Component to write random pixels into a WriteableBitmap on Windows 8

PixelBuffer.cs

/// <summary>
/// Allows for drawing random pixels into a writeable bitmap
/// </summary>
public class PixelBuffer
{
    private WriteableBitmap _source;
    private byte[] _pixels;

    /// <summary>
    /// Initializes a new instance of <see cref="PixelBuffer"/>
    /// </summary>
    /// <param name="source"></param>
    public PixelBuffer(WriteableBitmap source)
    {
        _source = source;
        _pixels = new byte[4* source.PixelWidth * source.PixelHeight];
    }

    public void SetPixel(int x, int y, Color color)
    {
        // Move to the correct index in the pixel buffer
        // Pixels are arranged on a col, row manner.
        int index = 4 * (y * _source.PixelWidth + x);

        // The pixel layout is always A, R, G, B.
        _pixels[index] = color.A;
        _pixels[index + 1] = color.R;
        _pixels[index + 2] = color.G;
        _pixels[index + 3] = color.B;
    }

    public void Flush()
    {
        var pixelStream = _source.PixelBuffer.AsStream();

        pixelStream.Seek(0, SeekOrigin.Begin);
        pixelStream.Write(_pixels,0,_pixels.Length);

        _source.Invalidate();
    }
}