JamesNK/Newtonsoft.Json · error · JsonSerializationException

Error setting value to

Error message

Error setting value to '{0}' on '{1}'.

What it means

Thrown by DynamicValueProvider.SetValue when the compiled dynamic-method setter fails while writing a value into a member (field or property) of the target object during serialization or deserialization. The '{0}' placeholder is the member name and '{1}' is the runtime type of the target. The original exception (e.g. TargetInvocationException, InvalidCastException) is preserved as the InnerException so the real cause is inspectable.

Solutions

1. Inspect ex.InnerException to find the real failure (cast, null-ref, setter validation) before changing anything.
2. Verify the JSON value type matches the target property type; add a custom JsonConverter or a property-level converter for mismatches.
3. If the property has no public setter, add one, use [JsonProperty] to map to a settable member, or use a constructor/[JsonConstructor] that accepts the value.
4. Catch JsonSerializationException around JsonConvert.DeserializeObject and log Path (ex.Path) to locate the offending JSON member.

Example fix

// before: deserializing incompatible type
public class Model { public int Age { get; set; } }
var m = JsonConvert.DeserializeObject<Model>("{\"Age\":\"twenty\"}");
// after: map via string property + conversion
public class Model {
    [JsonProperty("age")] private string _ageRaw;
    public int Age => int.Parse(_ageRaw);
}

if (value == null || !targetType.GetProperty(memberName)?.PropertyType.IsAssignableFrom(value.GetType()) == true) throw new ArgumentException("incompatible");

// guard the value type before serialize
static bool CanAssign(Type memberType, object value) => value == null ? !memberType.IsValueType || Nullable.GetUnderlyingType(memberType) != null : memberType.IsAssignableFrom(value.GetType());

try {
    var obj = JsonConvert.DeserializeObject<T>(json, settings);
} catch (JsonSerializationException ex) when (ex.Message.Contains("Error setting value")) {
    logger.Error(ex, "Deser failed at {Path}: {Inner}", ex.Path, ex.InnerException?.Message);
    throw;
}

When it happens

Trigger: Deserializing JSON into a property whose setter throws (e.g. a property with validation logic in the setter, a read-only auto-property with no setter, a type mismatch where the deserialized value cannot be assigned to the member type), or serializing where a getter is invoked but SetValue is used to populate during deserialization. Common during DeserializeObject<T> when the JSON contains a value incompatible with the target member type.

Common situations: Passing a JSON string into an int property, deserializing into a struct/record with init-only setters that the dynamic provider cannot bind, a property setter that throws ArgumentException on invalid input, value/type mismatches after a schema change, or a member that was renamed on the C# side but the JSON still carries the old name/type.

Related errors

• Error getting value from
• Error setting value to
• Could not create getter for
• Could not create getter for
• Could not get constructor for

AI-assisted analysis of JamesNK/Newtonsoft.Json@4f73e74372 (2026-08-07). Data as JSON: /api/errors/ddfd2c9700f3f85c. Report an issue: GitHub.

Appendix: source

Thrown at Src/Newtonsoft.Json/Serialization/DynamicValueProvider.cs:87

                if (_setter == null)
                {
                    _setter = DynamicReflectionDelegateFactory.Instance.CreateSet<object>(_memberInfo);
                }

#if DEBUG
                // dynamic method doesn't check whether the type is 'legal' to set
                // add this check for unit tests
                if (value != null && !ReflectionUtils.GetMemberUnderlyingType(_memberInfo).IsAssignableFrom(value.GetType()))
                {
                    throw new JsonSerializationException("Incompatible value. Cannot set {0} to type {1}.".FormatWith(CultureInfo.InvariantCulture, _memberInfo, value.GetType()));
                }
#endif

                _setter(target, value);
            }
            catch (Exception ex)
            {
                throw new JsonSerializationException("Error setting value to '{0}' on '{1}'.".FormatWith(CultureInfo.InvariantCulture, _memberInfo.Name, target.GetType()), ex);
            }
        }

        /// <summary>
        /// Gets the value.
        /// </summary>
        /// <param name="target">The target to get the value from.</param>
        /// <returns>The value.</returns>
        public object? GetValue(object target)
        {
            try
            {
                if (_getter == null)
                {
                    _getter = DynamicReflectionDelegateFactory.Instance.CreateGet<object>(_memberInfo);
                }

                return _getter(target);

View on GitHub (pinned to 4f73e74372)