我正在使用 JSON 作为配置文件,并且我想要一个数组的默认值。如果序列化的 JSON 等于 DefaultValueAttribute
,我想让序列化的 JSON 忽略数组,这样如果我决定在程序的第二个版本中更改 DefaultValues,将加载新的默认值而不是原始默认值的未修改副本。
我的问题是,如果数组引用没有更改,代码可以正常工作,但程序中的其他代码正在更改数组但保留其中的值。 (该程序维护了该类的许多克隆,所以这是不可避免的)。
这里是使用 c# interactive 显示的问题:
using System.ComponentModel;
using Newtonsoft.Json;
class A
{
[DefaultValue(new int[] { 4, 6, 12 })]
public int[] SomeArray;
}
var serializerSettings = new JsonSerializerSettings
{
DefaultValueHandling = DefaultValueHandling.IgnoreAndPopulate,
};
var a = new A();
JsonConvert.PopulateObject("{}", a, serializerSettings);
Console.WriteLine(JsonConvert.SerializeObject(a, serializerSettings));
// Prints {}
a.SomeArray = new int[] { 4, 6, 12 };
Console.WriteLine(JsonConvert.SerializeObject(a, serializerSettings));
// Prints {"SomeArray":[4,6,12]}
如您所见,第一个 SerializeObject 有效,但如果数组内容相同但不是相同的数组引用,它会将默认值写入 json,我希望避免这种情况。
有什么方法可以让 Json.net 在这种情况下忽略数组?
最佳答案
除了您已确定的问题之外,您当前的架构还有几个其他问题:
您忽略了 documented recommendations对于
DefaultValueAttribute
:A
DefaultValueAttribute
will not cause a member to be automatically initialized with the attribute's value. You must set the initial value in your code.您当前的实现导致所有具有默认值的
A
实例共享对int[3] { 4, 6, 12 } 的单个全局实例的引用
数组。由于数组并不是真正只读的,这意味着修改A
的一个实例将修改A
的所有其他当前和 future 实例默认值:var serializerSettings = new JsonSerializerSettings { DefaultValueHandling = DefaultValueHandling.IgnoreAndPopulate, }; var a1 = JsonConvert.DeserializeObject<A>("{}", serializerSettings); // The following succeeds Assert.IsTrue(a1.SomeArray.SequenceEqual(new int[] { 4, 6, 12 })); // Sime SomeArray is a globally shared pointer, this will modify all current and future instances of A! a1.SomeArray[0] = -999; var a2 = JsonConvert.DeserializeObject<A>("{}", serializerSettings); // The following now fails! Assert.IsTrue(a2.SomeArray.SequenceEqual(new int[] { 4, 6, 12 }));
避免这些问题的最简单方法是根本不对数组使用 DefaultValueHandling
,而是使用 conditional property serialization :
class A
{
static readonly int[] SomeArrayDefaultValue = new int[] { 4, 6, 12 };
// Disable global settings for NullValueHandling and DefaultValueHandling
[JsonProperty(NullValueHandling = NullValueHandling.Include, DefaultValueHandling = DefaultValueHandling.Include)]
public int[] SomeArray = (int[])SomeArrayDefaultValue.Clone();
public bool ShouldSerializeSomeArray()
{
return !(SomeArray != null && SomeArray.SequenceEqual(SomeArrayDefaultValue));
}
}
演示 fiddle #1 here .
如果您决定对数组使用 DefaultValueHandling
和 DefaultValueAttribute
,您将需要 custom contract resolver :
public class ArrayDefaultValueContractResolver : DefaultContractResolver
{
class ArrayDefaultValueProvider : IValueProvider
{
readonly IValueProvider baseProvider;
readonly System.Array defaultValue;
public ArrayDefaultValueProvider(IValueProvider baseProvider, System.Array defaultValue)
{
this.baseProvider = baseProvider;
this.defaultValue = defaultValue;
}
#region IValueProvider Members
public object GetValue(object target)
{
return baseProvider.GetValue(target);
}
public void SetValue(object target, object value)
{
// Make sure the default value is cloned since arrays are not truly read only.
if (value != null && object.ReferenceEquals(value, defaultValue))
value = defaultValue.Clone();
baseProvider.SetValue(target, value);
}
#endregion
}
static void AddArrayDefaultHandling<T>(JsonProperty property)
{
var defaultValue = (T [])property.DefaultValue;
// If the default value has length > 0, clone it when setting it back into the object.
if (defaultValue.Length > 0)
{
property.ValueProvider = new ArrayDefaultValueProvider(property.ValueProvider, defaultValue);
}
// Add a ShouldSerialize method that checks for memberwise array equality.
var valueProvider = property.ValueProvider;
var oldShouldSerialize = property.ShouldSerialize;
Predicate<object> shouldSerialize = target =>
{
var array = (T[])valueProvider.GetValue(target);
return !(array == null || array.SequenceEqual(defaultValue));
};
if (oldShouldSerialize == null)
property.ShouldSerialize = shouldSerialize;
else
property.ShouldSerialize = (target) => shouldSerialize(target) && oldShouldSerialize(target);
}
protected override JsonProperty CreateProperty(MemberInfo member, MemberSerialization memberSerialization)
{
var property = base.CreateProperty(member, memberSerialization);
if (property.PropertyType.IsArray && property.DefaultValue != null && property.DefaultValue.GetType() == property.PropertyType
&& property.PropertyType.GetArrayRank() == 1)
{
typeof(ArrayDefaultValueContractResolver)
.GetMethod("AddArrayDefaultHandling", BindingFlags.Static | BindingFlags.NonPublic)
.MakeGenericMethod(property.PropertyType.GetElementType())
.Invoke(null, BindingFlags.Static | BindingFlags.NonPublic, null, new [] { property }, null);
}
return property;
}
}
要使用它,在某处缓存一个静态实例以提高性能,例如
static IContractResolver resolver = new ArrayDefaultValueContractResolver();
并将其用作 JsonSerializerSettings.ContractResolver
序列化时:
var serializerSettings = new JsonSerializerSettings
{
DefaultValueHandling = DefaultValueHandling.IgnoreAndPopulate,
ContractResolver = resolver,
};
var a = new A();
JsonConvert.PopulateObject("{}", a, serializerSettings);
Console.WriteLine(JsonConvert.SerializeObject(a, serializerSettings));
Assert.IsTrue(JsonConvert.SerializeObject(a, serializerSettings) == "{}");
a.SomeArray = new int[] { 4, 6, 12 };
Console.WriteLine(JsonConvert.SerializeObject(a, serializerSettings));
Assert.IsTrue(JsonConvert.SerializeObject(a, serializerSettings) == "{}");
演示 fiddle #2 here .
注意事项:
契约解析器仅针对秩为 1 的数组实现。如果需要,您可以将其扩展到多维数组。
契约(Contract)解析器在将默认值数组实例设置为成员时会自动克隆它,以避免上述问题 #2。如果您不想这样,您可以删除
ArrayDefaultValueProvider
。尚不清楚支持数组值默认值是否是 Json.NET 的预期功能。
关于c# - 序列化时不忽略在 DefaultValue 中声明的数组,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/54066654/