c# - Json.net 集合大小固定

Question

我正在尝试使用 Json.net 序列化和反序列化以下类。

public class OperationBase
{
}

public class OperationCreate : OperationBase
{
    public string Entity
    {
        get;
        private set;
    }

    public IReadOnlyCollection<string> Attributes
    {
        get;
        private set;
    }

    public OperationCreate(string entity, params string[] attributes)
    {
        Contract.Requires(entity != null);
        Contract.Requires(attributes != null);

        Entity = entity;
        Attributes = attributes;
    }
}

public class OperationUpdate : OperationCreate
{
    public OperationUpdate(string entity, params string[] attributes)
        : base(entity, attributes)
    {
    }
}

public class OperationAssign : OperationUpdate
{
    public OperationAssign(string entity)
        : base(entity, "ownerid")
    {   
    }
}

使用以下代码。

public void SerialiseTest<T>(T t)
{
    string serialised = JsonConvert.SerializeObject(t, Formatting.Indented, new JsonSerializerSettings()
    {
        TypeNameHandling = TypeNameHandling.All,
    });

    ITraceWriter traceWriter = new MemoryTraceWriter();

    T deserialised = JsonConvert.DeserializeObject<T>(serialised, new JsonSerializerSettings()
    {
        TraceWriter = traceWriter,
        TypeNameHandling = TypeNameHandling.All,
    });

    deserialised.ShouldBeEquivalentTo(t);
}

[TestMethod]
public void Test()
{
    OperationCreate create = new OperationCreate("Create", new string[] { "ownerid" });

    OperationUpdate update = new OperationUpdate("Update", new string[] { "ownerid" });

    OperationAssign assign = new OperationAssign("Test");

    SerialiseTest(create);
    SerialiseTest(update);
    SerialiseTest(assign); //Exception!
}

我可以正确序列化和反序列化 OperationCreate 和 OperationUpdate，但是我在 OperationAssign 上收到此错误。

System.NotSupportedException:集合大小固定。

我不明白为什么或如何进一步调试它。 OperationAssign 基本上只是将参数传递给可以成功序列化和反序列化的基类(OperationCreate 和 OperationUpdate)。

我该如何解决这个问题？

序列化OperationAssign

{
  "$type": "Woodswork.Crm.Documenter.Data.Operations.OperationAssign, Woodswork.Crm.Documenter",
  "Entity": "Test",
  "Attributes": {
    "$type": "System.String[], mscorlib",
    "$values": [
      "ownerid"
    ]
  }
}

TraceWriter

2016-09-05T22:14:44.813 Verbose Resolved type 'Woodswork.Crm.Documenter.Data.Operations.OperationAssign, Woodswork.Crm.Documenter' to Woodswork.Crm.Documenter.Data.Operations.OperationAssign. Path '$type', line 2, position 95.
2016-09-05T22:14:44.813 Info Deserializing Woodswork.Crm.Documenter.Data.Operations.OperationAssign using creator with parameters: Entity. Path 'Entity', line 3, position 11.
2016-09-05T22:14:44.813 Verbose Resolved type 'System.String[], mscorlib' to System.String[]. Path 'Attributes.$type', line 5, position 40.
2016-09-05T22:14:44.813 Info Started deserializing System.String[]. Path 'Attributes.$values', line 6, position 16.
2016-09-05T22:14:44.813 Info Finished deserializing System.String[]. Path 'Attributes.$values', line 8, position 5.
2016-09-05T22:14:44.815 Info Started deserializing Woodswork.Crm.Documenter.Data.Operations.OperationAssign. Path '', line 10, position 1.
2016-09-05T22:14:44.824 Error Error deserializing Woodswork.Crm.Documenter.Data.Operations.OperationAssign. Collection was of a fixed size. Path '', line 10, position 1.

替代OperationAssign构造函数

这也会导致同样的错误。

public OperationAssign(string entity)
    : base(entity, new string[] { "ownerid" })
{   
}

Answer 1

问题是您正在序列化然后尝试反序列化 OperationAssign 的 Attributes 集合 - 但 Json.NET 无法反序列化它，因为集合已读取-only 并且该属性不可公开设置。

Json.NET 能够成功反序列化基类 OperationUpdate，因为它有一个参数化的构造函数，并且有一个与属性同名(模数大小写)的参数，即 属性。在这种情况下，Json.NET 将调用构造函数并传入从 JSON 文件反序列化的“属性”值。不幸的是，派生类省略了这个构造函数，因此反序列化失败。

您有几种方法可以解决此问题:

1. 添加一个带有适当参数的构造函数，并用 [JsonConstructor] 标记.它可以是私有(private)的:

   public class OperationAssign : OperationUpdate
   {
       [JsonConstructor]
       OperationAssign(string entity, params string[] attributes) : this(entity)
       {
       }
   
       public OperationAssign(string entity)
           : base(entity, "ownerid")
       {   
       }
   }
   

   如果需要，您可以忽略反序列化属性的值。该参数只需存在即可。

2. 使用 conditional property serialization 抑制派生类中属性的序列化:

   public class OperationCreate : OperationBase
   {
       public string Entity
       {
           get;
           private set;
       }
   
       public IReadOnlyCollection<string> Attributes
       {
           get;
           private set;
       }
   
       public virtual bool ShouldSerializeAttributes() { return true; }
   
       public OperationCreate(string entity, params string[] attributes)
       {
           Contract.Requires(entity != null);
           Contract.Requires(attributes != null);
   
           Entity = entity;
           Attributes = attributes;
       }
   }
   
   public class OperationAssign : OperationUpdate
   {
       public OperationAssign(string entity)
           : base(entity, "ownerid")
       {   
       }
   
       public override bool ShouldSerializeAttributes() { return false; }
   }
   

   基类必须通过虚拟 ShouldSerializeAttributes() 方法支持这一点。

3. 在基类中用[JsonProperty]标记属性。这将强制调用私有(private) setter :

   public class OperationCreate : OperationBase
   {
       public string Entity
       {
           get;
           private set;
       }
   
       [JsonProperty]
       public IReadOnlyCollection<string> Attributes
       {
           get;
           private set;
       }
   
       public OperationCreate(string entity, params string[] attributes)
       {
           Contract.Requires(entity != null);
           Contract.Requires(attributes != null);
   
           Entity = entity;
           Attributes = attributes;
       }
   }
   

样本 fiddle显示选项。