Scenario2Textbox->Text + "\" was written to sample.txt";
                        });
                        flushOperation->Start();
                    });
                    writeOperation->Start();
                }
            });
            openOperation->Start();
        }
        else
        {
            page->Scenario3Output_textblock->Text = "The file sample.txt does not exist. Use scenario one to create this file.";
        }
    });
    fileOperation->Start();
}

Thanks

 

Answer 1

Hi Steven,

You can seek to the end of the string before writing it. In your sample code you explicitly get the output stream from the beginning:

 IOutputStream^ outputStream = accessStream->GetOutputStreamAt(0);
                   

If you want the end, look at the accessStream's size and call GetOutputStreamAt(accessSream->Size) to get the outputStream from the end.

--Rob

 

Answer 2

Thank you Rob，it Works.