{"record":{"id":"4911131a4ccc19a2","repo":"JamesNK/Newtonsoft.Json","slug":"could-not-get-constructor-for-0","errorCode":null,"errorMessage":"Could not get constructor for {0}.","messagePattern":"Could not get constructor for (.+?)\\.","errorType":"validation","errorClass":"ArgumentException","httpStatus":null,"severity":"error","filePath":"Src/Newtonsoft.Json/Utilities/DynamicReflectionDelegateFactory.cs","lineNumber":287,"sourceCode":"            if (type.IsValueType())\n            {\n                generator.DeclareLocal(type);\n                generator.Emit(OpCodes.Ldloc_0);\n\n                // only need to box if the delegate isn't returning the value type\n                if (type != delegateType)\n                {\n                    generator.Emit(OpCodes.Box, type);\n                }\n            }\n            else\n            {\n                ConstructorInfo? constructorInfo =\n                    type.GetConstructor(BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance, null, ReflectionUtils.EmptyTypes, null);\n\n                if (constructorInfo == null)\n                {\n                    throw new ArgumentException(\"Could not get constructor for {0}.\".FormatWith(CultureInfo.InvariantCulture, type));\n                }\n\n                generator.Emit(OpCodes.Newobj, constructorInfo);\n            }\n\n            generator.Return();\n        }\n\n        [RequiresDynamicCode(MiscellaneousUtils.AotWarning)]\n        public override Func<T, object?> CreateGet<T>(PropertyInfo propertyInfo)\n        {\n            DynamicMethod dynamicMethod = CreateDynamicMethod(\"Get\" + propertyInfo.Name, typeof(object), new[] { typeof(T) }, propertyInfo.DeclaringType!);\n            ILGenerator generator = dynamicMethod.GetILGenerator();\n\n            GenerateCreateGetPropertyIL(propertyInfo, generator);\n\n            return (Func<T, object?>)dynamicMethod.CreateDelegate(typeof(Func<T, object?>));\n        }","sourceCodeStart":269,"sourceCodeEnd":305,"githubUrl":"https://github.com/JamesNK/Newtonsoft.Json/blob/4f73e74372445108d2c1bda37b36e6f5e43402e0/Src/Newtonsoft.Json/Utilities/DynamicReflectionDelegateFactory.cs#L269-L305","documentation":"GenerateCreateDefaultConstructorIL (DynamicReflectionDelegateFactory.cs:263-294) builds a delegate that news up an instance of `type`. For reference types it resolves a parameterless constructor (public or non-public) via GetConstructor; if none exists it throws ArgumentException at DynamicReflectionDelegateFactory.cs:287. Value types don't hit this (they use InitObj).","triggerScenarios":"The serializer tries to create an instance of a reference type that has no parameterless constructor, and no [JsonConstructor]/ObjectConstructor has been configured to use a parameterized one.","commonSituations":"DTOs with only parameterized constructors; immutable record-like types; classes whose only ctor is non-public and accessibility settings exclude it; AOT/trimmed scenarios where the ctor was stripped.","solutions":["Add a parameterless constructor to the type (public or accessible non-public).","Annotate the constructor you want used with [JsonConstructor].","Register a custom JsonConverter whose ReadJson constructs the type from its arguments.","Configure an IContractResolver that supplies a custom ObjectFactory/default-creator.","If trimming/AOT, ensure the constructor is preserved via DynamicallyAccessedMembers."],"exampleFix":"// before\npublic class Foo { public Foo(int x) { X = x; } public int X { get; } }\n// after\npublic class Foo {\n    [JsonConstructor] public Foo(int x) { X = x; }\n    public Foo() {}\n    public int X { get; set; }\n}","handlingStrategy":"validation","validationCode":"// Confirm a parameterless constructor (or [JsonConstructor]) exists before serializing.\nstatic bool HasDefaultCtor(Type t) {\n    if (t.IsValueType) return true;\n    return t.GetConstructor(System.Reflection.BindingFlags.Public | System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Instance, null, Type.EmptyTypes, null) != null;\n}","typeGuard":"static bool NeedsCtorHelp(Type t) => !t.IsAbstract && !HasDefaultCtor(t) && !t.GetConstructors().Any(c => c.GetCustomAttribute<JsonConstructorAttribute>() != null);","tryCatchPattern":"try { JsonConvert.DeserializeObject<T>(json); } catch (ArgumentException ex) when (ex.Message.Contains(\"Could not get constructor\")) { /* add [JsonConstructor] or a parameterless ctor */ }","preventionTips":["Ensure reference types have a parameterless constructor or a [JsonConstructor].","Register a custom JsonConverter for immutable types.","For trimmed/AOT builds, preserve constructors via DynamicallyAccessedMembers."],"tags":["reflection","serialization","constructor","newtonsoft-json"],"analyzedSha":"4f73e74372445108d2c1bda37b36e6f5e43402e0","analyzedAt":"2026-08-07T06:10:08.596Z","schemaVersion":2},"datasetVersion":"2026-08-07T07:17:06.508Z"}