Question

我正在尝试使用Json.net对以下类进行序列化和反序列化。

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!
}

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

System.NotSupportedException: Collection was of a fixed size.

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基类（OperationCreate和OperationUpdate），这些基类可以成功序列化和反序列化。

如何更正此问题？

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.

备用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Attributes OperationAssign的集合 - 但Json.NET无法反序列化它，因为集合是只读的，属性是不可公开设定。

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，并且具有与属性相同的名称（模数大小写）的参数，即attributes 。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”的值。不幸的是，派生类省略了这个构造函数，因此反序列化失败。

您可以通过以下几种方法解决此问题：

使用适当的参数添加构造函数，并使用[JsonConstructor]标记。它可能是私人的：

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。该参数只需要存在。

使用conditional property serialization禁止对派生类中的属性进行序列化：

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()方法支持此功能。

在基类中使用[JsonProperty]标记属性。这将强制调用私有的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;
    }
}

显示选项的示例fiddle。

Answer 2

我需要一个额外的构造函数来从Json.net获取数组输入。

[JsonConstructor]
public OperationAssign(string entity, params string[] attributes)
    : base(entity, attributes)
{
}

Json.net Collection的大小固定

2 个答案: