最近遇到的一個問題，就是關於 Json.NET 的  SerializeObject 的顯示問題

其實一般狀況下我就是直接無腦的  JsonConvert.SerializeObject(object)

但是，最近在維護一個系統因為物件要改成 Nullable 就有點麻煩，所以看看能不能直接再 SerializeObject

解決盡量的不使用 Attribute 或是改寫 Nullable 去解決

先來說說物件這是我的物件

User.cs:

public class User { public string Id { get; set; } public string Name { get; set; } public string FriendId { get; set; } public int Age { get; set; } public DateTime Birth { get; set; } public List<User> Colleague { get; set; } public User CloseFriend { get; set; } public User() { Colleague = new List<User>(); } }

因為我只填寫了 Id 還有 Name 就直接無腦的 SerializeObject 後你會得到

{"Id":"D1","Name":"DONMA","FriendId":null,"Age":0,"Birth":"0001-01-01T00:00:00","Colleague":[],"CloseFriend":null}

但是，我不希望 null 的值出現，這問題還很簡單 直接使用  NullValueHandling = NullValueHandling.Ignore

var u1 = new User { Id = "D1", Name = "DONMA" }; Result += JsonConvert.SerializeObject(u1, Newtonsoft.Json.Formatting.None, new JsonSerializerSettings { NullValueHandling = NullValueHandling.Ignore }); //{"Id":"D1","Name":"DONMA","Age":0,"Birth":"0001-01-01T00:00:00","Colleague":[]}

這時候結果變成

{"Id":"D1","Name":"DONMA","Age":0,"Birth":"0001-01-01T00:00:00","Colleague":[]}

接下來就是萬惡的 DateTime 預設是 0001-01-01T00:00:00 的問題 還有預設  int 是 0 的問題

這時候可以使用 DefaultValueHandling = DefaultValueHandling.Ignore 來解決

var u1 = new User { Id = "D1", Name = "DONMA" }; Result += JsonConvert.SerializeObject(u1, Newtonsoft.Json.Formatting.None, new JsonSerializerSettings { NullValueHandling = NullValueHandling.Ignore, DefaultValueHandling = DefaultValueHandling.Ignore, }); //{"Id":"D1","Name":"DONMA","Colleague":[]}

結果:

{"Id":"D1","Name":"DONMA","Colleague":[]}

這時候精彩的來了，對方希望 Colleague 雖然是物件  array 如果是空陣列的時候也不要出現，但是其實我個人習慣

List<object> 在 Ctor 的時候先 new 出來，這時候怎麼辦只好動用 DefaultContractResolver

EmptyListResolver.cs

public class EmptyListResolver : DefaultContractResolver { protected override JsonProperty CreateProperty(MemberInfo member, MemberSerialization memberSerialization) { JsonProperty property = base.CreateProperty(member, memberSerialization); Type propType = ((PropertyInfo)member).PropertyType; property.ShouldSerialize = obj => { if (propType.IsGenericType && propType.GetGenericTypeDefinition() == typeof(List<>)) { return (property.ValueProvider.GetValue(obj) as dynamic).Count > 0; } return true; }; return property; } }

這時候 需要在 SerializeObject 加上  ContractResolver = new EmptyListResolver()

var u1 = new User { Id = "D1", Name = "DONMA" }; Result += JsonConvert.SerializeObject(u1, Newtonsoft.Json.Formatting.None, new JsonSerializerSettings { NullValueHandling = NullValueHandling.Ignore, DefaultValueHandling = DefaultValueHandling.Ignore, ContractResolver = new EmptyListResolver() }); //{"Id":"D1","Name":"DONMA"}

結果:

{"Id":"D1","Name":"DONMA"}

大概就是這樣了，花了我一點時間處理 Happy Coding.

reference:

https://stackoverflow.com/questions/46977905/overriding-a-property-value-in-custom-json-net-contract-resolver

https://stackoverflow.com/questions/64552981/newtonsoft-json-defaultcontractresolver-remove-key-from-dictionary

https://stackoverflow.com/questions/37389884/removing-empty-array-members-from-a-json-string