How to Convert a WriteableBitmap image to Byte array in WinRt App

WriteableBitmap exposes PixelBuffer property of type IBuffer which is a Windows Runtime interface so it's a case of converting IBuffer to a byte array and this can be done through regular .NET Streams

byte[] ConvertBitmapToByteArray(WriteableBitmap bitmap)
{
    WriteableBitmap bmp = bitmap;

    using (Stream stream = bmp.PixelBuffer.AsStream())
    {
        MemoryStream memoryStream = new MemoryStream();
        stream.CopyTo(memoryStream);
        return memoryStream.ToArray();
    }
}

AsStream() is an extension method on IBuffer declared in WindowsRuntimeBufferExtensions class from System.Runtime.InteropServices.WindowsRuntime namespace.