在開發web api的時候 遇到一個要把string未賦值默認為null的情況改成默認為空字符串的需求
這種情況就需要自定義json序列話的 解析協議了

Newtonsoft.Json默認的解析協議是DefaultContractResolver

需要繼承DefaultContractResolver 自定義解析協議了

 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; // 特殊處理            }        }        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);        }    }

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