double question mark (??) public ControllerParameter2D Parameters{ get { return _overrideParameters ?? DefaultParameters; }

Questions & Answers
1

using UnityEngine;
using System.Collections;

public class CharacterController2D : MonoBehaviour {

public ControllerParameters2D DefaultParameters;

public ControllerParameters2D Parameters { get { return _overrideParameters ?? DefaultParameters;}}

private ControllerParameters2D _overrideParameters;

}

2

http://en.wikipedia.org/wiki/Null_coalescing_operator