JamesNK/Newtonsoft.Json · error · ArgumentNullException

type

Error message

type

What it means

CamelCasePropertyNamesContractResolver.ResolveContract throws an ArgumentNullException when its 'type' argument is null (line 66). The resolver cannot build or look up a JsonContract for an unknown type, so the very first statement guards against a null Type. This is the standard argument-validation contract for the method.

Source

Thrown at Src/Newtonsoft.Json/Serialization/CamelCasePropertyNamesContractResolver.cs:66

        public CamelCasePropertyNamesContractResolver()
        {
            NamingStrategy = new CamelCaseNamingStrategy
            {
                ProcessDictionaryKeys = true,
                OverrideSpecifiedNames = true
            };
        }

        /// <summary>
        /// Resolves the contract for a given type.
        /// </summary>
        /// <param name="type">The type to resolve a contract for.</param>
        /// <returns>The contract for a given type.</returns>
        public override JsonContract ResolveContract(Type type)
        {
            if (type == null)
            {
                throw new ArgumentNullException(nameof(type));
            }

            // for backwards compatibility the CamelCasePropertyNamesContractResolver shares contracts between instances
            StructMultiKey<Type, Type> key = new StructMultiKey<Type, Type>(GetType(), type);
            Dictionary<StructMultiKey<Type, Type>, JsonContract>? cache = _contractCache;
            if (cache == null || !cache.TryGetValue(key, out JsonContract? contract))
            {
                contract = CreateContract(type);

                // avoid the possibility of modifying the cache dictionary while another thread is accessing it
                lock (TypeContractCacheLock)
                {
                    cache = _contractCache;
                    Dictionary<StructMultiKey<Type, Type>, JsonContract> updatedCache = (cache != null)
                        ? new Dictionary<StructMultiKey<Type, Type>, JsonContract>(cache)
                        : new Dictionary<StructMultiKey<Type, Type>, JsonContract>();
                    updatedCache[key] = contract;

View on GitHub (pinned to 4f73e74372)

Solutions

1. Null-check the Type before calling ResolveContract and surface a clearer error upstream.
2. Fix the caller that produces a null Type (ensure typeof(T) / GetType() never yields null before reaching the resolver).
3. If the type genuinely depends on input, fall back to a default type (e.g. typeof(object)) instead of null.

Example fix

// before
JsonContract contract = resolver.ResolveContract(maybeNullType);

// after
if (maybeNullType == null) throw new InvalidOperationException("Type was not resolved from input.");
JsonContract contract = resolver.ResolveContract(maybeNullType);

if (type == null) throw new ArgumentNullException(nameof(type));
var contract = resolver.ResolveContract(type);

static bool IsResolvableType(Type? t) => t != null && !t.IsPointer && !t.IsByRef;

try { var contract = resolver.ResolveContract(type); }
catch (ArgumentNullException) when (type == null)
{
    // log and return a safe default contract or rethrow with context
}

When it happens

Trigger: Calling resolver.ResolveContract(null) directly, or passing a null Type that flows from reflection/serialization settings into the resolver. Also reachable when a generic serializer helper loses its type argument (e.g. typeof(T) where T is unconstrained and resolves to null at runtime through dynamic dispatch).

Common situations: Helpers like SerializeObject<T> where T is inferred as a non-type, deserialization pipelines that derive the type from JSON and pass null when $type is absent, or manually constructing the resolver and forgetting the type argument.

Related errors

• An ObjectId must be 12 bytes
• Cannot convert date value that is before Unix epoch of 00:00
• Namespace attribute must have a value.
• Delimiter must be a single or double quote.
• Value must be positive.

AI-assisted analysis of JamesNK/Newtonsoft.Json@4f73e74372 (2026-08-07). Data as JSON: /api/errors/9976fe243e20fe75. Report an issue: GitHub.