how to derive xml element name from an attribute value of a class using annotations?

Question

I have properties that have ids and values and a name. Can I represent all those with a single class using XmlElement/XmlArray C# annotations? I would like to derive the xml element name from the class attribute name;

my class would look like:

public class Property {
   public string name; //could be enum
   public int id; 
   public string value;
}

e.g:

new Property("property1name",2,"testvalue");
new Property("property2name",10,"anothervalue");

I would like to have xml that looks like:

<property1name><id>2</id><value>testvalue</value></property1name>
<property2name><id>10</id><value>anothervalue</value></property2name>

instead of the usual

<property><name>property1name</name><id>2</id><value>testvalue</value></property>
<property><name>property2name</name><id>10</id><value>anothervalue</value></property>

In other words the xmlelement gets its name from attribute name of the class Property

if there are 100 properties that just differ by name. I wouldn't want to have 100 classes for those just to change the name of the element. I know that could make things difficult for deserialization but I wonder if my requirement could be tackled somehow — Michail Michailidis
– Michail Michailidis, Commented Feb 27, 2015 at 16:31
Why not put the name as an attribute if you don't want is as an element? [XmlAttribute("name")] — BLoB
– BLoB, Commented Feb 27, 2015 at 16:33
If these are in a list, you might try something like this: stackoverflow.com/questions/28587155/… — dbc
– dbc, Commented Feb 27, 2015 at 16:35
There's no way to do this with XmlSerializer using just attributes. You would need to implement IXmlSerializable on a custom collection. — dbc
– dbc, Commented Feb 27, 2015 at 16:37

dbc · Accepted Answer · 2015-02-27 18:50:22Z

Update

And here's a quick adaptation to handle your Property class. First, a List<T> subclass that implements IXmlSerializable:

public interface IHasElementName
{
    string ElementName { get; set; }
}

public class XmlNamedElementList<T> : List<T>, IXmlSerializable where T : IHasXmlElementName
{
    // https://msdn.microsoft.com/en-us/library/System.Xml.Serialization.XmlSerializer%28v=vs.110%29.aspx
    // Any serializer created with the "XmlSerializer(typeof(T), rootAttribute)" must be cached
    // to avoid resource & memory leaks.
    class ValueSerializerCache
    {
        // By using a nested class with a static constructor, we defer generation of the XmlSerializer until it's actually required.
        static ValueSerializerCache()
        {
            var rootAttribute = new XmlRootAttribute();
            rootAttribute.ElementName = ValueTypeName;
            rootAttribute.Namespace = ValueTypeNamespace;
            serializer = new XmlSerializer(typeof(T), rootAttribute);
        }

        static readonly XmlSerializer serializer;

        internal static XmlSerializer Serializer { get { return serializer; } }
    }

    static string ValueTypeName { get { return typeof(T).DefaultXmlElementName(); } }

    static string ValueTypeNamespace { get { return typeof(T).DefaultXmlElementNamespace(); } }

    #region IXmlSerializable Members

    XmlSchema IXmlSerializable.GetSchema()
    {
        return null;
    }

    void IXmlSerializable.ReadXml(XmlReader reader)
    {
        if (reader.IsEmptyElement)
        {
            reader.Read();
            return;
        }

        var typeName = ValueTypeName;
        reader.ReadStartElement(); // Advance to the first sub element of the list element.
        while (reader.NodeType == XmlNodeType.Element)
        {
            var name = reader.Name;
            using (var subReader = reader.ReadSubtree())
            {
                var doc = XDocument.Load(subReader);
                if (doc != null && doc.Root != null)
                {
                    doc.Root.Name = doc.Root.Name.Namespace + typeName;
                    using (var docReader = doc.CreateReader())
                    {
                        var obj = ValueSerializerCache.Serializer.Deserialize(docReader);
                        if (obj != null)
                        {
                            T value = (T)obj;
                            value.ElementName = XmlConvert.DecodeName(name);
                            Add(value);
                        }
                    }
                }
            }
            // Move past the end of item element
            reader.Read();
        }
        // Move past the end of the list element
        reader.ReadEndElement();
    }

