F# - what does match...with | :? X as x mean?

Question

Super simple question I'm sure but I don't do F# usually so a bit new to the syntax.

How do I read the following fragment of code:

        match shape.Fill with
          | :? PictureBrush as pb ->
              //....

Specifically, I'm not sure what PictureBrush refers to. There's not really any reference to it anywhere else in this file

Answer 1

This the Type Test Pattern.

PictureBrush is a type. shape.Fill matches :? PictureBrush when the Fill property is assignable to that type.

Answer 2

Also note that the as allows for binding of a variable to be of that type. So pb will be of type PictureBrush, but will have the exact same value as what shape.Fill resolved to.

Answer 3

Its a type comparison. Like typeof(xx) in C#. So match to type of PictureBrush.