C#XML序列化覆盖IXmlSerializable类的类型

最初的问题

我想序列化List< IXmlSerializable>动态更改IXmlSerializable类的XmlType(所以我不能使用attribute标记来做到这一点)

到目前为止,我尝试使用XmlAttributeOverrides来完成此操作,但没有成功.

这是说明问题的示例代码:
IXmlSerializable类(从MSDN开始):

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(使用控制台进行输出):

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>

但是我在运行时收到此错误:

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类未实现IXmlSerializable时,一切正常.

有人可以帮我吗?

选择的解决方案(基于@dbc answer)

正如@dbc所指出的,使用“代理”类是完成我想要的最简单的方法.但是正如我所说,我需要动态更改Person类型,这意味着我不能使用attribute标签.
所以我仍然在最终设计中使用XmlAttributeOverrides,这里是:

代理名单<人>类(与不带属性标签的@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 };
    }
}

使用替代测试类:

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);
    }
}

最后,我要非常感谢@dbc,您的回答非常有帮助,而且内容丰富,我学到了很多东西.我不能支持你,但我希望社区能够做到!

最佳答案
获得所需XML的最简单方法是序列化“代理”类,如下所示:

[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,您可以将XmlAttributeOverrides与XmlAttributes.XmlElements一起使用,以动态指定EmployeeList的XML名称.

就是说,当您尝试将[XmlType]应用于还实现IXmlSerializable的类型时引发InvalidOperationException的原因是XmlSerializer要求通过完全不同的机制(即在[XmlSchemaProvider]属性中指定的XmlSchemaProviderAttribute.MethodName方法)返回类型名称. .

将[XmlSchemaProvider]应用于IXmlSerializable类型时,XmlSerializer将查找其名称在属性构造函数中指定且具有以下签名的该类型的公共静态方法:

    public static XmlQualifiedName GetSchemaMethod(XmlSchemaSet xs)
    {
    }

此方法的目的是双重的:

>在序列化类型的实例时,应使用所需的模式填充XmlSchemaSet.通过测试,我发现它必须填充一些有效的东西.不能仅将其留空,否则将引发异常.

(我不知道XmlSerializer在序列化时实际上对架构进行验证的程度.通过xsd.exe导出架构信息时也会调用该方法.)
>它应返回该类型的XML类型名称.

这似乎就是Microsoft引发您所看到的异常的原因:由于架构属性提供程序应返回类型名称,因此XmlType属性将发生冲突.

因此,如果我将您的Person类修改如下:

[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,已成功指定Person的XML类型名称.

但是,您想通过XmlAttributeOverrides动态覆盖Person的XML类型名称,而不是将其设置为编译类型.为此,似乎有必要在XmlAttributes中指定XmlSchemaProviderAttribute.不幸的是,在XmlAttributes中找不到XmlSchemaProvider属性.看来Microsoft从未实现过这种功能.因此,如果您想进一步进行这种设计,则需要做一些麻烦的事情:在创建替代序列化器时,临时替代GetSchemaMethod()的返回值.请记住两件事:

>在后台,XmlSerializer通过创建动态程序集来工作.如果使用新的XmlSerializer(Type)或新的XmlSerializer(Type,String)构造XmlSerializer,则.Net将缓存程序集并在第二次构造序列化器时重用它.

因此,在使用这两种方法构造序列化程序时,尝试暂时覆盖GetSchemaMethod()的返回将失败或产生意外的结果.
>否则,动态程序集将不会被缓存,因此您的代码必须手动缓存序列化程序,否则会造成严重的资源泄漏.参见Memory Leak using StreamReader and XmlSerializer.

在这些情况下,暂时覆盖GetSchemaMethod()的返回可能有效.

综上所述,下面将生成您需要的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);

但是正如您所看到的,这比使用最初显示的代理要复杂得多.

示例fiddle显示了这两个选项.

转载注明原文:C#XML序列化覆盖IXmlSerializable类的类型 - 代码日志