XmlSerializer attribute namespace for element type

Question

I'm looking for the XmlSerializer functionality to re-create some namespace/type info in my output XML.

So I have to replicate XML output like this to an old COM application:

<Amount dt:dt="int">500</Amount>  
<StartTime dt:dt="dateTime">2014-12-30T12:00:00.000</StartTime>      

I currently set the attributes of my properties like so:

[XmlElement(ElementName = "Amount", Namespace = "urn:schemas-microsoft-com:datatypes",  
DataType = "int", Type = typeof(int))]  
public int Amount{ get; set; }  

With my XmlSerializer and Namespaces set like this:

XmlSerializerNamespaces namespaces = new XmlSerializerNamespaces();
namespaces.Add("dt", "urn:schemas-microsoft-com:datatypes");
s.Serialize(writer, group, namespaces);

But this only gives me output like:

<dt:Amount>500</dt:Amount> 

Anyone have an idea where I'm going wrong?

Answer 1

The XmlElementAttribute.Namespace specifies the namespace of the element itself, not an attribute of the element. That's why you are seeing the dt: prefix. And the DataType = "int" isn't helping you here; it's for specifying the type of polymorphic fields, and won't auto-generate a dt data type attribute. In fact, there's no built-in functionality in XmlSerializer to auto-generate an XDR data type attribute values, which belong in the namespace "urn:schemas-microsoft-com:datatypes".

Thus, it's necessary to do it manually, using a wrapper struct with the necessary attribute. The following implementation is typed rather than polymorphic:

public struct XdrTypeWrapper<T>
{
    class XdrTypeWrapperTypeDictionary
    {
        static XdrTypeWrapperTypeDictionary instance;

        static XdrTypeWrapperTypeDictionary() { instance = new XdrTypeWrapper<T>.XdrTypeWrapperTypeDictionary(); }

        public static XdrTypeWrapperTypeDictionary Instance { get { return instance; } }

        readonly Dictionary<Type, string> dict;

        XdrTypeWrapperTypeDictionary()
        {
            // Taken from https://msdn.microsoft.com/en-us/library/ms256121.aspx
            // https://msdn.microsoft.com/en-us/library/ms256049.aspx
            // https://msdn.microsoft.com/en-us/library/ms256088.aspx
            dict = new Dictionary<Type, string>
            {
                { typeof(string), "string" },
                { typeof(sbyte), "i1" },
                { typeof(byte), "u1" },
                { typeof(short), "i2" },
                { typeof(ushort), "u2" },
                { typeof(int), "int" }, // Could have used i4
                { typeof(uint), "ui4" },
                { typeof(long), "i8" },
                { typeof(ulong), "ui8" },
                { typeof(DateTime), "dateTime" },
                { typeof(bool), "boolean" },
                { typeof(double), "float" }, // Could have used r8
                { typeof(float), "r4" },
            };
        }

        public string DataType(Type type)
        {
            return dict[type];
        }
    }

    public XdrTypeWrapper(T value) { this.value = value; }

    public static implicit operator XdrTypeWrapper<T>(T value) { return new XdrTypeWrapper<T>(value); }

    public static implicit operator T(XdrTypeWrapper<T> wrapper) { return wrapper.Value; }

    [XmlAttribute("dt", Namespace = "urn:schemas-microsoft-com:datatypes")]
    public string DataType
    {
        get
        {
            return XdrTypeWrapperTypeDictionary.Instance.DataType(typeof(T));
        }
        set
        {
            // Do nothing.
        }
    }

    T value;

    [XmlText]
    public T Value { get { return value; } set { this.value = value; } }
}

Then use it in your classes with proxy properties as follows:

public class TestClass
{
    [XmlIgnore]
    public int Amount { get; set; }

    [XmlElement("Amount")]
    [Browsable(false), EditorBrowsable(EditorBrowsableState.Never), DebuggerBrowsable(DebuggerBrowsableState.Never)]
    public XdrTypeWrapper<int> XmlAmount { get { return Amount; } set { Amount = value; } }

    [XmlIgnore]
    public DateTime StartTime { get; set; }

    [XmlElement("StartTime")]
    [Browsable(false), EditorBrowsable(EditorBrowsableState.Never), DebuggerBrowsable(DebuggerBrowsableState.Never)]
    public XdrTypeWrapper<DateTime> XmlStartTime { get { return StartTime; } set { StartTime = value; } }

    [XmlIgnore]
    public double Double { get; set; }

    [XmlElement("Double")]
    [Browsable(false), EditorBrowsable(EditorBrowsableState.Never), DebuggerBrowsable(DebuggerBrowsableState.Never)]
    public XdrTypeWrapper<double> XmlDouble { get { return Double; } set { Double = value; } }
}

Which produces the following XML:

<TestClass xmlns:dt="urn:schemas-microsoft-com:datatypes">
  <Amount dt:dt="int">101</Amount>
  <StartTime dt:dt="dateTime">2015-10-06T20:35:18.2308848+00:00</StartTime>
  <Double dt:dt="float">101.23</Double>
</TestClass>

Prototype fiddle.