The default value is an empty array.
Auto Layout uses the UIView.Constraints of a UIKit.UIView to lay out its UIView.Subviews. These constraints are set with UIView.AddConstraint or UIView.AddConstraints.
The NSLayoutConstraint.DebugDescription property can be helpful in debugging Auto Layout issues.
C# Example
foreach(var c in mainView.Constraints)
{
Console.WriteLine(c.DebugDescription);
}