C# XML 序列化覆盖 IXmlSerializable 类的类型

Question

初始问题

我要serialize一个List<IXmlSerializable>更改 IXmlSerializable 的 XmlType动态分类(所以我不能使用属性标签来做到这一点)

我已经尝试使用 XmlAttributeOverrides 来做到这一点，但到目前为止没有成功。

下面是一个示例代码来说明这个问题:
IXmlSerializable 类(来自 MSDN):

public class Person : IXmlSerializable
{
    // Private state
    private string personName;


    // Constructors
    public Person(string name)
    {
        personName = name;
    }

    public Person()
    {
        personName = null;
    }


    // Xml Serialization Infrastructure
    public void WriteXml(XmlWriter writer)
    {
        writer.WriteString(personName);
    }

    public void ReadXml(XmlReader reader)
    {
        personName = reader.ReadString();
    }

    public XmlSchema GetSchema()
    {
        return (null);
    }


    // Print
    public override string ToString()
    {
        return (personName);
    }
}

测试类(使用控制台输出):

class Program
{
    static void Main(string[] args)
    {
        List<Person> lPersonList = new List<Person> {
            new Person("First"),
            new Person("Second"),
            new Person("Third")
        };
        XmlAttributeOverrides lOverrides = new XmlAttributeOverrides();
        XmlAttributes lAttributes = new XmlAttributes { XmlType = new XmlTypeAttribute("Employee") };
        lOverrides.Add(typeof(Person), lAttributes);

        XmlSerializer lSerialiser = new XmlSerializer(typeof(List<Person>), lOverrides, null, new XmlRootAttribute("Employees"), null);
        XmlSerializerNamespaces lNamespaces = new XmlSerializerNamespaces();
        lNamespaces.Add("", "");
        lSerialiser.Serialize(Console.Out, lPersonList, lNamespaces);

        System.Console.WriteLine("Enter any key to close.");
        System.Console.ReadKey();
    }
}

这是我想要得到的:

<Employees>
<Employee>First</Employee>
<Employee>Second</Employee>
<Employee>Third</Employee>
</Employees>

但是我在运行时遇到这个错误:

System.InvalidOperationException: Only XmlRoot attribute may be specified for the type Person. Please use XmlSchemaProviderAttribute to specify schema type.

注意 当我的Person 类 没有实现IXmlSerializable 时, 一切正常...

有人可以帮我吗？

---

选择解决方案(基于@dbc answer)

正如@dbc 所指出的，使用“代理”类是实现我所想的最简单方法。但正如我所说，我需要动态更改 Person 类型，这意味着我不能使用属性标签。
所以我还是用XmlAttributeOverrides在我的最终设计中，它是:

代理 List<Person>类(与没有属性标签的@dbc 相同):

public class EmployeesListSurrogate
{
    public List<Person> EmployeeList { get; set; }

    public static implicit operator List<Person>(EmployeesListSurrogate surrogate)
    {
        return surrogate == null ? null : surrogate.EmployeeList;
    }

    public static implicit operator EmployeesListSurrogate(List<Person> employees)
    {
        return new EmployeesListSurrogate { EmployeeList = employees };
    }
}

使用代理的测试类:

class Program
{
    static void Main(string[] args)
    {
        List<Person> lPersonList = new List<Person> {
            new Person("First"),
            new Person("Second"),
            new Person("Third")
        };

        XmlAttributeOverrides lOverrides = new XmlAttributeOverrides();
        XmlAttributes lEmployeesListAttributes = new XmlAttributes { XmlRoot = new XmlRootAttribute("Employees") };
        lOverrides.Add(typeof(EmployeesListSurrogate), lEmployeesListAttributes);
        XmlAttributes lEmployeeAttributes = new XmlAttributes { XmlElements = { new XmlElementAttribute("Employee") } };
        lOverrides.Add(typeof(EmployeesListSurrogate), "EmployeeList", lEmployeeAttributes);

        XmlSerializer lSerializer = new XmlSerializer(typeof(EmployeesListSurrogate), lOverrides);
        XmlSerializerNamespaces lNamespaces = new XmlSerializerNamespaces();
        lNamespaces.Add("", "");
        lSerializer.Serialize(Console.Out, (EmployeesListSurrogate)lPersonList, lNamespaces);
    }
}

我想以非常感谢@dbc 来结束这篇文章，你的回答非常有帮助和信息丰富，我学到了很多东西。我不能给你投票，但我希望社区会这样做!