    void IXmlSerializable.WriteXml(XmlWriter writer)
    {
        foreach (var value in this)
        {
            XDocument doc = new XDocument();
            using (var subWriter = doc.CreateWriter())
            {
                // write xml into the writer
                ValueSerializerCache.Serializer.Serialize(subWriter, value);
            }

            if (doc.Root == null)
                continue;
            doc.Root.Name = doc.Root.Name.Namespace + XmlConvert.EncodeName(value.ElementName);
            // Remove redundant namespaces.
            foreach (var attr in doc.Root.Attributes().ToList())
            {
                if (!attr.IsNamespaceDeclaration || string.IsNullOrEmpty(attr.Value))
                    continue;
                var prefix = writer.LookupPrefix(attr.Value);
                if ((prefix == attr.Name.LocalName)
                    || (prefix == string.Empty && attr.Name == "xmlns"))
                    attr.Remove();
            }

            doc.Root.WriteTo(writer);
        }
    }

    #endregion
}

public static class XmlSerializationHelper
{
    static Attribute GetCustomAttribute(MemberInfo element, Type attributeType)
    {
        return Attribute.GetCustomAttribute(element, attributeType);
    }

    static T GetCustomAttribute<T>(MemberInfo element) where T : Attribute
    {
        return (T)GetCustomAttribute(element, typeof(T));
    }

    public static string DefaultXmlElementName(this Type type)
    {
        var xmlType = GetCustomAttribute<XmlTypeAttribute>(type);
        if (xmlType != null && !string.IsNullOrEmpty(xmlType.TypeName))
            return xmlType.TypeName;
        var xmlRoot = GetCustomAttribute<XmlRootAttribute>(type);
        if (xmlRoot != null && !string.IsNullOrEmpty(xmlRoot.ElementName))
            return xmlRoot.ElementName;
        return type.Name;
    }

    public static string DefaultXmlElementNamespace(this Type type)
    {
        var xmlType = GetCustomAttribute<XmlTypeAttribute>(type);
        if (xmlType != null && !string.IsNullOrEmpty(xmlType.Namespace))
            return xmlType.Namespace;
        var xmlRoot = GetCustomAttribute<XmlRootAttribute>(type);
        if (xmlRoot != null && !string.IsNullOrEmpty(xmlRoot.Namespace))
            return xmlRoot.Namespace;
        return string.Empty;
    }

    public static string GetXml<T>(this T obj)
    {
        return GetXml(obj, false);
    }

    public static string GetXml<T>(this T obj, bool omitNamespace)
    {
        return GetXml(obj, new XmlSerializer(obj.GetType()), omitNamespace);
    }

    public static string GetXml<T>(this T obj, XmlSerializer serializer)
    {
        return GetXml(obj, serializer, false);
    }

    public static string GetXml<T>(T obj, XmlSerializer serializer, bool omitStandardNamespaces)
    {
        XmlSerializerNamespaces ns = null;
        if (omitStandardNamespaces)
        {
            ns = new XmlSerializerNamespaces();
            ns.Add("", ""); // Disable the xmlns:xsi and xmlns:xsd lines.
        }
        return GetXml(obj, serializer, ns);
    }

    public static string GetXml<T>(T obj, XmlSerializerNamespaces ns)
    {
        return GetXml(obj, new XmlSerializer(obj.GetType()), ns);
    }

    public static string GetXml<T>(T obj, XmlSerializer serializer, XmlSerializerNamespaces ns)
    {
        using (var textWriter = new StringWriter())
        {
            XmlWriterSettings settings = new XmlWriterSettings();
            settings.Indent = true;        // For cosmetic purposes.
            settings.IndentChars = "    "; // For cosmetic purposes.
            using (var xmlWriter = XmlWriter.Create(textWriter, settings))
            {
                if (ns != null)
                    serializer.Serialize(xmlWriter, obj, ns);
                else
                    serializer.Serialize(xmlWriter, obj);
            }
            return textWriter.ToString();
        }
    }
}

