How to serialize Pen using XmlElementAttribute.Type property

Question

I am trying to serialize some properties defined as Pen in my class. As I want to find the simplest and most elegant way to do that, I've tried to apply the solution described in this answer to the Pen type. I only need to serialize the Color, DashStyle and Width properties of a Pen, so I've come up with the following class:

public class XmlPen {
    private Pen pen = new Pen(Color.Black, 1.0F);

    [XmlAttribute]
    public String ColorHtml {
        get { return ColorTranslator.ToHtml(this.pen.Color); }
        set { this.pen.Color = ColorTranslator.FromHtml(value); }
    }

    [XmlAttribute]
    public DashStyle Style {
        get { return this.pen.DashStyle; }
        set { this.pen.DashStyle = value; }
    }

    [XmlAttribute]
    public float Width {
        get { return this.pen.Width; }
        set { this.pen.Width = value; }
    }

    public XmlPen() {
    }

    public XmlPen(Pen pen) {
        this.pen = pen;
    }

    public static implicit operator Pen(XmlPen xmlPen) {
        return xmlPen.pen;
    }

    public static implicit operator XmlPen(Pen pen) {
        return new XmlPen(pen);
    }
}

According to the mentionned answer, I only need to add the following attribute in front of each Penproperty I want to serialize:

[XmlElement(Type = typeof(XmlPen))]
public Pen SomePen { get; set; }

But that doesn't work, I get a InvalidOperationException when trying to serialize my object:

System.Drawing.Pen cannot be serialized because it does not have a parameterless constructor.

My questions are:

Why am I getting this error? Doesn't the [XmlElement(Type = typeof(XmlPen))] suggests that the XmlPen class should be used when (de)serializing the Pen property?
Is there a trick that would allow me to use this solution on types that do not have a parameterless constructor?

PS: I am not interested in wrapping the Pen class in another one if I have to reference this new class everywhere in my code, nor am I interested to add a hidden property in my class to be used when (de)serializing.

As explained in the accepted answer, this method doesn't work. In the end, I used a [Surrogate](http://msdn.microsoft.com/en-us/library/ms733064%28v=vs.110%29.aspx) to serialize my Pen. — Otiel, Oct 16 '14 at 14:14

BRAHIM Kamel · Accepted Answer · 2014-10-14T14:48:23.333

Why am I getting this error?

because you don't have a parameter-less constructor in your Pen class

Doesn't the [XmlElement(Type = typeof(XmlPen))] suggests that the XmlPen class should be used when (de)serializing the Pen property?

No Pen is an independent object and Deserialization will require a public Parameter-less Constructor in your Pen Class

EDIT

Comment question

Isn't the serializer supposed to use the parameterless constructor of the XmlPen class?

No because still your public static implicit operator XmlPen(Pen pen) require a Pen object that needs to be constructed at runtime when serializing or deserializing the object

How to serialize Pen using XmlElementAttribute.Type property

1 Answers1