Introduce `ControlExtensions` to simplify frequently needed iterating tasks

[KlausLoeffelmann]

Background and motivation

There are a lot of cases, where we need to recursively traverse through the controls collection (up or down), to either process a collection of certain controls or find a specific control in the tree. This is, where the typical WinForms developer invents the wheel for their apps for the gazilion-th time.

To that end, we suggest introducing the following extension class to make those tasks super-convenient, and make sure, that we do not allocate anything during operation.

Note:
PR would be a side-product of one of the .NET Conf Demos and ready to go immediately.
@merriemcgaw, @lonitra

@JeremyKuhne: If we want to further discuss the concept of a more granular base-layout type, this API would be super-easy to adapt to that and also the perfect icing on that cake.

API Proposal

public static IEnumerable<Control> ChildControls(this Control control)
public static IEnumerable<T> ChildControls<T>(this Control control) where T : Control
public static Control FirstChild(this Control control, Func<Control, bool>? predicate = default)
public static T FirstChild<T>(this Control control, Func<T, bool>? predicate = default) where T : Control
public static T? FirstChildOrDefault<T>(this Control control, Func<T, bool>? predicate = default) where T : Control
public static IEnumerable<Control> AscendantControls(this Control control)
public static IEnumerable<T> AscendantControls<T>(this Control control, Func<Control, bool>? predicate = default) where T : Control
public static Control FirstAscendant(this Control control, Func<Control, bool>? predicate = default)
public static Control? FirstAscendantOrDefault(this Control control, Func<Control, bool>? predicate = default)
public static T FirstAscendant<T>(this Control control, Func<T, bool>? predicate = default) where T : Control
public static T? FirstAscendantOrDefault<T>(this Control control, Func<T, bool>? predicate = default) where T : Control
public static IEnumerable<Control> DescendantControls(this Control control)
public static IEnumerable<T> DescendantControls<T>(this Control control, Func<Control, bool>? predicate = default) where T : Control
public static Control FirstDescendant(this Control control, Func<Control, bool>? predicate = default)
public static T FirstDescendant<T>(this Control control, Func<T, bool>? predicate = default) where T : Control
public static Control? FirstDescendantOrDefault(this Control control, Func<Control, bool>? predicate = default)
public static T? FirstDescendantOrDefault<T>(this Control control, Func<T, bool>? predicate
public static Control Root(this Control control)

API Usage

Self-Explaining.

Alternative Designs

No response

Risks

None.

Will this feature affect UI controls?

Only easier usage and help with discoverability.

[elachlan]

@KlausLoeffelmann my implementation is a recursive call that returns a flat List. It takes a type parameter as we sometimes want specific types. We also allow filtering by type.

Private Function GetAllControlsInternal(Of T As Control)(Control As Control, Optional ControlsExclusionFilter As Type() = Nothing) As List(Of T)
    Dim result As New List(Of T)()

    For Each child As Control In Control.Controls
        If ControlsExclusionFilter IsNot Nothing AndAlso ControlsExclusionFilter.Any(Function(y) y.IsAssignableFrom(child.GetType())) Then
            Continue For
        End If
        If TypeOf child Is T Then
            result.Add(child)
        End If
        result.AddRange(GetAllControlsInternal(Of T)(child, ControlsExclusionFilter))
    Next

    Return result
End Function

I think the language on your APIs might be confusing. Ascendant/Descendant imply inheritance structure and not parent/child. Is that what you are proposing?

[JeremyKuhne]

@KlausLoeffelmann Why are these extension methods? They all take Control, which we own.

These APIs need some comments on what they do. I think I know what some of them do, but I'm not 100% sure. If this is "all parents" or "all children", what is the order of enumeration for children (presuming not depth first)?

I'm also unclear on why there are so many overloads. It also doesn't seem right to replicate and not push towards the normal IEnumerable extensions.

public class Control
{
    public IEnumerable<Control> ChildControls()
    public IEnumerable<Control> AscendantControls()
    public IEnumerable<Control> DescendantControls()
    // What is this?
    public Control Root()
}

[KlausLoeffelmann]

Yep - this needs some reconsideration.
Moving to .NET Next.