Unity5.6 Collision2D contact points array index out of range error - unity3d

Question

When I updated my Unity version 5.5 to 5.6, an error occured about the Collision2D.contacts array. When I try to access the contacts array, I can not get contact point info now.
void OnCollisionExit2D(Collision2D col)
{
if (col.gameObject.CompareTag("Ground"))
{
if ((_transform.position.y - col.contacts[0].point.y) > colliderHeight / 2 + .15f)
{
Debug.Log ("Contact count = " + col.contacts.Length);
_onGround = false;
ParticleController.PlayDustEffect ();
}
}
}
Error Log :
IndexOutOfRangeException: Array index is out of range.
Player.OnCollisionExit2D (UnityEngine.Collision2D col) (at Assets/Scripts/CharacterController/Player.cs:759)
How can I fix the error?
Thanks for your time.

Answer 1

It seems the exit point of collision is not computed anymore in Unity 5.6. Some changes about collisions detection have been changed in this version:
Physics: The internal 2D contact processing has been completely re-written, providing a more robust and reliable reporting of contacts.
I guess the logic for it is the following: why would there be a contact point if the two colliders are not touching each other anymore? As there is no contact point anymore, col.contacts is empty there. So when you are trying to access col.contacts[0], the element doesn't exist, triggering the IndexOutOfRangeException.
From the Collision.contacts documentation (not the Collision2D.contacts one, but I suppose the behavior is the same):
Every contact contains a contact point, normal and the two colliders that collided (see ContactPoint). From inside OnCollisionStay or OnCollisionEnter you can always be sure that contacts has at least one element.
So OnCollisionExit does not, in any case, guarantee the presence of at least one point in col.contacts.

Answer 2

For those searching for ArrayIndexOutOfRange errors while trying to access the ContactPoint2D on a Collision2D, it seems that the API is now:
collision.getContacts(myContacts)
where you initialize and pass in an empty array myContacts. See more about this in the documentation about Collision2D.contacts which says:
The specific points of contact with the incoming Collider2D. You
should avoid using this as it produces memory garbage. Use
GetContacts instead.