Example of using autolayout programmatically in Xamarin.iOS

ViewController.cs

public partial class ViewController : UIViewController
    {
        #region properties
        // Top Views setup
        private readonly UITextView textView = new UITextView
        { TranslatesAutoresizingMaskIntoConstraints = false };
        private readonly UIView topImageContainer = new UIView
        { TranslatesAutoresizingMaskIntoConstraints = false };
        private readonly UIImageView imageView = new UIImageView {
            TranslatesAutoresizingMaskIntoConstraints = false,
            Image = UIImage.FromFile("clover.png"),
            ContentMode = UIViewContentMode.ScaleAspectFit //Mantian aspect ratio
        };
        //Bottom Controls Layout Setup
        private readonly UIButton previousButton = new UIButton(UIButtonType.System)
        { TranslatesAutoresizingMaskIntoConstraints = false };
        private readonly UIButton nextButton = new UIButton(UIButtonType.System)
        { TranslatesAutoresizingMaskIntoConstraints = false };
        private readonly UIPageControl uIPageControl = new UIPageControl()
        {
            CurrentPage = 0,
            Pages = 4,
            CurrentPageIndicatorTintColor = UIColor.Green,
            PageIndicatorTintColor = UIColor.LightGray
        };
        private UIStackView bottomControlsStackView;
        #endregion

        public override void ViewDidLoad()
        {
            base.ViewDidLoad();

            this.SetUpBottomControlls();
            this.SetupLayout();
        }

        /// <summary>
        /// Generates the top layout
        /// </summary>
        private void SetupLayout()
        {
            View.AddSubview(textView);
            View.AddSubview(topImageContainer);
            topImageContainer.AddSubview(imageView);

            //Add the text and the sub text.
            NSMutableAttributedString attributedText = //NS'Mutable'AttributedString can be appended to.
                new NSMutableAttributedString( //Constructor with multiple optional params
                    str: "Join us for some fun!",
                    font: UIFont.BoldSystemFontOfSize(18));
            //This attributed string gets appended to attributedText so that only one UITextView is needed.
            NSAttributedString attribtedSubText =
                new NSAttributedString(
                str: "\n\n\nAre you ready for loads and loads of fun? Don't wait any longer! We hope to see you in our stroes soon.",
                font: UIFont.SystemFontOfSize(13),
                foregroundColor: UIColor.DarkGray);
            attributedText.Append(attribtedSubText); //One single string now has multiple subs strings with diferent styles.
            textView.AttributedText = attributedText; //Make sure that the string is already appended before adding it to the textview.
            textView.TextAlignment = UITextAlignment.Center;
            textView.Editable = false;
            textView.ScrollEnabled = false;

            // Make imageViewContainer occupy the top half of the screen.
            topImageContainer.TopAnchor.ConstraintEqualTo(View.TopAnchor).Active = true;
            topImageContainer.TrailingAnchor.ConstraintEqualTo(View.TrailingAnchor).Active = true;
            topImageContainer.LeadingAnchor.ConstraintEqualTo(View.LeadingAnchor).Active = true;
            topImageContainer.HeightAnchor.ConstraintEqualTo(View.HeightAnchor, 0.5f).Active = true; //Second argument acts as a percentage float multiplier.
            // Layout imageView in realtion to the XYcenter anchors of topImageViewContainer.
            imageView.CenterYAnchor.ConstraintEqualTo(topImageContainer.CenterYAnchor).Active = true;
            imageView.CenterXAnchor.ConstraintEqualTo(topImageContainer.CenterXAnchor).Active = true;
            imageView.HeightAnchor.ConstraintEqualTo(topImageContainer.HeightAnchor, 0.6f).Active = true;

            textView.TopAnchor.ConstraintEqualTo(topImageContainer.BottomAnchor).Active = true;
            textView.LeadingAnchor.ConstraintEqualTo(View.LeadingAnchor, 24).Active = true; //Anchor with offset
            textView.TrailingAnchor.ConstraintEqualTo(View.TrailingAnchor, -24).Active = true; //Negative means to the left.
        }

        /// <summary>
        /// Generates the navigation controlls at the bottom.
        /// </summary>
        private void SetUpBottomControlls()
        {
            previousButton.SetTitle("PREV", UIControlState.Normal);
            previousButton.TitleLabel.Font = UIFont.BoldSystemFontOfSize(14);
            previousButton.SetTitleColor(UIColor.DarkGray, UIControlState.Normal);
            nextButton.SetTitle("NEXT", UIControlState.Normal);
            nextButton.TitleLabel.Font = UIFont.BoldSystemFontOfSize(14);
            nextButton.SetTitleColor(UIColor.Green, UIControlState.Normal);
            bottomControlsStackView = new UIStackView //Child views need to be added during assignation.
                (views: new UIView[] { previousButton, uIPageControl, nextButton })
            {   
                TranslatesAutoresizingMaskIntoConstraints = false,
                Distribution = UIStackViewDistribution.FillEqually
            };
            View.AddSubview(bottomControlsStackView);

            //Alternative way of activating constraints.
            NSLayoutConstraint.ActivateConstraints(new NSLayoutConstraint[] {
                //Top Stack View
                bottomControlsStackView.BottomAnchor.ConstraintEqualTo(View.SafeAreaLayoutGuide.BottomAnchor),//Safe Area Bottom Anchor
                bottomControlsStackView.LeadingAnchor.ConstraintEqualTo(View.SafeAreaLayoutGuide.LeadingAnchor),
                bottomControlsStackView.TrailingAnchor.ConstraintEqualTo(View.SafeAreaLayoutGuide.TrailingAnchor),
                bottomControlsStackView.HeightAnchor.ConstraintEqualTo(50)
            });
        }

        public ViewController(IntPtr handle) : base(handle) { }

        public override void DidReceiveMemoryWarning()
        {
            base.DidReceiveMemoryWarning();
            // Release any cached data, images, etc that aren't in use.
        }
    }