File-based save system in Unity - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Unity C# code saving a player's score?

Consider this Unity C# code that saves a player's score to a file and then reads it back. What will be printed in the console?

Unity

using System.IO;
using UnityEngine;

public class SaveLoadExample : MonoBehaviour
{
    void Start()
    {
        string path = Application.persistentDataPath + "/score.txt";
        File.WriteAllText(path, "150");
        string scoreText = File.ReadAllText(path);
        Debug.Log($"Score loaded: {scoreText}");
    }
}

A Score loaded: 150

B Score loaded: 0

C FileNotFoundException

D Score loaded: null

💡 Hint

Think about what File.WriteAllText and File.ReadAllText do.

The code writes the string "150" to the file, then reads it back and prints it. So the output is "Score loaded: 150".

🧠 Conceptual

intermediate

Which file path is best for saving game data in Unity?

In Unity, where should you save persistent game data to ensure it works on all platforms?

A Application.persistentDataPath

B Application.streamingAssetsPath

C Application.dataPath

D Application.temporaryCachePath

💡 Hint

Look for the path meant for permanent data storage.

Application.persistentDataPath is the recommended location for saving files that should persist between app sessions on all platforms.

🔧 Debug

advanced

Why does this Unity save code throw an exception?

Look at this code snippet that tries to save player data. Why does it throw an exception?

Unity

using System.IO;
using UnityEngine;

public class SaveDebug : MonoBehaviour
{
    void SaveScore(int score)
    {
        string path = Application.persistentDataPath + "/save.txt";
        FileStream fs = new FileStream(path, FileMode.Open);
        StreamWriter writer = new StreamWriter(fs);
        writer.Write(score);
        writer.Close();
        fs.Close();
    }
}

A FileStream needs FileAccess.ReadWrite specified

B StreamWriter.Write cannot write integers

C Application.persistentDataPath is read-only

D FileMode.Open throws exception if file does not exist

💡 Hint

Check what happens if the file is missing when opening with FileMode.Open.

FileMode.Open requires the file to exist. If it doesn't, an exception is thrown. Use FileMode.Create or FileMode.OpenOrCreate to avoid this.

📝 Syntax

advanced

Which option correctly serializes and saves a PlayerData object to JSON in Unity?

Given a PlayerData class, which code snippet correctly saves it as JSON to a file?

Unity

public class PlayerData
{
    public int level;
    public float health;
}

A

string json = JsonUtility.FromJson(playerData);
File.WriteAllText(path, json);

B

string json = JsonUtility.ToJson(playerData);
File.ReadAllText(path, json);

C

string json = JsonUtility.ToJson(playerData);
File.WriteAllText(path, json);

D

string json = JsonUtility.ToString(playerData);
File.WriteAllText(path, json);

💡 Hint

Remember the method to convert an object to JSON is ToJson.

JsonUtility.ToJson converts an object to a JSON string. Then File.WriteAllText saves it to a file. Other options misuse methods or use wrong functions.

🚀 Application

expert

How many files are created after running this Unity save system code?

This code saves multiple player profiles to separate files. How many files will be created after running it?

Unity

using System.IO;
using UnityEngine;

public class MultiSave : MonoBehaviour
{
    void SaveProfiles()
    {
        for (int i = 0; i < 3; i++)
        {
            string path = Application.persistentDataPath + $"/player_{i}.json";
            string json = JsonUtility.ToJson(new PlayerData { level = i, health = 100f });
            File.WriteAllText(path, json);
        }
    }
}

public class PlayerData
{
    public int level;
    public float health;
}

A 1

B 3

C 0

D 4

💡 Hint

Count how many times File.WriteAllText is called with different file names.

The loop runs 3 times, each time creating a file named player_0.json, player_1.json, and player_2.json. So 3 files are created.

Practice

1. What is the main purpose of using a file-based save system in Unity games?

easy

A. To store game data on the device so progress is not lost

B. To improve game graphics performance

C. To increase the game's frame rate

D. To connect the game to online multiplayer servers

Solution

1. Step 1: Understand the role of saving data

   Saving data means keeping player progress or settings so they are not lost when the game closes.

2. Step 2: Identify file-based save system purpose

   A file-based save system stores this data on the device's storage, allowing the game to load it later.