Answer 1

获取所需 XML 的最简单方法是按如下方式序列化“代理”类:

[XmlRoot("Employees")]
public class EmployeesListSurrogate
{
    [XmlElement("Employee")]
    public List<Person> EmployeeList { get; set; }

    public static implicit operator List<Person>(EmployeesListSurrogate surrogate)
    {
        return surrogate == null ? null : surrogate.EmployeeList;
    }

    public static implicit operator EmployeesListSurrogate(List<Person> employees)
    {
        return new EmployeesListSurrogate { EmployeeList = employees };
    }
}

这完全消除了对 XmlAttributeOverrides 的需要.或者你可以使用 XmlAttributeOverrides连同 XmlAttributes.XmlElements 为 EmployeeList 指定 XML 名称动态地。

也就是说，InvalidOperationException 的原因当您尝试申请 [XmlType] 时抛出到也实现了 IXmlSerializable 的类型是那个XmlSerializer需要通过完全不同的机制返回类型名称，即 XmlSchemaProviderAttribute.MethodName [XmlSchemaProvider] 中指定的方法属性。

当 [XmlSchemaProvider]应用于 IXmlSerializable类型，XmlSerializer将查找其名称在属性构造函数中指定并具有以下签名的类型的公共(public)静态方法:

    public static XmlQualifiedName GetSchemaMethod(XmlSchemaSet xs)
    {
    }

这个方法有两个目的:

1. 应该填写XmlSchemaSet在序列化类型的实例时使用预期的模式。通过测试，我发现它必须填充有效的东西。不能留空，否则会抛出异常。

   (我不知道 XmlSerializer 在序列化时实际在多大程度上验证了架构。在通过 xsd.exe 导出架构信息时也会调用该方法。)

2. 它应该返回类型的 XML 类型名称。

   这似乎就是 Microsoft 抛出您所看到的异常的原因:因为模式属性提供程序应该返回类型名称，一个 XmlType属性会发生冲突。

因此，如果我修改你的 Person类如下:

[XmlSchemaProvider("GetSchemaMethod")]
public class Person : IXmlSerializable
{
    // Private state
    private string personName;

    // Constructors
    public Person(string name)
    {
        personName = name;
    }

    public Person()
    {
        personName = null;
    }

    // This is the method named by the XmlSchemaProviderAttribute applied to the type.
    public static XmlQualifiedName GetSchemaMethod(XmlSchemaSet xs)
    {
        string EmployeeSchema = @"<?xml version=""1.0"" encoding=""utf-16""?>
<xs:schema elementFormDefault=""qualified"" xmlns:xs=""http://www.w3.org/2001/XMLSchema"">
  <xs:import namespace=""http://www.w3.org/2001/XMLSchema"" />
  <xs:element name=""Employee"" nillable=""true"" type=""Employee"" />
  <xs:complexType name=""Employee"" mixed=""true"">
  <xs:sequence>
    <xs:any />
  </xs:sequence>
  </xs:complexType>
</xs:schema>";

        using (var textReader = new StringReader(EmployeeSchema))
        using (var schemaSetReader = System.Xml.XmlReader.Create(textReader))
        {
            xs.Add("", schemaSetReader);
        }
        return new XmlQualifiedName("Employee");
    }

    // Xml Serialization Infrastructure
    public void WriteXml(XmlWriter writer)
    {
        writer.WriteString(personName);
    }

    public void ReadXml(XmlReader reader)
    {
        reader.MoveToContent();
        var isEmpty = reader.IsEmptyElement;
        reader.ReadStartElement();
        if (!isEmpty)
        {
            personName = reader.ReadContentAsString();
            reader.ReadEndElement();
        }
    }

    public XmlSchema GetSchema()
    {
        return (null);
    }

    // Print
    public override string ToString()
    {
        return (personName);
    }
}

并序列化您的 List<Person>到 XML，我得到以下结果:

<ArrayOfEmployee xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
    <Employee>First</Employee>
    <Employee>Second</Employee>
    <Employee>Third</Employee>
</ArrayOfEmployee>

如您所见，Person 的 XML 类型名称已经指定成功。

但是，您希望动态覆盖 Person 的 XML 类型名称通过XmlAttributeOverrides而不是将其设置为编译类型。为此，似乎有必要指定一个 XmlSchemaProviderAttribute。里面 XmlAttributes .不幸的是，没有 XmlSchemaProvider在里面找到属性(property) XmlAttributes . Microsoft 似乎从未实现过此类功能。因此，如果你想进一步追求这个设计，你将需要做一些笨拙的事情:暂时覆盖 GetSchemaMethod() 的返回。创建覆盖序列化程序时。需要牢记两点:

1. 在幕后，XmlSerializer通过创建动态程序集来工作。如果你构造一个 XmlSerializer与 new XmlSerializer(Type)或 new XmlSerializer(Type, String) , 然后 .Net 将缓存程序集并在第二次构造序列化程序时重用它。

   因此，试图暂时覆盖 GetSchemaMethod() 的返回使用其中任何一个构建序列化程序时都会失败或产生意外结果。

2. 否则，动态程序集不会被缓存，因此您的代码必须手动缓存序列化程序，否则会出现严重的资源泄漏。参见 Memory Leak using StreamReader and XmlSerializer .

