JSON net how to deserialize without using the default constructor

Question

I have a class that has a default constructor and also an overloaded constructor that takes in a set of parameters   These parameters match to fields on the object and are assigned on construction   At this point i need the default constructor for other purposes so i would like to keep it if i can  My Problem   If I remove the default constructor and pass in the JSON string  the object deserializes correctly and passes in the constructor parameters without any issues  I end up getting back the object populated the way I would expect   However  as soon as I add the default constructor into the object  when i call JsonConvert DeserializeObject lt Result gt  jsontext  the properties are no longer populated  At this point I have tried adding new JsonSerializerSettings   CheckAdditionalContent   true  to the deserialization call   That did not do anything  Another note   the constructor parameters do match the names of the fields exactly except that the parameters are start with a lowercase letter   I wouldn t think this would matter since  like i mentioned  the deserialization works fine with no default constructor  Here is a sample of my constructors  public Result        public Result int  code  string format  Dictionary lt string  string gt  details   null        Code   code    ERROR CODE      Format   format       if  details    null          Details   new Dictionary lt string  string gt         else         Details   details

User · Answer

Solution   public Response Get string jsonData        var json   JsonConvert DeserializeObject lt modelname gt  jsonData       var data   StoredProcedure procedureName json Parameter  json Parameter  json Parameter  json Parameter       return data      Model   public class modelname       public long parameter  get  set        public int parameter  get  set        public int parameter  get  set        public string parameter  get  set

User · Answer

A bit late and not exactly suited here  but I m gonna add my solution here  because my question had been closed as a duplicate of this one  and because this solution is completely different    I needed a general way to instruct Json NET to prefer the most specific constructor for a user defined struct type  so I can omit the JsonConstructor attributes which would add a dependency to the project where each such struct is defined   I ve reverse engineered a bit and implemented a custom contract resolver where I ve overridden the CreateObjectContract method to add my custom creation logic   public class CustomContractResolver   DefaultContractResolver        protected override JsonObjectContract CreateObjectContract Type objectType                var c   base CreateObjectContract objectType           if   IsCustomStruct objectType   return c           IList lt ConstructorInfo gt  list   objectType GetConstructors BindingFlags Instance   BindingFlags Public   BindingFlags NonPublic  OrderBy e   gt  e GetParameters   Length  ToList            var mostSpecific   list LastOrDefault            if  mostSpecific    null                        c OverrideCreator   CreateParameterizedConstructor mostSpecific               c CreatorParameters AddRange CreateConstructorParameters mostSpecific  c Properties                       return c             protected virtual bool IsCustomStruct Type objectType                return objectType IsValueType  amp  amp   objectType IsPrimitive  amp  amp   objectType IsEnum  amp  amp   objectType Namespace IsNullOrEmpty    amp  amp   objectType Namespace StartsWith  System                private ObjectConstructor lt object gt  CreateParameterizedConstructor MethodBase method                method ThrowIfNull  method            var c   method as ConstructorInfo          if  c    null              return a   gt  c Invoke a           return a   gt  method Invoke null  a             I m using it like this   public struct Test     public readonly int A    public readonly string B     public Test int a  string b        A   a      B   b         var json   JsonConvert SerializeObject new Test 1   Test    new JsonSerializerSettings     ContractResolver   new CustomContractResolver       var t   JsonConvert DeserializeObject lt Test gt  json   t A ShouldEqual 1   t B ShouldEqual  Test

