ConstrainToParentCenterExtensionMethod.cs

public static void ConstrainToParentCenter(this UIView view, nfloat height = default(nfloat), nfloat width = default(nfloat))
{
    if (view?.Superview == null) throw new InvalidOperationException("Must add view to a superview before calling this method");

    view.TranslatesAutoresizingMaskIntoConstraints = false;
    
    var verticalFormat = string.Format("V:[super]-(<=1)-[view{0}]", height == default(nfloat) ? string.Empty : $"({height})");

    var horizontalFormat = string.Format("H:[super]-(<=1)-[view{0}]", width == default(nfloat) ? string.Empty : $"({width})");

    var viewsAndMetrics = new object [] { "super", view.Superview, "view", view }; 

    view.Superview.AddConstraints(NSLayoutConstraint.FromVisualFormat(verticalFormat, NSLayoutFormatOptions.AlignAllCenterX, viewsAndMetrics));

    view.Superview.AddConstraints(NSLayoutConstraint.FromVisualFormat(horizontalFormat, NSLayoutFormatOptions.AlignAllCenterY, viewsAndMetrics));
}