var settings = new JsonSerializerSettings() { ContractResolver= new NullToEmptyStringResolver() };var str = JsonConvert.SerializeObject(yourObj, settings);

    public class NullToEmptyStringResolver : Newtonsoft.Json.Serialization.DefaultContractResolver    {        protected override IList<JsonProperty> CreateProperties(Type type, MemberSerialization memberSerialization)        {            return type.GetProperties()                    .Select(p =>                    {                        var jp = base.CreateProperty(p, memberSerialization);                        jp.ValueProvider = new NullToEmptyStringValueProvider(p);                        return jp;                    }).ToList();        }    }    public class NullToEmptyStringValueProvider : IValueProvider    {        PropertyInfo _MemberInfo;        public NullToEmptyStringValueProvider(PropertyInfo memberInfo)        {            _MemberInfo = memberInfo;        }        public object GetValue(object target)        {            object result = _MemberInfo.GetValue(target, null);            if (_MemberInfo.PropertyType == typeof(string) && result == null) result = "";            return result;        }        public void SetValue(object target, object value)        {            _MemberInfo.SetValue(target, value,null);        }    }