如何使用 XmlSerializer 使用 DefaultValueAttribute 序列化属性？

Question

我正在使用 XmlSerializer 将 C# 对象序列化为 XML。我在类的某些属性上有 DefaultValueAttribute 我正在尝试序列化，当我尝试序列化它们时，似乎 XmlSerializer 不包含 xml 中的值，如果它等于默认值。看这个例子：

using System.IO;
using System.Xml;
using System.Xml.Serialization;

namespace Test
{
    public class Person
    {
        [System.Xml.Serialization.XmlAttribute()]
        [System.ComponentModel.DefaultValue("John")]
        public string Name { get; set; }
    }

    public static class Test
    {
        public static void Main()
        {
            var serializer = new XmlSerializer(typeof(Person));
            var person = new Person { Name = "John" };

            using (var sw = new StringWriter())
            {
                using (var writer = XmlWriter.Create(sw))
                {
                    serializer.Serialize(writer, person);
                    var xml = sw.ToString();
                }
            }
        }
    }
}

它将产生以下内容xml（注意名称属性不可用）：

<?xml version="1.0" encoding="utf-16"?>
<Person xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" />

我无法修改类的源代码，所以我无法删除 DefaultValueAttribute。有没有办法在不更改源代码的情况下使 XmlSerializer 序列化此属性？

Answer 1

您可以在创建时将 XmlAttributeOverrides 实例传递给 XmlSerializer 来实现，如下所示。您可以使用此方法将默认值更改为其他值，或将其设置为 null 以有效删除它。

XmlAttributeOverrides attributeOverrides = new XmlAttributeOverrides();

var attributes = new XmlAttributes()
{
    XmlDefaultValue = null,
    XmlAttribute = new XmlAttributeAttribute()
};

attributeOverrides.Add(typeof(Person), "Name", attributes);

var serializer = new XmlSerializer(typeof(Person), attributeOverrides);
var person = new Person { Name = "John" };

using (var sw = new StringWriter())
{
    using (var writer = XmlWriter.Create(sw))
    {
        serializer.Serialize(writer, person);
        var xml = sw.ToString();
    }
}

更新：以上意味着您必须在每个要更改的属性上再次提供其他不相关的属性。如果您有很多属性并且只想删除所有属性的默认值，这会有点麻烦。下面的 class 可用于保留其他自定义属性，同时仅删除一种类型的属性。如果需要只对某些属性等进行扩展，它可以进一步扩展。

public class XmlAttributeOverrideGenerator<T>
{
    private static XmlAttributeOverrides _overrides;
    private static Type[] _ignoreAttributes = new Type[] { typeof(DefaultValueAttribute) };

    static XmlAttributeOverrideGenerator()
    {
        _overrides = Generate();
    }

    public static XmlAttributeOverrides Get()
    {
        return _overrides;
    }

    private static XmlAttributeOverrides Generate()
    {
        var xmlAttributeOverrides = new XmlAttributeOverrides();

        Type targetType = typeof(T);
        foreach (var property in targetType.GetProperties())
        {
            XmlAttributes propertyAttributes = new XmlAttributes(new CustomAttribProvider(property, _ignoreAttributes));
            xmlAttributeOverrides.Add(targetType, property.Name, propertyAttributes);
        }

        return xmlAttributeOverrides;
    }

    public class CustomAttribProvider : ICustomAttributeProvider
    {
        private PropertyInfo _prop = null;
        private Type[] _ignoreTypes = null;            

        public CustomAttribProvider(PropertyInfo property, params Type[] ignoreTypes)
        {
            _ignoreTypes = ignoreTypes;
            _prop = property;
        }

        public object[] GetCustomAttributes(bool inherit)
        {
            var attribs = _prop.GetCustomAttributes(inherit);
            if (_ignoreTypes == null) return attribs;
            return attribs.Where(attrib => IsAllowedType(attrib)).ToArray();
        }

        private bool IsAllowedType(object attribute)
        {
            if (_ignoreTypes == null) return true;
            foreach (Type type in _ignoreTypes)
                if (attribute.GetType() == type)
                    return false;

            return true;
        }

        public object[] GetCustomAttributes(Type attributeType, bool inherit)
        {
            throw new NotImplementedException();
        }

        public bool IsDefined(Type attributeType, bool inherit)
        {
            throw new NotImplementedException();
        }
    }
}

用法：

XmlAttributeOverrides attributeOverrides = XmlAttributeOverrideGenerator<Person>.Get();            
var serializer = new XmlSerializer(typeof(Person), attributeOverrides);

Answer 2

我没有足够的声誉来评论 steve16351 的回答。但我觉得我对他的代码做了一点改进。

我的用例涉及通过 Microsoft 提供的 XSD.exe 生成的 XML 架构文件，然后从架构文件生成 class。所以 class 上有架构中的所有 DefaultValue 标签。还创建了许多嵌套类型。因此，为了使我的代码简洁，我修改了 'Generate' 方法以获取 top-level class 及其下方的所有方法的覆盖。然后所有内容都在单个 XmlAttributesOverrides 对象中返回。

static XmlAttributeOverrideGenerator()
{
    _overrides = Generate(typeof(T), new XmlAttributeOverrides());
}

private static XmlAttributeOverrides Generate(Type targetType, XmlAttributeOverrides Overrides)
{
    foreach (var property in targetType.GetProperties())
    {
        if (property.CustomAttributes.Any( CA => CA.AttributeType == typeof(DefaultValueAttribute)))
        {
            XmlAttributes propertyAttributes = new XmlAttributes(new CustomAttribProvider(property, _ignoreAttributes));
            Overrides.Add(targetType, property.Name, propertyAttributes);
        }
        else
        {
            Type propType = property.PropertyType;
            if (propType.IsClass && !propType.IsValueType && !propType.IsEnum && !propType.IsPrimitive && !propType.IsSealed)  // Check if this is a custom class
            {
                //If this is a nested class or other class type, generate its overrides as well
                Generate(propType, Overrides);
            }
        }
    }

    return Overrides;
}

如何使用 XmlSerializer 使用 DefaultValueAttribute 序列化属性？

How to serialize properties with DefaultValueAttribute using XmlSerializer?

c#

xml

attributes

xmlwriter

xmlserializer