And use it like:

public class Property : IHasElementName
{
    public Property()
    {
    }

    public Property(string name, int id, string value)
    {
        this.name = name;
        this.id = id;
        this.value = value;
    }

    [XmlIgnore]
    public string name; //could be enum

    public int id;
    public string value;

    #region IHasElementName Members

    [XmlIgnore]
    string IHasElementName.ElementName { get { return name; }  set { name = value; } }

    #endregion
}

public class RootObject
{
    public RootObject()
    {
        this.Properties = new XmlNamedElementList<Property>();
    }

    public XmlNamedElementList<Property> Properties { get; set; }
}

public static class TestClass
{
    public static void Test()
    {
        var root = new RootObject
        {
            // Characters " <> first" in the first element name are for testing purposes.

            Properties = new XmlNamedElementList<Property> { new Property { id = 1, value = "1", name = "first" }, new Property("property1name", 2, "testvalue"), new Property("property2name", 10, "anothervalue") }
        };

        var xml = root.GetXml();
        Debug.WriteLine(xml);
    }
}

Which produces XML as follows:

<?xml version="1.0" encoding="utf-16"?>
<RootObject xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
    <Properties>
        <_x0020__x003C__x003E__x0020_first>
            <id>1</id>
            <value>1</value>
        </_x0020__x003C__x003E__x0020_first>
        <property1name>
            <id>2</id>
            <value>testvalue</value>
        </property1name>
        <property2name>
            <id>10</id>
            <value>anothervalue</value>
        </property2name>
    </Properties>
</RootObject>

Original Answer

As requested, here's an implementation of IXmlSerializable on a List<KeyValuePair<string, T>> in which the Key string becomes the element name in the collection.

What you would probably want to do is to adapt this to serialize a List<IHasElementName> where:

public interface IHasElementName
{
    string ElementName { get; set; }
}

public class Property : IHasElementName
{
    [XmlIgnore]
    public string name; //could be enum

    public int id;
    public string value;

    #region IHasElementName Members

    [XmlIgnore]
    string IHasElementName.ElementName
    {
        get
        {
            return name;
        }
        set
        {
            name = value;
        }
    }

    #endregion
}

If the name is actually an Enum, you could return the enum string representation from HasElementName.ElementName.

The list looks like:

public class XmlKeyValueList<T> : List<KeyValuePair<string, T>>, IXmlSerializable
{
    // TODO: validate that the "Key" string using XmlConvert.VerifyName.

    // https://msdn.microsoft.com/en-us/library/System.Xml.Serialization.XmlSerializer%28v=vs.110%29.aspx
    // Any serializer created with the "XmlSerializer(typeof(T), rootAttribute)" must be cached
    // to avoid resource & memory leaks.
    class ValueSerializerCache
    {
        // By using a nested class with a static constructor, we defer generation of the XmlSerializer until it's actually required.
        static ValueSerializerCache()
        {
            var rootAttribute = new XmlRootAttribute();
            rootAttribute.ElementName = ValueTypeName;
            rootAttribute.Namespace = ValueTypeNamespace;
            serializer = new XmlSerializer(typeof(T), rootAttribute);
        }

        static readonly XmlSerializer serializer;

        internal static XmlSerializer Serializer { get { return serializer; } }
    }

    static string ValueTypeName { get { return typeof(T).DefaultXmlElementName(); } }

    static string ValueTypeNamespace { get { return typeof(T).DefaultXmlElementNamespace(); } }

