# contactId

> The unique Id of the contact. This contact is volatile and may be destroyed automatically when the world is modified or simulated therefore it should always be checked for validity with PhysicsShape.ContactId.isValid.

## Definition

* **Type:** Property
* **Namespace:** [UnityEngine.LowLevelPhysics2D](/engine/6000.3/script-reference/unityengine/lowlevelphysics2d.md)
* **Assembly:** UnityEngine.Physics2DModule

```csharp
public PhysicsShape.ContactId contactId { get; }
```