`Pixels` member doesn't exist

[beauxq]

I'm referencing these lines from the readme:

    // Line from P1(1, 2) to P2(30, 40) using the fastest draw line method 
   int[] pixels = writeableBmp.Pixels;

When I write code like that, I get this:

Error CS1061 'WriteableBitmap' does not contain a definition for 'Pixels' and no accessible extension method 'Pixels' accepting a first argument of type 'WriteableBitmap' could be found (are you missing a using directive or an assembly reference?)

How do I access the int array directly?