User · Answer

Based on some of the answers here  I have written a CustomConstructorResolver for use in a current project  and I thought it might help somebody else   It supports the following resolution mechanisms  all configurable    Select a single private constructor so you can define one private constructor without having to mark it with an attribute  Select the most specific private constructor so you can have multiple overloads  still without having to use attributes  Select the constructor marked with an attribute of a specific name - like the default resolver  but without a dependency on the Json Net package because you need to reference Newtonsoft Json JsonConstructorAttribute    public class CustomConstructorResolver   DefaultContractResolver       public string ConstructorAttributeName   get  set       JsonConstructorAttribute       public bool IgnoreAttributeConstructor   get  set      false      public bool IgnoreSinglePrivateConstructor   get  set      false      public bool IgnoreMostSpecificConstructor   get  set      false       protected override JsonObjectContract CreateObjectContract Type objectType                var contract   base CreateObjectContract objectType               Use default contract for non-object types          if  objectType IsPrimitive    objectType IsEnum  return contract              Look for constructor with attribute first  then single private  then most specific          var overrideConstructor                    this IgnoreAttributeConstructor   null   GetAttributeConstructor objectType                    this IgnoreSinglePrivateConstructor   null   GetSinglePrivateConstructor objectType                    this IgnoreMostSpecificConstructor   null   GetMostSpecificConstructor objectType                Set override constructor if found  otherwise use default contract          if  overrideConstructor    null                        SetOverrideCreator contract  overrideConstructor                      return contract             private void SetOverrideCreator JsonObjectContract contract  ConstructorInfo attributeConstructor                contract OverrideCreator   CreateParameterizedConstructor attributeConstructor           contract CreatorParameters Clear            foreach  var constructorParameter in base CreateConstructorParameters attributeConstructor  contract Properties                         contract CreatorParameters Add constructorParameter                        private ObjectConstructor lt object gt  CreateParameterizedConstructor MethodBase method                var c   method as ConstructorInfo          if  c    null              return a   gt  c Invoke a           return a   gt  method Invoke null  a              protected virtual ConstructorInfo GetAttributeConstructor Type objectType                var constructors   objectType              GetConstructors BindingFlags Instance   BindingFlags Public   BindingFlags NonPublic               Where c   gt  c GetCustomAttributes   Any a   gt  a GetType   Name    this ConstructorAttributeName   ToList             if  constructors Count    1  return constructors 0           if  constructors Count  gt  1              throw new JsonException   Multiple constructors with a  this ConstructorAttributeName               return null             protected virtual ConstructorInfo GetSinglePrivateConstructor Type objectType                var constructors   objectType              GetConstructors BindingFlags Instance   BindingFlags NonPublic            return constructors Length    1   constructors 0    null             protected virtual ConstructorInfo GetMostSpecificConstructor Type objectType                var constructors   objectType              GetConstructors BindingFlags Instance   BindingFlags Public   BindingFlags NonPublic               OrderBy e   gt  e GetParameters   Length            var mostSpecific   constructors LastOrDefault            return mostSpecific            Here is the complete version with XML documentation as a gist  https   gist github com maverickelementalch 80f77f4b6bdce3b434b0f7a1d06baa95  Feedback appreciated

User · Answer

The default behaviour of Newtonsoft Json is going to find the public constructors  If your default constructor is only used in containing class or the same assembly  you can reduce the access level to protected or internal so that Newtonsoft Json will pick your desired public constructor   Admittedly  this solution is rather very limited to specific cases   internal Result        public Result int  code  string format  Dictionary lt string  string gt  details   null        Code   code    ERROR CODE      Format   format       if  details    null          Details   new Dictionary lt string  string gt         else         Details   details

User · Answer

Json Net prefers to use the default  parameterless  constructor on an object if there is one   If there are multiple constructors and you want Json Net to use a non-default one  then you can add the  JsonConstructor  attribute to the constructor that you want Json Net to call    JsonConstructor  public Result int  code  string format  Dictionary lt string  string gt  details   null                It is important that the constructor parameter names match the corresponding property names of the JSON object  ignoring case  for this to work correctly   You do not necessarily have to have a constructor parameter for every property of the object  however   For those JSON object properties that are not covered by the constructor parameters  Json Net will try to use the public property accessors  or properties fields marked with  JsonProperty   to populate the object after constructing it   If you do not want to add attributes to your class or don t otherwise control the source code for the class you are trying to deserialize  then another alternative is to create a custom JsonConverter to instantiate and populate your object   For example   class ResultConverter   JsonConverter       public override bool CanConvert Type objectType                return  objectType    typeof Result               public override object ReadJson JsonReader reader  Type objectType  object existingValue  JsonSerializer serializer                   Load the JSON for the Result into a JObject         JObject jo   JObject Load reader               Read the properties which will be used as constructor parameters         int  code    int  jo  Code            string format    string jo  Format                Construct the Result object using the non-default constructor         Result result   new Result code  format                If anything else needs to be populated on the result object  do that here              Return the result         return result             public override bool CanWrite               get   return false               public override void WriteJson JsonWriter writer  object value  JsonSerializer serializer                throw new NotImplementedException              Then  add the converter to your serializer settings  and use the settings when you deserialize   JsonSerializerSettings settings   new JsonSerializerSettings    settings Converters Add new ResultConverter     Result result   JsonConvert DeserializeObject lt Result gt  jsontext  settings

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