1

I made a simple wrapper class for an array to be serialized into json.

[System.Serializable] public class ItemWrapper { public Item[] items; }

Then I serialize and deserialize the class. Why do I only get an error (JSON parse error: Invalid value) when I try reading it?

            ItemWrapper wrapper = new ItemWrapper();
            Item[] oldItems = new Item[0];
            oldItems = JsonUtility.FromJson<ItemWrapper>("Assets/items.json").items; //This is where I get the error
            wrapper.items = oldItems.Union(new Item[] { newItem }).ToArray();
            File.WriteAllText("Assets/items.json", JsonUtility.ToJson(wrapper, true));

I also made sure that the Item class is serializable.

2

The JsonUtility.FromJson method is expecting a json string, you have given it a path to a json file,

Read the contents of the file into a string and then pass that string into that method.