JamesNK/Newtonsoft.Json · error · JsonSerializationException

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

Error message

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

What it means

Thrown by ReflectionValueProvider.SetValue when ReflectionUtils.SetMemberValue throws while assigning a value to a member (field or property) of the target. '{0}' is the member name, '{1}' is the target runtime type; the original exception is the InnerException. ReflectionValueProvider is the fallback provider used when neither dynamic-method nor expression providers are available (e.g. limited trust / restricted reflection).

Source

Thrown at Src/Newtonsoft.Json/Serialization/ReflectionValueProvider.cs:63

        {
            ValidationUtils.ArgumentNotNull(memberInfo, nameof(memberInfo));
            _memberInfo = memberInfo;
        }

        /// <summary>
        /// Sets the value.
        /// </summary>
        /// <param name="target">The target to set the value on.</param>
        /// <param name="value">The value to set on the target.</param>
        public void SetValue(object target, object? value)
        {
            try
            {
                ReflectionUtils.SetMemberValue(_memberInfo, 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
            {
                // https://github.com/dotnet/corefx/issues/26053
                if (_memberInfo is PropertyInfo propertyInfo && propertyInfo.PropertyType.IsByRef)
                {
                    throw new InvalidOperationException("Could not create getter for {0}. ByRef return values are not supported.".FormatWith(CultureInfo.InvariantCulture, propertyInfo));
                }

View on GitHub (pinned to 4f73e74372)

Solutions

1. Inspect InnerException to identify the underlying setter/reflection failure.
2. Ensure the member has an accessible setter and the JSON value type is assignable to it.
3. Use a parameterized constructor/[JsonConstructor] to supply read-only members.
4. Register a custom JsonConverter to coerce the value to the correct type before assignment.

Example fix

// before: readonly field cannot be set via reflection
public class Model { public readonly int Id; }
JsonConvert.DeserializeObject<Model>("{\"Id\":5}");
// after: settable property or constructor
public class Model {
    public Model(int id) { Id = id; }
    public int Id { get; }
}

var prop = target.GetType().GetProperty(memberName); if (prop != null && !prop.CanWrite) throw new InvalidOperationException(memberName + " is read-only");

try { JsonConvert.DeserializeObject<T>(json); }
catch (JsonSerializationException ex) when (ex.Message.Contains("Error setting value")) {
    logger.Error(ex.InnerException, "reflection setter failed at {Path}", ex.Path); throw;
}

When it happens

Trigger: Deserializing JSON into a member via plain reflection where SetMemberValue fails: type mismatch, setter throws ArgumentException, attempt to set a const/read-only/initializer-only field, value is incompatible with the member's underlying type.

Common situations: Running under partial trust or restricted reflection where DynamicValueProvider/ExpressionValueProvider are unavailable; setting a readonly field; assigning a derived type to a property whose setter validates the value; version where ReflectionValueProvider is the active provider.

Related errors

• Property does not have a setter.
• Error setting value to '{0}' on '{1}'.
• Error setting value to '{0}' on '{1}'.
• Could not create getter for {0}. ByRef return values are not
• Error getting value from '{0}' on '{1}'.

AI-assisted analysis of JamesNK/Newtonsoft.Json@4f73e74372 (2026-08-07). Data as JSON: /api/errors/52f62c9180386797. Report an issue: GitHub.