Why does Xamarin.Forms return -1 for Height and Width ?

Xamarin.Forms returns -1 as the default value for these properties, and it remains -1 until Xamarin.Forms creates the native control, e.g. UIButton, and adds that native control to the layout.

In this link, you can see the Xamarin.Forms source code returning -1 as the default value: https://github.com/xamarin/Xamarin.Forms/blob/master/Xamarin.Forms.Core/VisualElement.cs

Best Way to Use Relative Layouts to Constrain Views

Option 1: Local Functions (Requires C# 7.0 or higher)

Use a Local Function to dynamically retrieve the Width and Height properties

var mainLayout = new RelativeLayout(); //Add the Switch to the center of the screen mainLayout.Children.Add(mySwitch, Constraint.RelativeToParent(parent => parent.Width / 2 - getWidth(parent, mySwitch)/ 2), Constraint.RelativeToParent(parent => parent.Height / 2 - getHeight(parent, mySwitch) / 2)); //Add a Label below the switch mainLayout.Children.Add(switchLabel, Constraint.RelativeToParent(parent => parent.Width / 2 - getWidth(parent, switchLabel) / 2), Constraint.RelativeToView(mySwitch, (parent, view) => view.Y + getHeight(parent, mySwitch) + 10)); Content = mainLayout; static double getWidth(RelativeLayout parent, View view) => view?.Measure(parent.Width, parent.Height).Request.Width ?? -1; static double getHeight(RelativeLayout parent, View view) => view?.Measure(parent.Width, parent.Height).Request.Height ?? -1;

Option 2: Func<RelativeLayout, double>

Use a Func to dynamically retrieve the Width and Height properties

var mainLayout = new RelativeLayout(); Func<RelativeLayout, double> getSwitchWidth = (parent) => mySwitch.Measure(parent.Width, parent.Height).Request.Width; Func<RelativeLayout, double> getSwitchHeight = (parent) => mySwitch.Measure(parent.Width, parent.Height).Request.Height; Func<RelativeLayout, double> getLabelWidth = (parent) => switchLabel.Measure(parent.Width, parent.Height).Request.Width; Func<RelativeLayout, double> getLabelHeight = (parent) => switchLabel.Measure(parent.Width, parent.Height).Request.Height; //Add the Switch to the center of the screen mainLayout.Children.Add(mySwitch, Constraint.RelativeToParent(parent => parent.Width / 2 - getSwitchWidth(parent)/ 2), Constraint.RelativeToParent(parent => parent.Height / 2 - getSwitchHeight(parent) / 2)); //Add a Label below the switch mainLayout.Children.Add(switchLabel, Constraint.RelativeToParent(parent => parent.Width / 2 - getLabelWidth(parent) / 2), Constraint.RelativeToView(mySwitch, (parent, view) => view.Y + getSwitchHeight(parent) + 10)); Content = mainLayout;

Thanks to @BrewMate for teaching me this trick!