3. Final Answer:

   To store game data on the device so progress is not lost -> Option A

4. Quick Check:

   Save system = store progress [OK]

Hint: Saving means keeping progress on device storage [OK]

Common Mistakes:

• Confusing saving with graphics or performance
• Thinking saving connects to online servers
• Assuming saving only affects frame rate

2. Which of the following is the correct way to write text data to a file in Unity using C#?

easy

A. File.WriteAllText(Application.persistentDataPath + "/savefile.txt", data);

B. File.ReadAllText(Application.persistentDataPath + "/savefile.txt", data);

C. File.WriteText(Application.dataPath + "/savefile.txt", data);

D. File.SaveText(Application.persistentDataPath + "/savefile.txt", data);

Solution

1. Step 1: Identify the correct method to write text

   The method to write text to a file is File.WriteAllText.

2. Step 2: Use the correct file path

   Application.persistentDataPath is the safe path for saving files on the device.

3. Final Answer:

   File.WriteAllText(Application.persistentDataPath + "/savefile.txt", data); -> Option A

4. Quick Check:

   WriteAllText + persistentDataPath = correct save [OK]

Hint: Use File.WriteAllText with persistentDataPath to save [OK]

Common Mistakes:

• Using File.ReadAllText to write data
• Using Application.dataPath which is not safe for saving
• Using non-existent methods like File.SaveText

3. What will be the output of the following code snippet if the file contains '{"score": 100}'?

string path = Application.persistentDataPath + "/savefile.json";
string json = File.ReadAllText(path);
PlayerData data = JsonUtility.FromJson<PlayerData>(json);
Debug.Log(data.score);

Assuming PlayerData is a class with a public int score field.

medium

A. null

B. 0

C. Error: File not found

D. 100

Solution

1. Step 1: Read JSON string from file

   The file contains '{"score": 100}', so json will hold this string.

2. Step 2: Deserialize JSON to PlayerData object

   JsonUtility.FromJson<PlayerData>(json) creates an object with score = 100.

3. Step 3: Output the score field

   Debug.Log(data.score) prints 100 to the console.

4. Final Answer:

   100 -> Option D

5. Quick Check:

   Deserialize JSON score = 100 [OK]

Hint: JsonUtility.FromJson reads JSON into object fields [OK]

Common Mistakes:

• Assuming score is zero if not initialized
• Confusing ReadAllText with WriteAllText
• Expecting null instead of a valid object

4. Identify the error in this code snippet for saving player data:

PlayerData data = new PlayerData();
data.score = 50;
string json = JsonUtility.ToJson(data);
File.WriteAllText("savefile.json", json);

medium

A. PlayerData.score must be private

B. Missing Application.persistentDataPath in file path

C. File.WriteAllText requires a byte array, not string

D. JsonUtility.ToJson cannot serialize PlayerData

Solution

1. Step 1: Check file path usage

   The code writes to "savefile.json" without specifying a full path, which may cause saving in an unsafe or unexpected location.

2. Step 2: Use Application.persistentDataPath for safe saving

   Unity recommends using Application.persistentDataPath + "/savefile.json" to save files safely on all platforms.

3. Final Answer:

   Missing Application.persistentDataPath in file path -> Option B

4. Quick Check:

   Always use persistentDataPath for file saving [OK]

Hint: Always include persistentDataPath in file paths [OK]

Common Mistakes:

• Saving files without full path causes errors
• Thinking JsonUtility cannot serialize simple classes
• Confusing WriteAllText input types

5. You want to save multiple player stats (score, level, health) in a file and load them later. Which approach correctly combines JSON serialization and file saving in Unity?

hard

A. Save each stat as a separate text file using File.WriteAllText without JSON

B. Serialize stats manually to a string and save using File.ReadAllText

C. Create a PlayerStats class with fields, serialize it with JsonUtility.ToJson, then save with File.WriteAllText using Application.persistentDataPath

D. Use JsonUtility.FromJson to save data and File.WriteAllText to load data

Solution

1. Step 1: Use a class to hold multiple stats

   A class like PlayerStats with fields score, level, health organizes data for easy serialization.

2. Step 2: Serialize and save correctly

   JsonUtility.ToJson converts the class to JSON string; File.WriteAllText with Application.persistentDataPath saves it safely.

3. Final Answer:

   Create a PlayerStats class with fields, serialize it with JsonUtility.ToJson, then save with File.WriteAllText using Application.persistentDataPath -> Option C

4. Quick Check:

   Class + JsonUtility + WriteAllText + persistentDataPath = correct [OK]

Hint: Use class + JsonUtility + persistentDataPath to save/load data [OK]

Common Mistakes:

• Saving stats separately without JSON causes complexity
• Mixing up ReadAllText and WriteAllText roles
• Using FromJson for saving instead of loading