[英]XmlSerializer filter by attribute
When deserializing an XML to an entity class using XmlSerializer, is it possible to filter by an attribute?使用 XmlSerializer 将 XML 反序列化为实体 class 时,是否可以按属性进行过滤? For example, let's say I have an item which can be of type "a" or type "b".
例如,假设我有一个可以是“a”类型或“b”类型的项目。 I want to deserialize all items but only those of type "a".
我想反序列化所有项目,但只有“a”类型的项目。
I need this because my real situation is that our endpoint receives very big XMLs (some can be upwards of 100MB) with hundreds of thousands of tags of type <item>
but I need only some of them - those of type "a".我需要这个,因为我的实际情况是我们的端点接收非常大的 XML(有些可能超过 100MB),其中包含数十万个
<item>
类型的标签,但我只需要其中的一些 - “a”类型的标签。 I want to avoid allocations for the rest (including their child tags which are not few).我想避免分配给 rest(包括他们的子标签,这些标签并不多)。
Example XML:示例 XML:
<root>
<item type="a"/>
<item type="a"/>
<item type="b"/>
<item type="c"/>
</root>
Entities:实体:
[XmlRoot("root")]
public class Root {
[XmlElement("item")]
public Item[] Items { get; set; }
}
public class Item {
[XmlAttribute("type")]
[DeserializeIfValueIs("a")] // <-- Is there something like this?
public string Type { get; set; }
}
Code:代码:
var serializer = new XmlSerializer(typeof(Root));
var dto = (Root) serializer.Deserialize(XmlReader.Create("input.xml"));
// Show the results - {"Items":[{"Type":"a"},{"Type":"a"},{"Type":"b"},{"Type":"c"}]}
Console.WriteLine(JsonConvert.SerializeObject(dto));
How do I make it allocate objects only for type "a" items?如何让它只为“a”类型的项目分配对象?
Obligatory note: This is neither an XY problem nor premature optimization.强制性说明:这既不是 XY 问题,也不是过早优化。 We have identified that we need to improve performance in this with profiling and so on.
我们已经确定我们需要通过分析等来提高这方面的性能。 Also filtering out the values post-deserialization doesn't help - by that time the allocations have already been made and will have to be garbage-collected.
同样过滤掉反序列化后的值也无济于事 - 到那时分配已经完成并且必须进行垃圾收集。
This is possible by handling the de-serialization process ourselves (at least for the root class)这可以通过我们自己处理反序列化过程来实现(至少对于根类)
Please let me remind you that the XML content you provided is insufficient to run unit tests on, so this is a very basic implementation which, however, should work for you directly or by just tweaking a little bit over here and there.请让我提醒您,您提供的 XML 内容不足以运行单元测试,所以这是一个非常基本的实现,但是应该直接为您工作,或者只是在这里和那里稍微调整一下。
First of all, we change our Item class XML serialization attribute to root.首先,我们将 Item class XML 序列化属性更改为 root。 The "Why" will be answered soon.
“为什么”很快就会得到回答。
[XmlRoot("item")]
public class Item
{
[XmlAttribute("type")]
public string Type { get; set; }
[XmlElement("prop1")]
public int Prop1 { get; set; }
}
I've also added a simple integer property to prove that the deserialization works as expected.我还添加了一个简单的 integer 属性来证明反序列化按预期工作。
I also changed the XML content to match the new type, for testing.我还更改了 XML 内容以匹配新类型,以进行测试。
<root>
<item type="b">
<prop1>5</prop1>
</item>
<item type="a">
<prop1>5</prop1>
</item>
<item type="a">
<prop1>5</prop1>
</item>
<item type="b">
<prop1>5</prop1>
</item>
<item type="c">
<prop1>5</prop1>
</item>
</root>
And now comes the Root class, which implements IXmlSerializable explicitly now:现在出现了 Root class,它现在显式地实现了 IXmlSerializable:
[XmlRoot("root")]
public class Root : IXmlSerializable
{
[XmlElement("item")]
public Item[] Items { get; set; }
// These two methods are not implemented for you need to deserialize only,
// and because you haven't provided the schema for your XML content
System.Xml.Schema.XmlSchema IXmlSerializable.GetSchema() { throw new NotImplementedException(); }
void IXmlSerializable.WriteXml(System.Xml.XmlWriter writer) { throw new NotImplementedException(); }
void IXmlSerializable.ReadXml(System.Xml.XmlReader reader)
{
// The element is <root> when here for the first time.
// Maintain a list to keep items with type "a"
List<Item> typeAItems = new List<Item>();
// Create a serializer for the type Item
XmlSerializer deserializer = new XmlSerializer(typeof(Item));
while (reader.Read())
{
// The code is self explanatory.
// Skip() will help omitting unnecessary reads
// if we are not interested in the Item
if (reader.IsStartElement() && reader.Name == "item")
{
if (reader.GetAttribute("type") == "a")
{
// This works, and deserializes the current node
// into an Item object. When the deserialization
// is completed, the reader is at the beginning
// of the next <Item> element
typeAItems.Add((Item)deserializer.Deserialize(reader));
}
else
{
// skip element with all its children
reader.Skip();
}
}
else
{
// skip element with all its children
reader.Skip();
}
}
Items = typeAItems.ToArray();
}
}
The deserialization logic is kept the same, like new XmlSerializer(typeof(Root)).Deserialize().反序列化逻辑保持不变,如 new XmlSerializer(typeof(Root)).Deserialize()。
The rest.. is to test. rest.. 用于测试。
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