Here's an example how to fill a `WriteableBitmap` with random colors:

    // Hardcoded just for example
    int width = 500;
    int height = 500;
    // Calculate stride
    var stride = (width * (PixelFormats.Bgra32.BitsPerPixel) + 7) / 8;
    // Create a WriteableBitmap
    WriteableBitmap wb = new WriteableBitmap(width, height, 300.0, 300.0, PixelFormats.Bgra32, null);
    // And a buffer for the bytes
    byte [] b = new byte[stride * height];
    // Randomly fill array
    for (int i = 0; i < height; i++)
    {
        for (int j = 0; j < width; j++)
        {
            b[(i * width) + (j * 4)] = 255;   // Alpha
            b[(i * width) + (j * 4) + 1] = (byte)r.Next(255);
            b[(i * width) + (j * 4) + 2] = (byte)r.Next(255);
            b[(i * width) + (j * 4) + 3] = (byte)r.Next(255);
        }
    }

    // Copy
    Int32Rect rect = new Int32Rect(0, 0, width, height);
    wb.WritePixels(rect, b, stride, 0, 0);
    // img is a WPF Image control
    img.Source = wb;