Question

In the following snippet my intention is to convert a System.Object (which could be an FSharpList) to a list of whatever generic type it is holding.

    match o with
    | :? list<_>              -> addChildList(o :?> list<_>)
    | _                       -> addChild(o)

Unfortunately only list<obj> is ever matched as a list. I would like list<Foo> to also be matched as a list.

For some context, I am trying to traverse an object structure by reflection in order to build a TreeView of the class and its children. Consider the following class:

type Entity = {
    Transform   : Matrix
    Components  : obj list
    Children    : Entity list
}

I would like to build a tree that shows me all the classes that is contained in the entity.
Through reflection, I can obtain all the properties of an object and also their values (The value is important, since I want to display the different elements in a list with the Name property of the element if it has one):

        let o = propertyInfo.GetValue(obj, null)

This value could be a list of some type, but the value return is just a System.Object
I run into problems when trying to convert this object to a list. I am forced to do the following:

        match o with
        | :? list<obj>              -> addChildList(o :?> list<obj>)
        | :? list<Entity>           -> addChildList(o :?> list<Entity>)
        | _                         -> addChild(o)

Here I have to specify exactly the type that I am trying to convert to.
I would really like to write this:

        match o with
        | :? list<_>              -> addChildList(o :?> list<_>)
        | _                       -> addChild(o)

Unfortunately this only ever matches on list< obj >

Answer 1

It turns out that either list<'a> or array<'a> can be matched as seq<obj>

    match o with
    | :? seq<obj> -> addChildCollection(o :?> seq<obj>)
    | _           -> addChild(o)

I don’t really care that it is a list. As long as I can iterate over it.