What causes "Tried to read {} json value as list" in Unity-Technologies/UnityCsReference?

Check IsList() before calling AsList(), or use AsList(true) which returns null on mismatch.

Unity-Technologies/UnityCsReference · error · JSONTypeException

Tried to read {} json value as list

Error message

Tried to read {} json value as list

What it means

JSONValue.AsList(bool nothrow) returns the value only when it is a List<JSONValue>; otherwise it builds a message of the form "Tried to read <ActualType> json value as list" using data.GetType().Name, so the placeholder in the message reflects the real wrapped CLR type (e.g. String, Dictionary`2, Single). The parameterless AsList() throws via the nothrow=false path. This fires whenever code indexes into something that the JSON modeled as an object or scalar rather than an array.

Source

Thrown at Editor/Mono/AssetStore/Json.cs:121

        {
            if (data is bool)
                return (bool)data;
            if (!nothrow)
                throw new JSONTypeException("Tried to read non-bool json value as bool");
            return false;
        }

        public bool AsBool()
        {
            return AsBool(false);
        }

        public List<JSONValue> AsList(bool nothrow)
        {
            if (data is List<JSONValue>)
                return (List<JSONValue>)data;
            if (!nothrow)
                throw new JSONTypeException("Tried to read " + data.GetType().Name + " json value as list");
            return null;
        }

        public List<JSONValue> AsList()
        {
            return AsList(false);
        }

        public Dictionary<string, JSONValue> AsDict(bool nothrow)
        {
            if (data is Dictionary<string, JSONValue>)
                return (Dictionary<string, JSONValue>)data;
            if (!nothrow)
                throw new JSONTypeException("Tried to read non-dictionary json value as dictionary");
            return null;
        }

        public Dictionary<string, JSONValue> AsDict()

View on GitHub (pinned to 225b0fbdb5)

Solutions

Check IsList() before calling AsList(), or use AsList(true) which returns null on mismatch.
Handle the single-object-vs-array variants explicitly by testing the wrapped type.
Confirm the field is present and is an array in the schema before parsing.

Example fix

// before
foreach (var item in node["items"].AsList()) { ... } // throws if items is an object

// after
var items = node["items"].AsList(true) ?? new List<JSONValue>();
// or, when a single object should be treated as a 1-element list:
if (!node["items"].IsList() && node["items"].IsDict())
    items = new List<JSONValue> { node["items"] };

Defensive patterns

Strategy: type-guard

Validate before calling

List<JSONValue> ReadList(JSONValue v) {
    var list = v.AsList(true);
    if (list != null) return list;
    if (v.IsDict()) return new List<JSONValue> { v }; // tolerate single-object form
    return new List<JSONValue>();
}

Type guard

static List<JSONValue> AsListOrEmpty(JSONValue v) => v.AsList(true) ?? new List<JSONValue>();

Try / catch

List<JSONValue> arr;
try { arr = v.AsList(); } catch (JSONTypeException) { arr = new List<JSONValue>(); }

Prevention

Handle the single-object-vs-array variants explicitly for fields known to collapse.
Use AsList(true) and null-check instead of letting the throw propagate.
Confirm array fields are present and typed as arrays in the schema.

When it happens

Trigger: Calling AsList() on a JSON object or scalar (e.g. node["items"].AsList() when "items" is an object keyed by id, or when the server omitted the array and returned null); iterating a field that is sometimes an array and sometimes a single object.

Common situations: API collapses a one-element array to a single object; missing field serialized as null; field renamed/repurposed.

Related errors

AI-assisted analysis of Unity-Technologies/UnityCsReference@225b0fbdb5 (2026-08-13). Data as JSON: /api/errors/0a907f3050aade0f. Report an issue: GitHub.