   在这些情况下，暂时覆盖 GetSchemaMethod() 的返回可以工作。

综上所述，以下生成您需要的 XML:

[XmlSchemaProvider("GetSchemaMethod")]
public class Person : IXmlSerializable
{
    // Private state
    private string personName;

    // Constructors
    public Person(string name)
    {
        personName = name;
    }

    public Person()
    {
        personName = null;
    }

    [ThreadStatic]
    static string personXmlTypeName;

    const string defaultXmlTypeName = "Person";

    static string PersonXmlTypeName
    {
        get
        {
            if (personXmlTypeName == null)
                personXmlTypeName = defaultXmlTypeName;
            return personXmlTypeName;
        }
        set
        {
            personXmlTypeName = value;
        }
    }

    public static IDisposable PushXmlTypeName(string xmlTypeName)
    {
        return new PushValue<string>(xmlTypeName, () => PersonXmlTypeName, val => PersonXmlTypeName = val);
    }

    // This is the method named by the XmlSchemaProviderAttribute applied to the type.
    public static XmlQualifiedName GetSchemaMethod(XmlSchemaSet xs)
    {
        string EmployeeSchemaFormat = @"<?xml version=""1.0"" encoding=""utf-16""?>
            <xs:schema elementFormDefault=""qualified"" xmlns:xs=""http://www.w3.org/2001/XMLSchema"">
              <xs:import namespace=""http://www.w3.org/2001/XMLSchema"" />
              <xs:element name=""{0}"" nillable=""true"" type=""{0}"" />
              <xs:complexType name=""{0}"" mixed=""true"">
              <xs:sequence>
                <xs:any />
              </xs:sequence>
              </xs:complexType>
            </xs:schema>";
        var EmployeeSchema = string.Format(EmployeeSchemaFormat, PersonXmlTypeName);

        using (var textReader = new StringReader(EmployeeSchema))
        using (var schemaSetReader = System.Xml.XmlReader.Create(textReader))
        {
            xs.Add("", schemaSetReader);
        }
        return new XmlQualifiedName(PersonXmlTypeName);
    }

    // Xml Serialization Infrastructure
    public void WriteXml(XmlWriter writer)
    {
        writer.WriteString(personName);
    }

    public void ReadXml(XmlReader reader)
    {
        reader.MoveToContent();
        var isEmpty = reader.IsEmptyElement;
        reader.ReadStartElement();
        if (!isEmpty)
        {
            personName = reader.ReadContentAsString();
            reader.ReadEndElement();
        }
    }

    public XmlSchema GetSchema()
    {
        return (null);
    }

    // Print
    public override string ToString()
    {
        return (personName);
    }
}

public struct PushValue<T> : IDisposable
{
    Action<T> setValue;
    T oldValue;

    public PushValue(T value, Func<T> getValue, Action<T> setValue)
    {
        if (getValue == null || setValue == null)
            throw new ArgumentNullException();
        this.setValue = setValue;
        this.oldValue = getValue();
        setValue(value);
    }

    #region IDisposable Members

    // By using a disposable struct we avoid the overhead of allocating and freeing an instance of a finalizable class.
    public void Dispose()
    {
        if (setValue != null)
            setValue(oldValue);
    }

    #endregion
}

public static class PersonEmployeeListSerializerFactory
{
    static Dictionary<Tuple<string, string>, XmlSerializer> serializers;
    static object padlock = new object();

    static PersonEmployeeListSerializerFactory()
    {
        serializers = new Dictionary<Tuple<string, string>, XmlSerializer>();
    }

    public static XmlSerializer GetSerializer(string rootName, string personName)
    {
        lock (padlock)
        {
            XmlSerializer serializer;
            var key = Tuple.Create(rootName, personName);
            if (!serializers.TryGetValue(key, out serializer))
            {
                using (Person.PushXmlTypeName(personName))
                {
                    var lOverrides = new XmlAttributeOverrides();
                    //var lAttributes = new XmlAttributes();
                    //lOverrides.Add(typeof(Person), lAttributes);

                    serializers[key] = serializer = new XmlSerializer(typeof(List<Person>), lOverrides, new Type[0], new XmlRootAttribute(rootName), null);
                }
            }
            return serializer;
        }
    }
}

然后做

var lSerialiser = PersonEmployeeListSerializerFactory.GetSerializer("Employees", "Employee");

var lNamespaces = new XmlSerializerNamespaces();
lNamespaces.Add("", "");

var sb = new StringBuilder();
using (var writer = new StringWriter(sb))
    lSerialiser.Serialize(writer, lPersonList, lNamespaces);

Console.WriteLine(sb);

但如您所见，这比使用最初显示的代理要复杂得多。

样本 fiddle显示两个选项。