Bug 57838 - SetBoundsConstraint cannot be used to preconfigure a view for adding to a RelativeLayout
Summary: SetBoundsConstraint cannot be used to preconfigure a view for adding to a Rel...
Alias: None
Product: Forms
Classification: Xamarin
Component: Forms ()
Version: 2.3.4
Hardware: Macintosh Mac OS
: --- normal
Target Milestone: ---
Assignee: Bugzilla
Depends on:
Reported: 2017-06-28 15:13 UTC by Mike Norman
Modified: 2017-07-12 22:51 UTC (History)
2 users (show)

Tags: ac relativelayout boundscontstraint
Is this bug a regression?: ---
Last known good build:

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Description Mike Norman 2017-06-28 15:13:54 UTC
The user should be able to call RelativeLayout.SetBoundsConstraint(someView), and then call someRelLayout.Add(someView) to add a preconfigured view to a RelativeLayout.

The following code does not compile:
public partial class App : Application
    public App()

        ContentPage cp = new ContentPage();
        RelativeLayout rl = new RelativeLayout();
        cp.Content = rl;

        Label downHereLabel = new Label { Text = "Down here!" , BackgroundColor = Color.Blue};

        BoundsConstraint bc = BoundsConstraint.FromExpression(() => new Rectangle(cp.Width / 2, cp.Height / 2, cp.Width / 2, cp.Height / 2), null);
        RelativeLayout.SetBoundsConstraint(downHereLabel, bc);
        rl.Children.Add(downHereLabel); // NOTE: this line will not compile

        MainPage = cp;

    protected override void OnStart()
        // Handle when your app starts

    protected override void OnSleep()
        // Handle when your app sleeps

    protected override void OnResume()
        // Handle when your app resumes

It appears that the Add() method overloads in IRelativeList<T> are ambiguous, and clobber the bare Add(TRestrict item) overload that seems to have been intended to have been inherited from the (internal) ObserveableWrapper<TTrack, TRestrict> class. In its absence, the compiler cannot choose between the Add() overload that has four default Constraint arguments and the one that has four default Expression<Func<double>> arguments. Whatever the reason, though, Add(someView) does not resolve.

As a workaround, the user can disambiguate by passing "(null as Constraint)", for example, but then should delay calling SetBoundsConstraint() until after the Add() call. However, this results in useless code being run that gets clobbered with the eventual SetBoundsConstraint() call.

Similar Add(someView) calls seem to work on other Layout subclasses, so perhaps guidance could be sought from the code in those classes.

My guess is that this is an untested regression from e6d5186c8 on 2017-03-07, when various types were given the internal access modifier. This would also explain things if, by chance, this scenario more or less works for XAML.
Comment 1 Jimmy [MSFT] 2017-06-29 16:59:53 UTC
I can confirm that I am seeing a build error when I try to add a View to the RelativeLayout as shown above. I get the error:

> The call is ambiguous between the following methods or properties: 'RelativeLayout.IRelativeList<T>.Add(T, Expression<Func<double>>, Expression<Func<double>>, Expression<Func<double>>, Expression<Func<double>>)' and 'RelativeLayout.IRelativeList<T>.Add(T, Constraint, Constraint, Constraint, Constraint)'

The commit you mentioned is included in 2.3.5 so I tested with several older versions to verify but the error still occurs. 

### Version Tests   BAD   BAD   BAD