处理有时没有数组的JSON响应 [英] Handling JSON response which sometimes doesn't have array
问题描述
我在C#控制台应用程序中使用了一些JSON，对于某些数据，有很多选项. JSON示例:
I am consuming some JSON in a C# console application and for some of the data, there is an array of options. Example JSON:
{
"FIELD_NAME": "Survey",
"VALUE": "",
"FIELD_ID": 1234,
"OPTIONS":[
{ "VALUE": "GENERAL", "DISPLAY_ORDER": 1, "DISPLAY": "GENERAL" },
{ "VALUE": "HPEFS", "DISPLAY_ORDER": 3, "DISPLAY": "HPEFS" },
{ "VALUE": "NONE", "DISPLAY_ORDER": 3, "DISPLAY": "NONE" }]
}
但有时对于JSON中的记录，选项为空:
But sometimes for records in the JSON the OPTIONS is empty:
{"FIELD_NAME":"Product_Node3","VALUE":"","FIELD_ID":1740,"OPTIONS":{}}
如您所见，选项设置为{}，但据我了解，{}是一个空对象，而不是一个空数组.
As you can see the options is set to {} but it is my understanding that {} is an empty object, not an empty array.
当我尝试反序列化为POCO时，我收到一个异常，抱怨它在OPTIONS属性中需要一个JSON数组.
When I try deserialize to a POCO I get an exception complaining that it requires a JSON array in the OPTIONS property.
我的领域课:
public class Field
{
public string FIELD_NAME { get; set; }
public string VALUE { get; set; }
public int FIELD_ID { get; set; }
public List<Option> OPTIONS { get; set;
}
}
和选项类:
public class Option
{
public string VALUE { get; set; }
public int DISPLAY_ORDER { get; set; }
public string DISPLAY { get; set; }
}
导致此异常的代码是:
var stringTest = File.ReadAllText("json.txt");
var data = JsonConvert.DeserializeObject<List<Field>>(stringTest);
例外是:
Cannot deserialize the current JSON object (e.g. {"name":"value"}) into type 'System.Collections.Generic.List`1[testproj.Option]' because the type requires a JSON array (e.g. [1,2,3]) to deserialize correctly.
推荐答案
当预期的JSON值类型(数组，集合或基元)与观察到的值类型不匹配时，Json.NET将引发异常.由于对于List<Option> OPTIONS，您希望跳过意外的值类型，因此需要创建
Json.NET will throw an exception when the expected JSON value type (array, collection or primitive) does not match the observed value type. Since, in the case of your List<Option> OPTIONS, you want unexpected value types to be skipped, you will need to create a custom JsonConverter such as the following:
public class TolerantCollectionConverter : JsonConverter
{
public override bool CanConvert(Type objectType)
{
if (objectType.IsPrimitive || objectType == typeof(string) || objectType.IsArray)
return false;
return objectType.GetCollectionItemTypes().Count() == 1;
}
public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
{
if (reader.TokenType == JsonToken.Null)
{
return null;
}
else if (reader.TokenType == JsonToken.StartArray)
{
existingValue = existingValue ?? serializer.ContractResolver.ResolveContract(objectType).DefaultCreator();
serializer.Populate(reader, existingValue);
return existingValue;
}
else
{
reader.Skip();
return existingValue;
}
}
public override bool CanWrite { get { return false; } }
public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
{
throw new NotImplementedException();
}
}
public static class TypeExtensions
{
/// <summary>
/// Return all interfaces implemented by the incoming type as well as the type itself if it is an interface.
/// </summary>
/// <param name="type"></param>
/// <returns></returns>
public static IEnumerable<Type> GetInterfacesAndSelf(this Type type)
{
if (type == null)
throw new ArgumentNullException();
if (type.IsInterface)
return new[] { type }.Concat(type.GetInterfaces());
else
return type.GetInterfaces();
}
public static IEnumerable<Type> GetCollectionItemTypes(this Type type)
{
foreach (Type intType in type.GetInterfacesAndSelf())
{
if (intType.IsGenericType
&& intType.GetGenericTypeDefinition() == typeof(ICollection<>))
{
yield return intType.GetGenericArguments()[0];
}
}
}
}
然后按如下所示将其应用于Field:
Then apply it to Field as follows:
public class Field
{
public string FIELD_NAME { get; set; }
public string VALUE { get; set; }
public int FIELD_ID { get; set; }
[JsonConverter(typeof(TolerantCollectionConverter))]
public List<Option> OPTIONS { get; set; }
}
或通过 JsonSerializerSettings 将其用于所有收藏集:
Or use it for all collections via JsonSerializerSettings:
var settings = new JsonSerializerSettings
{
Converters = { new TolerantCollectionConverter() },
};
var obj = JsonConvert.DeserializeObject<Field>(stringTest, settings);
注意:
-
转换器仅适用于可写的集合，因为它先分配集合，然后填充它.对于只读集合或数组，您需要先填充
List<T>，然后从中分配只读集合或数组.
The converter only works for collections that are writable, since it allocates the collection first and then populates it. For read-only collections or arrays you need to populate a
List<T>first then allocate the read-only collection or array from it.
我在这里的假设是，当需要数组值时，您想忽略一个空对象.相反，如果您想将对象反序列化为集合项，然后将其添加到返回的集合中，则可以使用
My assumption here is that you want to ignore an empty object when an array value is expected. If instead you want to deserialize the object into a collection item then add that to the returned collection you could use SingleOrArrayConverter<T> from How to handle both a single item and an array for the same property using JSON.net.
您的问题中显示的JSON根容器是一个对象-一个以{开头并以}结束的名称/值对的无序集合-而不是数组.因此，您需要将其反序列化为Field而不是List<Field>.
The root JSON container shown in your question is an object -- an unordered set of name/value pairs that begins with { and ends with } -- rather than an array. Thus you need to deserialize it as a Field not a List<Field>.
示例小提琴.
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