JamesNK/Newtonsoft.Json · error · ArgumentException

A name is required when setting property name state.

Error message

A name is required when setting property name state.

What it means

Thrown on the synchronous path (JsonWriter.cs) inside InternalWriteToken when writing a JsonToken.PropertyName whose value is not a string. JSON property names are required to be strings; a non-string value is rejected with ArgumentException to prevent malformed output. Identical guard to the async variant.

Solutions

1. Pass only string values to WritePropertyName.
2. Ensure PropertyName tokens in any JToken/WriteToken pipeline carry a string value.
3. Stringify non-string keys before writing (e.g. key.ToString()).

Example fix

// before
writer.WriteToken(JsonToken.PropertyName, 42);

// after
writer.WritePropertyName("42");

if (value is string name)
{
    writer.WritePropertyName(name);
}

static bool IsValidPropertyName(object value) => value is string;

When it happens

Trigger: Calling WritePropertyName with a non-string, or WriteToken/WriteTokenAsync routing a PropertyName token whose value is not a string; programmatic token construction with non-string names.

Common situations: Custom writers/token builders boxing numbers or objects as property names; dictionary serialization with non-string keys not pre-stringified; copy/paste of tokens from a non-conforming source.

Related errors

• A name is required when setting property name state.
• Accessed JObject values with invalid key value
• Argument is not a JToken.
• Array slice is not valid on
• Can not add to .

AI-assisted analysis of JamesNK/Newtonsoft.Json@4f73e74372 (2026-08-07). Data as JSON: /api/errors/44fb3acf877bcc54. Report an issue: GitHub.

Appendix: source

Thrown at Src/Newtonsoft.Json/JsonWriter.cs:1710

        /// <param name="token">The <see cref="JsonToken"/> being written.</param>
        /// <param name="value">The value being written.</param>
        protected void SetWriteState(JsonToken token, object value)
        {
            switch (token)
            {
                case JsonToken.StartObject:
                    InternalWriteStart(token, JsonContainerType.Object);
                    break;
                case JsonToken.StartArray:
                    InternalWriteStart(token, JsonContainerType.Array);
                    break;
                case JsonToken.StartConstructor:
                    InternalWriteStart(token, JsonContainerType.Constructor);
                    break;
                case JsonToken.PropertyName:
                    if (!(value is string s))
                    {
                        throw new ArgumentException("A name is required when setting property name state.", nameof(value));
                    }

                    InternalWritePropertyName(s);
                    break;
                case JsonToken.Comment:
                    InternalWriteComment();
                    break;
                case JsonToken.Raw:
                    InternalWriteRaw();
                    break;
                case JsonToken.Integer:
                case JsonToken.Float:
                case JsonToken.String:
                case JsonToken.Boolean:
                case JsonToken.Date:
                case JsonToken.Bytes:
                case JsonToken.Null:
                case JsonToken.Undefined:

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