Newtonsoft.Json 自定义 解析协议

在开发web api的时候 遇到一个要把string未赋值默认为null的情况改成默认为空字符串的需求
这种情况就需要自定义json序列话的 解析协议了

Newtonsoft.Json默认的解析协议是DefaultContractResolver

需要继承DefaultContractResolver 自定义解析协议了

 public class NullableValueProvider : IValueProvider
    {
        private readonly object _defaultValue;
        private readonly IValueProvider _underlyingValueProvider;


        public NullableValueProvider(MemberInfo memberInfo, Type underlyingType)
        {
            _underlyingValueProvider = new DynamicValueProvider(memberInfo);
            if (underlyingType != typeof(string))
            {
                _defaultValue = Activator.CreateInstance(underlyingType);
            }
            else
            {
                _defaultValue = string.Empty; // 特殊处理
            }
        }

        public void SetValue(object target, object value)
        {
            _underlyingValueProvider.SetValue(target, value);
        }

        public object GetValue(object target)
        {
            return _underlyingValueProvider.GetValue(target) ?? _defaultValue;
        }
    }

    public class SpecialContractResolver : DefaultContractResolver
    {
        protected override IValueProvider CreateMemberValueProvider(MemberInfo member)
        {
            if (member.MemberType == MemberTypes.Property)
            {
                var pi = (PropertyInfo)member;
                
                if (pi.PropertyType == typeof(string))
                {
                    return new NullableValueProvider(member, pi.PropertyType);
                }
            }
            else if (member.MemberType == MemberTypes.Field)
            {
                var fi = (FieldInfo)member;
             
                if (fi.FieldType == typeof(string))
                {
                    return new NullableValueProvider(member, fi.FieldType);
                }
            }

            return base.CreateMemberValueProvider(member);
        }
    }

参考