How to: Convert between .NET Framework and Windows Runtime streams (Windows only)

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.NET Framework for UWP apps is a subset of the full .NET Framework. Because of security and other requirements for UWP apps, you can't use the full set of .NET Framework APIs to open and read files. For more information, see .NET for UWP apps overview. However, you may want to use .NET Framework APIs for other stream manipulation operations. To manipulate these streams, you can convert between a .NET Framework stream type, such as MemoryStream or FileStream, and a Windows Runtime stream, such as IInputStream, IOutputStream, or IRandomAccessStream.

The System.IO.WindowsRuntimeStreamExtensions class contains methods that make these conversions easy. However, underlying differences between .NET Framework and Windows Runtime streams affect the results of using these methods, as described in the following sections:

Convert from a Windows Runtime to a .NET Framework stream

To convert from a Windows Runtime stream to a .NET Framework stream, use one of the following System.IO.WindowsRuntimeStreamExtensions methods:

The Windows Runtime offers stream types that support reading only, writing only, or reading and writing. These capabilities are maintained when you convert a Windows Runtime stream to a .NET Framework stream. Furthermore, if you convert a Windows Runtime stream to a .NET Framework stream and back, you get the original Windows Runtime instance back.

It's best practice to use the conversion method that matches the capabilities of the Windows Runtime stream you want to convert. However, since IRandomAccessStream is readable and writeable (it implements both IOutputStream and IInputStream), the conversion methods maintain the capabilities of the original stream. For example, using WindowsRuntimeStreamExtensions.AsStreamForRead to convert an IRandomAccessStream doesn't limit the converted .NET Framework stream to being readable. It's also writable.

Example: Convert Windows Runtime random-access to .NET Framework stream

To convert from a Windows Runtime random-access stream to a .NET Framework stream, use the WindowsRuntimeStreamExtensions.AsStream method.

The following code example prompts you to select a file, opens it with Windows Runtime APIs, and then converts it to a .NET Framework stream. It reads the stream and outputs it to a text block. You would typically manipulate the stream with .NET Framework APIs before outputting the results.

// Create a file picker.
FileOpenPicker picker = new FileOpenPicker();
picker.SuggestedStartLocation = PickerLocationId.DocumentsLibrary;
picker.ViewMode = PickerViewMode.List;
picker.FileTypeFilter.Add(".txt");

// Show picker, enabling user to pick one file.
StorageFile result = await picker.PickSingleFileAsync();
if (result != null)
{
    try
    {
        // Retrieve the stream. This method returns a IRandomAccessStreamWithContentType.
        var stream = await result.OpenReadAsync();

        // Convert the stream to a .NET stream using AsStream, pass to a
        // StreamReader and read the stream.
        using (StreamReader sr = new StreamReader(stream.AsStream()))
        {
            TextBlock1.Text = sr.ReadToEnd();
        }
    }
    catch (Exception ex)
    {
        // ...
    }
}

Convert from a .NET Framework to a Windows Runtime stream

To convert from a .NET Framework stream to a Windows Runtime stream, use one of the following System.IO.WindowsRuntimeStreamExtensions methods:

When you convert a .NET Framework stream to a Windows Runtime stream, the capabilities of the converted stream depend on the original stream. For example, if the original stream supports both reading and writing, and you call WindowsRuntimeStreamExtensions.AsInputStream to convert the stream, the returned type is an IRandomAccessStream. IRandomAccessStream implements IInputStream and IOutputStream, and supports reading and writing.

.NET Framework streams don't support cloning, even after conversion. If you convert a .NET Framework stream to a Windows Runtime stream and call GetInputStreamAt or GetOutputStreamAt, which call CloneStream, or if you call CloneStream directly, an exception occurs.

Example: Convert .NET Framework to Windows Runtime random-access stream

To convert from a .NET Framework stream to a Windows Runtime random-access stream, use the AsRandomAccessStream method, as shown in the following example:

Important

Make sure that the .NET Framework stream you are using supports seeking, or copy it to a stream that does. You can use the Stream.CanSeek property to determine this.

// Create an HttpClient and access an image as a stream.
var client = new HttpClient();
Stream stream = await client.GetStreamAsync("https://video2.skills-academy.com/en-us/dotnet/images/hub/featured-1.png");
// Create a .NET memory stream.
var memStream = new MemoryStream();
// Convert the stream to the memory stream, because a memory stream supports seeking.
await stream.CopyToAsync(memStream);
// Set the start position.
memStream.Position = 0;
// Create a new bitmap image.
var bitmap = new BitmapImage();
// Set the bitmap source to the stream, which is converted to a IRandomAccessStream.
bitmap.SetSource(memStream.AsRandomAccessStream());
// Set the image control source to the bitmap.
Image1.Source = bitmap;

See also