How to Use OnCollisionEnter in Unity: Simple Collision Detection

In Unity, use the OnCollisionEnter method inside a script attached to a GameObject with a Collider and Rigidbody to detect collisions. This method is called automatically by Unity when the GameObject collides with another Collider during gameplay.

Syntax

The OnCollisionEnter method has this syntax:

• void OnCollisionEnter(Collision collision): Unity calls this method when the GameObject's Collider hits another Collider.
• Collision collision: This parameter contains information about the collision, like the other object involved.

csharp

void OnCollisionEnter(Collision collision) {
    // Code to run when collision happens
}

Example

This example shows a script that prints the name of the object it collides with. Attach this script to a GameObject with a Rigidbody and Collider to see it work.

csharp

using UnityEngine;

public class CollisionDetector : MonoBehaviour {
    void OnCollisionEnter(Collision collision) {
        Debug.Log("Collided with " + collision.gameObject.name);
    }
}

Output

Console output when collision occurs: "Collided with [OtherObjectName]"

Common Pitfalls

• Missing Rigidbody: OnCollisionEnter only works if at least one object has a Rigidbody component.
• Collider setup: Both objects must have Colliders, and they must not be set as triggers.
• Method signature: The method must be spelled exactly OnCollisionEnter with the correct parameter to be called by Unity.

csharp

/* Wrong: Missing Rigidbody, so no collision detected */
void OnCollisionEnter(Collision collision) {
    Debug.Log("Collision detected");
}

/* Right: Ensure Rigidbody is attached to this GameObject */

Quick Reference

• Attach script with OnCollisionEnter to a GameObject with Collider and Rigidbody.
• Use collision.gameObject to access the other object involved.
• Do not use IsTrigger on Colliders if you want OnCollisionEnter to fire (use OnTriggerEnter instead).

Key Takeaways

OnCollisionEnter detects collisions when a GameObject with Rigidbody and Collider hits another Collider.

The method must have the exact signature: void OnCollisionEnter(Collision collision).

At least one object involved in the collision must have a Rigidbody component.

Colliders must not be set as triggers for OnCollisionEnter to work.

Use collision.gameObject to get information about the other object in the collision.