Windows Phone 8 shared core with Windows 8 – File IO

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For developers, the biggest news for Windows Phone 8 is that it shares a core with Windows 8. It means that your code has more chances than ever before to be compatible on both platforms.

One of the easier components and surely the most used in applications is the File IO component. In this post, I’ll also cover the Windows Phone 7 code compatibility.

In Windows Phone 7

To write a file, you use the IsolatedStorageFile: 

private void WriteFile(string fileName, string content)
{
    using (IsolatedStorageFile isolatedStorageFile = IsolatedStorageFile.GetUserStoreForApplication())
    {
        using (IsolatedStorageFileStream isolatedStorageFileStream = isolatedStorageFile.CreateFile(fileName))
        {
            using (StreamWriter streamWriter = new StreamWriter(isolatedStorageFileStream))
            {
                streamWriter.Write(content);
            }
        }
    }
}

To read a file:

private string ReadFile(string fileName)
{
    string text;

    using (IsolatedStorageFile isolatedStorageFile = IsolatedStorageFile.GetUserStoreForApplication())
    {
        using (IsolatedStorageFileStream isolatedStorageFileStream = isolatedStorageFile.OpenFile(fileName, FileMode.Open))
        {
            using (StreamReader streamReader = new StreamReader(isolatedStorageFileStream))
            {
                text = streamReader.ReadToEnd();
            }
        }
    }

    return text;
}

In Windows Phone 8

To write a file, you use the Windows.Storage component. The file and folder objects are respectively IStorageFile and IStorageFolder. The usage of the await and async keywords come in.

To write a file:

public async Task WriteFile(string fileName, string text)
{
    IStorageFolder applicationFolder = ApplicationData.Current.LocalFolder;

    IStorageFile storageFile = await applicationFolder.CreateFileAsync(fileName, CreationCollisionOption.ReplaceExisting);

    using (Stream stream = await storageFile.OpenStreamForWriteAsync())
    {
        byte[] content = Encoding.UTF8.GetBytes(text);
        await stream.WriteAsync(content, 0, content.Length);
    }
}

To read a file:

public async Task<string> ReadFile(string fileName)
{
    string text;
    IStorageFolder applicationFolder = ApplicationData.Current.LocalFolder;

    IStorageFile storageFile = await applicationFolder.GetFileAsync(fileName);

    IRandomAccessStream accessStream = await storageFile.OpenReadAsync();

    using (Stream stream = accessStream.AsStreamForRead((int)accessStream.Size))
    {
        byte[] content = new byte[stream.Length];
        await stream.ReadAsync(content, 0, (int) stream.Length);

        text = Encoding.UTF8.GetString(content, 0, content.Length);
    }

    return text;
}

You can use these methods this way:

await WriteFile("Dummy.txt", "I love the Windows Phone");
string text = await ReadFile("Dummy.txt");

You can see that the code between Windows Phone 7 and Windows Phone 8 is quite different.

Compatibility

As we know already, Microsoft announced that the Windows Phone 7 applications will run on Windows Phone 8. This is great news. What is even better is that your Windows Phone 7 code that uses the IsolatedStorage will compile without any changes in Windows Phone 8. You can take the Windows Phone 7 code above and it will work.

You might be wondering if the internal folder structure is different between WP7 and WP8… The difference is minor.

In WP7, if you create a file at the root, the file will point to:

C:DataUsersDefaultAppAccountAppData{9679D8C4-24B4-41DF-85C5-5E099947F109}LocalIsolatedStoreDummy.txt

In WP8, the same file created at the root will be in:

C:DataUsersDefaultAppAccountAppData{9679D8C4-24B4-41DF-85C5-5E099947F109}LocalDummy.txt

The difference is the IsolatedStore folder added in WP7. It means that if you have old WP7 code that you would like to use with new WP8 code, you just have to get the IsolatedStore folder:

IStorageFolder applicationFolder = 
    await ApplicationData.Current.LocalFolder.GetFolderAsync("IsolatedStore");

In the end, you can mix both WP7 and WP8 code in your next Windows Phone 8 application. However, I would encourage you to use only the Windows Phone 8 version, as it will then be easier for you to use the same code in Windows 8.