How to add a property to a json object on the fly?

Question

We are using Json.Net in our project to serialize and deserialize json objects.

Our entities have some DateTime properties and I would like to be able to convert them into PersianCalender DateTime and to provide them as string in my json object:

for example we have this entity :

public class PersonCertificate
{
      public DateTime CertificateDate{get;set;}
}

I would like to have a json object like this :

{
    "PersianCertificateDate":"1395/10/10"
}

So I thought that would be great to have an attribute named "AsPersianDate" for example so that I could do something like this:

public class PersonCertificate
{
      [JsonIgnore]
      [AsPersianDate]
      public DateTime CertificateDate{get;set;}
}

I know that I can have a custom contract resolver to intercept json property creation process but I don't know how should I tell Json.Net to deserialize PersianCertificateDate into CertificateDate ?

You could add a 2nd property, and it would be left empty when you do json-to-object, because the json doesn't have that property. But then you can calculate it according to the Date, and then your object-to-json will have that extra piece of data. — YoryeNathan
– YoryeNathan, Commented Dec 25, 2016 at 12:13
that is what I am doing right now the problem is that I have to add these extra properties for all of my entities but I only need them when I want to serialize and deserialize my objects. — Beatles1692
– Beatles1692, Commented Dec 25, 2016 at 12:18
So make a derived class that only adds those properties, and is only used for serialize and deserialize, and your "real" classes remain clean. The derived classes can even be private within a class, so they don't even add junk to the intellisense throughout your project. — YoryeNathan
– YoryeNathan, Commented Dec 25, 2016 at 13:09

Beatles1692 · Accepted Answer · 2016-12-25 13:42:36Z

OK it was far more easier than I thought.Actually ContractResolver is responsible for getting and setting all property values so here's what I have done:

public class EntityContractResolver:DefaultContractResolver
    {
        private class PersianDateValueProvider:IValueProvider
        {
            private readonly PropertyInfo _propertyInfo;

            public PersianDateValueProvider(PropertyInfo propertyInfo)
            {
                _propertyInfo = propertyInfo;
            }


            public void SetValue(object target, object value)
            {
                try
                {
                    var date = value as string;
                    if(value==null && _propertyInfo.PropertyType==typeof(DateTime))
                        throw new InvalidDataException();
                    _propertyInfo.SetValue(target,date.ToGregorianDate());
                }
                catch (InvalidDataException)
                {
                    throw new ValidationException(new[]
                    {
                        new ValidationError
                        {
                            ErrorMessage = "Date is not valid",
                            FieldName = _propertyInfo.Name,
                            TypeName = _propertyInfo.DeclaringType.FullName
                        }
                    });
                }
            }

            public object GetValue(object target)
            {
                if(_propertyInfo.PropertyType.IsNullable() && _propertyInfo.GetValue(target)==null) return null;
                try
                {
                    return ((DateTime) _propertyInfo.GetValue(target)).ToPersian();
                }
                catch
                {
                    return string.Empty;
                }

            }
        }


        protected override IList<JsonProperty> CreateProperties(Type type, MemberSerialization memberSerialization)
        {
            var list= base.CreateProperties(type, memberSerialization).ToList();
            list.AddRange(type.GetProperties()
                .Where(pInfo => IsAttributeDefined(pInfo,typeof(AsPersianDateAttribute))&& (pInfo.PropertyType == typeof (DateTime) || pInfo.PropertyType == typeof (DateTime?)))
                .Select(CreatePersianDateTimeProperty));
            return list;
        }

        private JsonProperty CreatePersianDateTimeProperty(PropertyInfo propertyInfo)
        {
            return new JsonProperty
            {
                PropertyName = "Persian"+propertyInfo.Name ,
                PropertyType = typeof (string),
                ValueProvider = new PersianDateValueProvider(propertyInfo),
                Readable = true,
                Writable = true
            };
        }

        private bool IsAttributeDefined(PropertyInfo propertyInfo,Type attribute)
        {
            var metaDataAttribute = propertyInfo.DeclaringType.GetCustomAttribute<MetadataTypeAttribute>(true);
            var metaDataProperty = metaDataAttribute?.MetadataClassType?.GetProperty(propertyInfo.Name);
            var metaDataHasAttribute = metaDataProperty != null && Attribute.IsDefined(metaDataProperty, attribute);

            return metaDataHasAttribute || Attribute.IsDefined(propertyInfo, attribute);
        }
    }

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