Why Are My Unity Class Instances Null Despite Proper Parameter Assignment?

Troubleshooting Null Class Instances in Unity

Unity developers often encounter null class instances, even with seemingly correct parameter assignments. This usually stems from the unique behavior of classes inheriting from MonoBehaviour.

The MonoBehaviour Conundrum

Scripts inheriting from MonoBehaviour become Unity components, subject to its lifecycle management. This means directly using the new keyword to create instances is incorrect. Unity mandates using specific methods for instantiation.

Correct Instantiation: GameObject.AddComponent()

For a MonoBehaviour-derived class like 'Rule', instantiation should leverage GameObject.AddComponent():

Rule rule2 = null;
void Start() {
    rule2 = gameObject.AddComponent<Rule>();
}

This ensures proper integration with Unity's system.

Instantiating Non-MonoBehaviour Classes

If 'Rule' doesn't inherit from MonoBehaviour, the new keyword is acceptable:

Rule rule2 = null;
void Start() {
    rule2 = new Rule(3);
}

Here, direct instantiation and parameter assignment work as expected.

Callback Function Considerations

Older Unity versions exhibited subtle issues when using new with MonoBehaviour-derived classes; callback functions like Start() might not trigger correctly. While this might be less apparent in newer versions, best practices still dictate using the correct instantiation methods.

Key Takeaway

Proper instantiation hinges on whether your class inherits from MonoBehaviour. Using GameObject.AddComponent() for MonoBehaviour-derived classes and new for others prevents null reference errors and ensures correct script behavior within Unity.

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