Why DontDestroyOnLoad usage in Unity? - Purpose & Use Cases

The Big Idea

What if your game's music and score could magically survive every scene change without extra code?

The Scenario

Imagine you are making a game where the player's score or music should keep playing even when moving between different game levels or scenes.

Without special handling, every time you load a new scene, all objects from the previous scene disappear, and you lose the score or music.

The Problem

Manually copying data or restarting music every time a new scene loads is slow and error-prone.

You might forget to save or restore something, causing bugs or a bad player experience.

The Solution

DontDestroyOnLoad lets you mark important game objects so they stay alive and keep working across scene changes automatically.

This means your music keeps playing and your score stays intact without extra work.

Before vs After

✗ Before

SceneManager.LoadScene("Level2"); // music stops, score resets

✓ After

DontDestroyOnLoad(gameObject); // music and score persist across scenes

What It Enables

You can create smooth, continuous gameplay experiences where key objects survive scene changes effortlessly.

Real Life Example

In a game, the background music player object uses DontDestroyOnLoad so the music never stops even when the player moves from the main menu to the game level.

Key Takeaways

Scene changes normally destroy all objects, losing data or effects.

DontDestroyOnLoad keeps marked objects alive across scenes.

This makes managing persistent data or effects easy and reliable.

Practice

1. What is the main purpose of using DontDestroyOnLoad in Unity?

easy

A. To pause the game during scene loading

B. To destroy a GameObject immediately

C. To load a new scene asynchronously

D. To keep a GameObject alive when switching scenes

Solution

1. Step 1: Understand what DontDestroyOnLoad does

   This method prevents a GameObject from being destroyed when a new scene loads.

2. Step 2: Identify the purpose in scene management

   It keeps important objects like music players or managers alive across scenes.

3. Final Answer:

   To keep a GameObject alive when switching scenes -> Option D

4. Quick Check:

   DontDestroyOnLoad keeps objects alive = A [OK]

Hint: Remember: DontDestroyOnLoad means object stays between scenes [OK]

Common Mistakes:

• Thinking it destroys objects instead of preserving them
• Confusing it with scene loading functions
• Assuming it pauses the game

2. Which of the following is the correct way to use DontDestroyOnLoad in a Unity script?

easy

A. DontDestroyOnLoad(this);

B. DontDestroyOnLoad(Scene);

C. DontDestroyOnLoad(gameObject);

D. DontDestroyOnLoad(SceneManager);

Solution

1. Step 1: Identify the correct parameter for DontDestroyOnLoad

   The method requires a GameObject to keep alive, usually referenced as gameObject in scripts.

2. Step 2: Check each option

   DontDestroyOnLoad(gameObject); uses gameObject, which is correct. DontDestroyOnLoad(this); uses this (the script), which is incorrect. Options B and C use unrelated types.

3. Final Answer:

   DontDestroyOnLoad(gameObject); -> Option C

4. Quick Check:

   Use gameObject with DontDestroyOnLoad = D [OK]

Hint: Use gameObject, not this or Scene, with DontDestroyOnLoad [OK]

Common Mistakes:

• Passing 'this' instead of 'gameObject'
• Trying to pass Scene or SceneManager objects
• Calling DontDestroyOnLoad without parameters

3. Consider this Unity C# script attached to a GameObject:

void Awake() {
    DontDestroyOnLoad(gameObject);
}

void Start() {
    Debug.Log("Object alive");
}

What will happen when the scene changes?

medium

A. The GameObject will persist and "Object alive" will print once

B. The GameObject will be destroyed and "Object alive" will not print again

C. The GameObject will persist and "Object alive" will print every scene load

D. The GameObject will be duplicated on each scene load

Solution

1. Step 1: Understand when Awake and Start run

   Awake runs once when the object is created. Start runs once after Awake. Both run only once per object lifetime.

2. Step 2: Effect of DontDestroyOnLoad on scene change

   The GameObject is kept alive across scenes, so Awake and Start do not run again on scene load.

3. Final Answer:

   The GameObject will persist and "Object alive" will print once -> Option A

4. Quick Check:

   DontDestroyOnLoad keeps object alive, Awake/Start run once = A [OK]

Hint: Awake/Start run once; object stays alive with DontDestroyOnLoad [OK]

Common Mistakes:

• Thinking Start runs on every scene load
• Assuming object duplicates on scene change
• Believing object is destroyed on scene change

4. You wrote this code in a Unity script:

void Start() {
    DontDestroyOnLoad(this);
}

What is the problem and how to fix it?

medium

A. Error: DontDestroyOnLoad must be called in Awake, not Start

B. Error: 'this' is not a GameObject; fix by using DontDestroyOnLoad(gameObject);

C. No problem; code works fine as is

D. Error: DontDestroyOnLoad requires a Scene parameter

Solution

1. Step 1: Identify the parameter type for DontDestroyOnLoad

   DontDestroyOnLoad requires a GameObject, but 'this' refers to the script instance, not the GameObject.

2. Step 2: Correct the parameter

   Use gameObject to pass the GameObject the script is attached to.

3. Final Answer:

   Error: 'this' is not a GameObject; fix by using DontDestroyOnLoad(gameObject); -> Option B

4. Quick Check:

   DontDestroyOnLoad needs gameObject, not this = C [OK]

Hint: Pass gameObject, not this, to DontDestroyOnLoad [OK]

Common Mistakes:

• Passing 'this' instead of 'gameObject'
• Thinking DontDestroyOnLoad must be in Awake only
• Passing wrong types like Scene or SceneManager

5. You want to keep a music player GameObject alive across scenes but avoid duplicates if the scene reloads. Which code snippet correctly implements this using DontDestroyOnLoad?

hard

A. void Awake() {
if (FindObjectsOfType<MusicPlayer>().Length > 1) {
Destroy(gameObject);
} else {
DontDestroyOnLoad(gameObject);
}
}

B. void Awake() {
DontDestroyOnLoad(gameObject);
}

C. void Start() {
Destroy(gameObject);
DontDestroyOnLoad(gameObject);
}

D. void Awake() {
Destroy(gameObject);
}

Solution

1. Step 1: Understand the problem of duplicates

   Using DontDestroyOnLoad alone keeps the object alive, but reloading the scene creates duplicates.

2. Step 2: Use a check to destroy duplicates

   void Awake() { if (FindObjectsOfType<MusicPlayer>().Length > 1) { Destroy(gameObject); } else { DontDestroyOnLoad(gameObject); } } checks if more than one MusicPlayer exists and destroys the extra one, preventing duplicates.

3. Step 3: Verify other options

   void Awake() { DontDestroyOnLoad(gameObject); } keeps duplicates. void Start() { Destroy(gameObject); DontDestroyOnLoad(gameObject); } destroys then calls DontDestroyOnLoad (wrong order). void Awake() { Destroy(gameObject); } destroys always.

4. Final Answer:

   void Awake() { if (FindObjectsOfType<MusicPlayer>().Length > 1) { Destroy(gameObject); } else { DontDestroyOnLoad(gameObject); } } -> Option A

5. Quick Check:

   Check duplicates before DontDestroyOnLoad to avoid duplicates = B [OK]

Hint: Check existing objects before DontDestroyOnLoad to prevent duplicates [OK]

Common Mistakes:

• Not checking for duplicates causing multiple music players
• Destroying object before calling DontDestroyOnLoad
• Calling DontDestroyOnLoad without duplicate check