JamesNK/Newtonsoft.Json · error · JsonException

Multiple constructors with the JsonConstructorAttribute.

Error message

Multiple constructors with the JsonConstructorAttribute.

What it means

Only one constructor per type may carry [JsonConstructor]. GetAttributeConstructor enumerates constructors marked with JsonConstructorAttribute and throws a JsonException (line 617) if a second one is found. The attribute tells Json.NET which constructor to use for deserialization, so an ambiguous choice is rejected at contract build time.

Source

Thrown at Src/Newtonsoft.Json/Serialization/DefaultContractResolver.cs:617

                }
            }

            IEnumerator IEnumerable.GetEnumerator()
            {
                return GetEnumerator();
            }
        }

        private ConstructorInfo? GetAttributeConstructor(Type objectType)
        {
            IEnumerator<ConstructorInfo> en = objectType.GetConstructors(BindingFlags.Instance | BindingFlags.Public | BindingFlags.NonPublic).Where(c => c.IsDefined(typeof(JsonConstructorAttribute), true)).GetEnumerator();

            if (en.MoveNext())
            {
                ConstructorInfo conInfo = en.Current;
                if (en.MoveNext())
                {
                    throw new JsonException("Multiple constructors with the JsonConstructorAttribute.");
                }

                return conInfo;
            }

            // little hack to get Version objects to deserialize correctly
            if (objectType == typeof(Version))
            {
                return objectType.GetConstructor(new[] { typeof(int), typeof(int), typeof(int), typeof(int) });
            }

            return null;
        }

        private ConstructorInfo? GetImmutableConstructor(Type objectType, JsonPropertyCollection memberProperties)
        {
            IEnumerable<ConstructorInfo> constructors = objectType.GetConstructors();
            IEnumerator<ConstructorInfo> en = constructors.GetEnumerator();

View on GitHub (pinned to 4f73e74372)

Solutions

  1. Remove [JsonConstructor] from all but one constructor.
  2. If you need a specific deserialization path, keep the single most appropriate constructor and restructure the others without the attribute.

Example fix

// before
public class Widget {
    [JsonConstructor] public Widget(int a) {}
    [JsonConstructor] public Widget(int a, int b) {} // throws
}

// after
public class Widget {
    public Widget(int a) {}
    [JsonConstructor] public Widget(int a, int b) {}
}
Defensive patterns

Strategy: validation

Validate before calling

// Fail fast in a unit test if more than one constructor is marked [JsonConstructor].
var tagged = typeof(T).GetConstructors(BindingFlags.Instance | BindingFlags.Public | BindingFlags.NonPublic)
    .Where(c => c.IsDefined(typeof(JsonConstructorAttribute), true)).ToList();
if (tagged.Count > 1)
    throw new InvalidOperationException("Multiple [JsonConstructor] constructors on " + typeof(T));

Type guard

static bool HasSingleJsonConstructor(Type t) =>
    t.GetConstructors(BindingFlags.Instance | BindingFlags.Public | BindingFlags.NonPublic)
     .Count(c => c.IsDefined(typeof(JsonConstructorAttribute), true)) == 1;

Try / catch

try { JsonConvert.DeserializeObject<T>(json); }
catch (JsonException ex) when (ex.Message.Contains("Multiple constructors"))
{
    // remove [JsonConstructor] from all but one constructor
}

Prevention

When it happens

Trigger: Decorating two (or more) constructors of the same class with [JsonConstructor], e.g. both a parameterized and a default constructor.

Common situations: Adding [JsonConstructor] to a new constructor while forgetting to remove it from the old one, or merging two classes during refactoring.

Related errors

AI-assisted analysis of JamesNK/Newtonsoft.Json@4f73e74372 (2026-08-07). Data as JSON: /api/errors/255edadd5d34e8a2. Report an issue: GitHub.