    #region IXmlSerializable Members

    XmlSchema IXmlSerializable.GetSchema()
    {
        return null;
    }

    void IXmlSerializable.ReadXml(XmlReader reader)
    {
        var typeName = ValueTypeName;

        reader.ReadStartElement(); // Advance to the first sub element of the list element.
        while (reader.NodeType == XmlNodeType.Element)
        {
            var name = reader.Name;
            using (var subReader = reader.ReadSubtree())
            {
                var doc = XDocument.Load(subReader);
                if (doc != null && doc.Root != null)
                {
                    doc.Root.Name = typeName;
                    using (var docReader = doc.CreateReader())
                    {
                        var obj = ValueSerializerCache.Serializer.Deserialize(docReader);
                        if (obj != null)
                        {
                            Add(new KeyValuePair<string, T>(name, (T)obj));
                        }
                    }
                }
            }
            // Move past the XmlNodeType.Element
            if (reader.NodeType == XmlNodeType.EndElement)
                reader.Read();
        }
    }

    void IXmlSerializable.WriteXml(XmlWriter writer)
    {
        foreach (var pair in this)
        {
            XDocument doc = new XDocument();
            using (var subWriter = doc.CreateWriter())
            {
                // write xml into the writer
                ValueSerializerCache.Serializer.Serialize(subWriter, pair.Value);
            }
            if (doc.Root == null)
                continue;
            doc.Root.Name = pair.Key;
            // Remove redundant namespaces.
            foreach (var attr in doc.Root.Attributes().ToList())
            {
                if (!attr.IsNamespaceDeclaration || string.IsNullOrEmpty(attr.Value))
                    continue;
                if (writer.LookupPrefix(attr.Value) == attr.Name.LocalName)
                    attr.Remove();
            }

            doc.Root.WriteTo(writer);
        }
    }

    #endregion
}

public static class XmlSerializationHelper
{
    public static string DefaultXmlElementName(this Type type)
    {
        var xmlType = type.GetCustomAttribute<XmlTypeAttribute>();
        if (xmlType != null && !string.IsNullOrEmpty(xmlType.TypeName))
            return xmlType.TypeName;
        var xmlRoot = type.GetCustomAttribute<XmlRootAttribute>();
        if (xmlRoot != null && !string.IsNullOrEmpty(xmlRoot.ElementName))
            return xmlRoot.ElementName;
        return type.Name;
    }

    public static string DefaultXmlElementNamespace(this Type type)
    {
        var xmlType = type.GetCustomAttribute<XmlTypeAttribute>();
        if (xmlType != null && !string.IsNullOrEmpty(xmlType.Namespace))
            return xmlType.Namespace;
        var xmlRoot = type.GetCustomAttribute<XmlRootAttribute>();
        if (xmlRoot != null && !string.IsNullOrEmpty(xmlRoot.Namespace))
            return xmlRoot.Namespace;
        return string.Empty;
    }
}

@MichailMichailidis - Yes, because the XmlIgnore applied to the name attribute will prevent the element name from being serialized standalone.
@MichailMichailidis - woops, those are from XmlSerializationHelper in the original answer. Will edit.
@MichailMichailidis - GetXml() just some extension method I use for testing, that generates an XML string from an object. Presumably you already have something like that but I could add it if you want.
@MichailMichailidis - type.GetCustomAttribute<XmlTypeAttribute>() is a standard .Net extension: msdn.microsoft.com/en-us/library/hh194292%28v=vs.110%29.aspx . I added the GetXml() methods.
@MichailMichailidis - OK, I eliminated the use of the .Net 4.5 extension. I also fixed a couple bugs with namespaces in XmlNamedElementList so you should recopy that.

Redwerk · Accepted Answer · 2016-01-04 14:18:52Z

1

If UnitItem is changed into

  public class UnitItem
    {
        public string AAA;
        public string BBB;
    }

then the XML will be

<Items>
   <UnitItem>
         <AAA>testa</AAA>
         <BBB>testb</BBB>
   </UnitItem>
</Items>

answered Jan 4, 2016 at 14:18

Redwerk

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