JamesNK/Newtonsoft.Json · error · JsonException

Could not determine new value to add to '{0}'.

Error message

Could not determine new value to add to '{0}'.

What it means

Thrown by IBindingList.AddNew on a JContainer when AddingNewEventArgs.NewObject is null after raising the AddingNew event. AddNew expects an event handler to supply the new object; if no handler is attached or the handler leaves NewObject null, Newtonsoft.Json cannot construct a value and throws JsonException.

Source

Thrown at Src/Newtonsoft.Json/Linq/JContainer.cs:1132

                return _syncRoot;
            }
        }
        #endregion

        #region IBindingList Members
#if HAVE_COMPONENT_MODEL
        void IBindingList.AddIndex(PropertyDescriptor property)
        {
        }

        object IBindingList.AddNew()
        {
            AddingNewEventArgs args = new AddingNewEventArgs();
            OnAddingNew(args);

            if (args.NewObject == null)
            {
                throw new JsonException("Could not determine new value to add to '{0}'.".FormatWith(CultureInfo.InvariantCulture, GetType()));
            }

            if (!(args.NewObject is JToken newItem))
            {
                throw new JsonException("New item to be added to collection must be compatible with {0}.".FormatWith(CultureInfo.InvariantCulture, typeof(JToken)));
            }

            Add(newItem);

            return newItem;
        }

        bool IBindingList.AllowEdit => true;

        bool IBindingList.AllowNew => true;

        bool IBindingList.AllowRemove => true;

View on GitHub (pinned to 4f73e74372)

Solutions

1. Subscribe to the container's AddingNew event and set e.NewObject to a new JToken (e.g. new JObject() or new JValue(0)).
2. Do not rely on IBindingList.AddNew for JContainer; create and Add the JToken explicitly.
3. If binding through BindingSource, handle its AddingNew event instead and supply the object.

Example fix

// before
((IBindingList)jArray).AddNew(); // JsonException: cannot determine new value

// after
jArray.AddingNew += (s, e) => e.NewObject = new JObject();
((IBindingList)jArray).AddNew();

// Ensure an AddingNew handler always supplies a JToken before enabling AddNew.
jArray.AddingNew += (s, e) =>
{
    e.NewObject ??= new JObject();
};

try { ((IBindingList)container).AddNew(); }
catch (JsonException ex) when (ex.Message.Contains("Could not determine new value"))
{
    var token = new JObject();
    container.Add(token);
}

When it happens

Trigger: Calling ((IBindingList)container).AddNew() (directly or via BindingSource.AddNew on a JObject/JArray) without subscribing to the AddingNew event, or with a handler that does not set e.NewObject to a JToken instance.

Common situations: Using a JContainer as a bindable list in WinForms/WPF and relying on default 'add new' behavior; a BindingSource whose AddingNew handler was removed or never wired; UI that allows the user to add rows.

Related errors

• New item to be added to collection must be compatible with {
• Newtonsoft.Json support for System.ComponentModel is not com
• Argument is not a JToken.
• Unexpected merge array handling when merging JSON.
• Can not add {0} to {1}.

AI-assisted analysis of JamesNK/Newtonsoft.Json@4f73e74372 (2026-08-07). Data as JSON: /api/errors/8b0ffd1499e8f8d3. Report an issue: GitHub.