2

为了让字典序列化的 XML 更清晰，我编写了一个自定义类，它实现了IXmlSerializable.

我的自定义类是这样定义的：

public class MyCollection : System.Collections.Generic.Dictionary<string, string>, IXmlSerializable
{
    private const string XmlElementName = "MyData";
    private const string XmlAttributeId = "Id";

    public XmlSchema GetSchema()
    {
        return null;
    }

    public void ReadXml(XmlReader reader)
    {
        reader.MoveToContent();
        while (reader.Read())
        {
            if(reader.LocalName == XmlElementName)
            {
                var tag = reader.GetAttribute(XmlAttributeId);
                var content = reader.ReadElementContentAsString(); 

                this.Add(tag, content);
            }
        }
    }

    public void WriteXml(System.Xml.XmlWriter writer)
    {
        foreach (string key in this.Keys)
        {
            writer.WriteStartElement(XmlElementName);
            writer.WriteAttributeString(XmlAttributeId, key);
            writer.WriteString(this[key]);
            writer.WriteEndElement();
        }
    }
}

我的代码适用于这个 XML 片段：

<MyCollection xmlns="http://schemas.datacontract.org/2004/07/MyProject" xmlns:i="http://www.w3.org/2001/XMLSchema-instance">
    <MyData Id="1">some content</MyData>
    <MyData Id="2">some other content</MyData>
</MyCollection>

但是，当我拥有这个缩小的 XML 时，我的代码会引发异常：

<MyCollection xmlns="http://schemas.datacontract.org/2004/07/MyProject" xmlns:i="http://www.w3.org/2001/XMLSchema-instance"><MyData Id="1">some content </MyData><MyData Id="2">some other content</MyData></MyCollection>

例外是：

System.InvalidOperationException: The ReadElementContentAsString method is not supported on node type EndElement

它在调用ReadElementContentAsString.

如何修复我的代码？

我可以使用以下方法重现问题：

var xml = @"<MyCollection xmlns=""http://schemas.datacontract.org/2004/07/MyProject"" xmlns:i=""http://www.w3.org/2001/XMLSchema-instance""><MyData Id=""1"">some content </MyData><MyData Id=""2"">some other content</MyData></MyCollection>";

var raw = Encoding.UTF8.GetBytes(xml);

var serializer = new DataContractSerializer(typeof(MyCollection));

using (var ms = new MemoryStream(raw))
{
    var result = serializer.ReadObject(ms); // Exception throws here
}
4

1 回答 1

2

您的问题是reader.ReadElementContentAsString()将阅读器定位在下一个节点的开头，而不是当前节点的结尾。然后，您随后的无条件调用会reader.Read()消耗下一个节点。当该节点是空白时，不会造成任何伤害，但是当该节点是一个元素时，将跳过该元素。

您的以下版本MyCollection修复了此问题：

public class MyCollection : System.Collections.Generic.Dictionary<string, string>, IXmlSerializable
{
    public XmlSchema GetSchema()
    {
        return null;
    }

    public void ReadXml(XmlReader reader)
    {
        using (var subReader = reader.ReadSubtree())
        {
            XmlKeyValueListHelper.ReadKeyValueXml(subReader, this);
        }
        // Consume the EndElement also (or move past the current element if reader.IsEmptyElement).
        reader.Read();
    }

    public void WriteXml(System.Xml.XmlWriter writer)
    {
        XmlKeyValueListHelper.WriteKeyValueXml(writer, this);
    }
}

public static class XmlKeyValueListHelper
{
    private const string XmlElementName = "MyData";
    private const string XmlAttributeId = "Id";

    public static void WriteKeyValueXml(System.Xml.XmlWriter writer, ICollection<KeyValuePair<string, string>> collection)
    {
        foreach (var pair in collection)
        {
            writer.WriteStartElement(XmlElementName);
            writer.WriteAttributeString(XmlAttributeId, pair.Key);
            writer.WriteString(pair.Value);
            writer.WriteEndElement();
        }
    }

    public static void ReadKeyValueXml(System.Xml.XmlReader reader, ICollection<KeyValuePair<string, string>> collection)
    {
        if (reader.IsEmptyElement)
        {
            reader.Read();
            return;
        }

        reader.ReadStartElement(); // Advance to the first sub element of the list element.
        while (reader.NodeType != XmlNodeType.EndElement)
        {
            if (reader.NodeType == XmlNodeType.Element && reader.LocalName == XmlElementName)
            {
                var tag = reader.GetAttribute(XmlAttributeId);
                string content;
                if (reader.IsEmptyElement)
                {
                    content = string.Empty;
                    // Move past the end of item element
                    reader.Read();
                }
                else
                {
                    // Read content and move past the end of item element
                    content = reader.ReadElementContentAsString();
                }
                collection.Add(new KeyValuePair<string, string>(tag, content));
            }
            else
            {
                // For instance a comment.
                reader.Skip();
            }
        }
        // Move past the end of the list element
        reader.ReadEndElement();
    }
}

一些注意事项：

  • 通过使用XmlReader.ReadSubtree()，我确保ReadXml()不会读到元素的末尾，MyCollection从而破坏未来的元素——在实现时很容易犯错误IXmlSerializable。

  • 通过检查reader.NodeType == XmlNodeType.Element && reader.LocalName == XmlElementName我忽略了意外类型的节点，例如评论。

工作.Net 小提琴。

于 2017-08-23T16:47:05.547 回答