如何使用XmlSerializer反序列化一个可能是基类或派生类的对象，而不事先知道类型？

Question

在C#中，如何使用XmlSerializer来反序列化一个可能是基类或多个派生类之一的对象，而不需要预先知道类型？

我所有的派生类都添加了额外的数据成员。我制作了一个简单的GUI，可以将类对象序列化和反序列化。它会根据用户选择填充的字段将对象序列化为适当的继承类（甚至只是基类）。

我对序列化没有问题；问题在于反序列化。如何让XmlSerializer根据不事先知道的类正确地反序列化数据？目前，我创建了一个XmlReader来读取XML文件的第一个节点并从中确定类，这似乎对我的目的有用，但看起来非常不优雅。

我在下面发布了一些示例代码。有什么建议吗？

BaseType objectOfConcern = new BaseType();
XmlSerializer xserializer;
XmlTextReader xtextreader = new XmlTextReader(DEFAULT_FILENAME);

do { xtextreader.Read(); } while (xtextreader.NodeType != XmlNodeType.Element);

string objectType = xtextreader.Name;
xtextreader.Close();

FileStream fstream = new FileStream(DEFAULT_FILENAME, FileMode.Open);

switch (objectType)
    {
case "type1":
    xserializer = new XmlSerializer(typeof(DerivedType));

    objectOfConcern = (DerivedType)xserializer.Deserialize(fstream);

    //Load fields specific to that derived type here
    whatever = (objectOfConcern as DerivedType).NoOfstreamubordinates.ToString();

    case "xxx_1":
        //code here

    case "xxx_2":
        //code here

    case "xxx_n":
        //code here

        //and so forth

    case "BaseType":
    xserializer = new XmlSerializer(typeof(BaseType));
    AssignEventHandler(xserializer);
    objectOfConcern = (BaseType)xserializer.Deserialize(fstream);
}

//Assign all deserialized values from base class common to all derived classes here

//Close the FileStream
fstream.Close();

- 8bitcartridge

Answer 1

您是否有一个包含派生类型的根类/标记？ 如果是，您可以使用XmlElementAttribute将标记名称映射到类型：

public class RootElementClass
{
    [XmlElement(ElementName = "Derived1", Type = typeof(Derived1BaseType))]
    [XmlElement(ElementName = "Derived2", Type = typeof(Derived2BaseType))]
    [XmlElement(ElementName = "Derived3", Type = typeof(Derived3BaseType))]
    public BaseType MyProperty { get; set; }
}

public class BaseType { }
public class Derived1BaseType : BaseType { }
public class Derived2BaseType : BaseType { }
public class Derived3BaseType : BaseType { }

- SeeSharp

Answer 2

我最近为基类T及其任何派生类编写了这个通用的序列化器/反序列化器。 到目前为止似乎工作得很好。

Type[]数组存储了T及其所有派生类型。 反序列化器会尝试它们中的每一个，当找到正确的类型时返回。

/// <summary>
/// A generic serializer\deserializer
/// </summary>
/// <typeparam name="T"></typeparam>
public static class Serializer<T>
{
    /// <summary>
    /// serialize an instance to xml
    /// </summary>
    /// <param name="instance"> instance to serialize </param>
    /// <returns> instance as xml string </returns>
    public static string Serialize(T instance)
    {
        StringBuilder sb = new StringBuilder();
        XmlWriterSettings settings = new XmlWriterSettings();

        using (XmlWriter writer = XmlWriter.Create(sb, settings))
        {
            XmlSerializer serializer = new XmlSerializer(instance.GetType());
            serializer.Serialize(writer, instance);
        }

        return sb.ToString();
    }

    /// <summary>
    /// deserialize an xml into an instance
    /// </summary>
    /// <param name="xml"> xml string </param>
    /// <returns> instance </returns>
    public static T Deserialize(string xml)
    {
        using (XmlReader reader = XmlReader.Create(new StringReader(xml)))
        {
            foreach (Type t in types)
            {
                XmlSerializer serializer = new XmlSerializer(t);
                if (serializer.CanDeserialize(reader))
                    return (T)serializer.Deserialize(reader);
            }
        }

        return default(T);
    }

    /// <summary>
    /// store all derived types of T:
    /// is used in deserialization
    /// </summary>
    private static Type[] types = AppDomain.CurrentDomain.GetAssemblies()
                                        .SelectMany(s => s.GetTypes())
                                        .Where(t => typeof(T).IsAssignableFrom(t)
                                            && t.IsClass
                                            && !t.IsGenericType)
                                            .ToArray();
}

- Bartel

---

正是我在寻找的，谢谢！ - whywhywhy

太好了！你救了我的一天！ - Lamelas84

Answer 3

如果您不打算使用 XmlSerializer，您可以改用 DataContractSerializer 并在每个子类的父类中添加 KnownType 属性。

只需为每个子类的父类添加一个 KnownType 属性即可，DataContractSerializer 将完成其余的工作。 DataContractSerializer 在序列化为 XML 时会添加类型信息，并在反序列化时使用该类型信息创建正确的类型。

例如，以下代码：

[KnownType( typeof( C2 ) )]
[KnownType( typeof( C3 ) )]
public class C1 {public string P1 {get;set;}}
public class C2 :C1 {public string P2 {get;set;}}
public class C3 :C1 {public string P3 {get;set;}}

class Program
{
  static void Main(string[] args)
  {
    var c1 = new C1{ P1="c1"};
    var c2 = new C2{ P1="c1", P2="c2"};
    var c3 = new C3{ P1="c1", P3="c3"};

    var s = new DataContractSerializer( typeof( C1 ) );
    Test( c1, s );
    Test( c2, s );
    Test( c3, s );
  }

  static void Test( C1 objectToSerialize, DataContractSerializer serializer )
  {
    using ( var stream = new MemoryStream() )
    {
      serializer.WriteObject( stream, objectToSerialize );
      stream.WriteTo( Console.OpenStandardOutput() );
      stream.Position = 0;
      var deserialized = serializer.ReadObject( stream );
      Console.WriteLine( Environment.NewLine + "Deserialized Type: " + deserialized.GetType().FullName );              
    }
  }
}

将输出：

<C1 xmlns="http://schemas.datacontract.org/2004/07/ConsoleApplication1" xmlns:i="http://www.w3.org/2001/XMLSchema-instance">
<P1>c1</P1></C1>

Deserialized Type: ConsoleApplication1.C1

<C1 i:type="C2" xmlns="http://schemas.datacontract.org/2004/07/ConsoleApplication1" xmlns:i="http://www.w3.org/2001/XMLSchema-instance">
<P1>c1</P1><P2>c2</P2></C1>

Deserialized Type: ConsoleApplication1.C2

<C1 i:type="C3" xmlns="http://schemas.datacontract.org/2004/07/ConsoleApplication1" xmlns:i="http://www.w3.org/2001/XMLSchema-instance">
<P1>c1</P1><P3>c3</P3></C1>

Deserialized Type: ConsoleApplication1.C3

在输出中，您会注意到c2和c3的xml包含了额外的类型信息，这使得能够创建正确的类型。

- Aaron Carlson

Answer 4

你可以尝试使用构造函数XmlSerializer(Type type, Type[] extraTypes)来创建一个能够处理所有相关类型的序列化器。

- Jordão

Answer 5

您可以使用XmlInclude

[XmlInclude(typeof(MyClass))]
public abstract class MyBaseClass
{
   //...
}

否则，如果您想在序列化时添加类型：

Type[] types = new Type[]{ typeof(MyClass) }

XmlSerializer serializer = new XmlSerializer(typeof(MyBaseClass), types);

- lupok