Here's an example XML:
<Device>Each device has a shape which is a list of different drawing objects. The simple way to do this would be to have a list for each object type (Line, Ellipse, Triangle) but that's not what I wanted. The order of the XML is also the order of drawing on the screen, so I wanted these to remain in a single list as a grouping of objects, derived from a simple object class.
<Shape>
<Line Colour="blue">
<Point x="29" y="55"/>
<Point x="43" y="55"/>
</Line>
<Ellipse Colour="yellow">
<Point x="44" y="50"/>
<Point x="53" y="59"/>
</Ellipse>
<Triangle Colour="red">
<Point x="1456" y="191"/>
<Point x="1456" y="201"/>
<Point x="1465" y="201"/>
</Triangle>
</Shape>
</Device>
Just being lazy and using [XmlElement] on the Shape list in a C# class did not work, so I had to go deeper. First, let's have a look at my objects. I defined my own Point class, instead of using System.Drawing.Point, just so they could be represented as attributes in my XML (a design decision).
public sealed class PointI then created a base drawing object, with a colour and a list of points. Because the size of the Point array changes based on each derived object, the XML Serialiser ignores the Point array in the base class.
{
[XmlAttribute]
public int x
{
get;
set;
}
[XmlAttribute]
public int y
{
get;
set;
}
}
public class DrawingObjectNow, I derive each specific object from this base class. To set the size of the Point array, I use a private field and then modify the base array to become a getter, using the 'new' keyword. The XmlElement is defined in these derived classes for the Serialiser (and yes, I realise the Ellipse is the same as a Line, but there's other code I removed for this example. It still serves the point of showing different derived classes).
{
[XmlAttribute]
public string Colour
{
get;
set;
}
[XmlIgnore]
public Point[] Points;
}
public sealed class Line : DrawingObjectVery good. Now, let's make a list of the base object and force the XML Serialiser to add the different element names (Line, Triangle, Ellipse) to the single list. This is when we hit our first slightly different XML definition. To get this to work, .NET makes us add an enumeration which is ignored by the XML. The XML Serialiser then uses this to help detect what object type it is (http://msdn.microsoft.com/en-us/library/system.xml.serialization.xmlchoiceidentifierattribute%28v=vs.100%29.aspx).
{
private Point[] _points = new Point[2];
[XmlElement("Point")]
new public Point[] Points
{
get
{
return _points;
}
set
{
_points = value;
}
}
}
public sealed class Triangle : DrawingObject
{
private Point[] _points = new Point[3];
[XmlElement("Point")]
new public Point[] Points
{
get
{
return _points;
}
set
{
_points = value;
}
}
}
public sealed class Ellipse : DrawingObject
{
private Point[] _points = new Point[2];
[XmlElement("Point")]
new public Point[] Points
{
get
{
return _points;
}
set
{
_points = value;
}
}
}
So we define a public enumeration of the different object types:
[XmlType(IncludeInSchema = false)]Then in our serializing Shape class we add an array of this enumeration, so it can be matched with the list being serialised. But we get the Serialiser to ignore it.
public enum ShapeChoiceType
{
Line,
Triangle,
Ellipse
}
// Do not serialize this next field:Finally we add the List! We have to use the XmlChoiceIdentifier, pointing to our List of ItemTypes, to help cast the objects. In our XmlElement definition, we specify the name of each object type, as well as what the C# type will be.
[XmlIgnore]
public ListItemType;
[XmlElement("Line", typeof(Line))]This builds all fine! But the first time you try to deserialise the XML in the application, we get an error! Oh dear. With XML, the CLR tends to compile the XML classes at run-time.
[XmlElement("Triangle", typeof(Triangle))]
[XmlElement("Ellipse", typeof(Ellipse))]
[XmlChoiceIdentifier("ItemType")]
public ListDrawingObjects
{
get;
set;
}
System.InvalidOperationException was caughtSo, what does this mean? For reasons I'm not going into, I use List
Message=Unable to generate a temporary class (result=1).
error CS1061: 'System.Collections.Generic.List' does not contain a definition for 'Length' and no extension method 'Length' accepting a first argument of type 'System.Collections.Generic.List ' could be found (are you missing a using directive or an assembly reference?)
Final class definitions below!
public sealed class Point
{
[XmlAttribute]
public int x
{
get;
set;
}
[XmlAttribute]
public int y
{
get;
set;
}
}
public class DrawingObject
{
[XmlAttribute]
public string Colour
{
get;
set;
}
[XmlIgnore]
public Point[] Points;
}
public sealed class Line : DrawingObject
{
private Point[] _points = new Point[2];
[XmlElement("Point")]
new public Point[] Points
{
get
{
return _points;
}
set
{
_points = value;
}
}
}
public sealed class Triangle : DrawingObject
{
private Point[] _points = new Point[3];
[XmlElement("Point")]
new public Point[] Points
{
get
{
return _points;
}
set
{
_points = value;
}
}
}
public sealed class Ellipse : DrawingObject
{
private Point[] _points = new Point[2];
[XmlElement("Point")]
new public Point[] Points
{
get
{
return _points;
}
set
{
_points = value;
}
}
}
[XmlType(IncludeInSchema = false)]
public enum ShapeChoiceType
{
Line,
Triangle,
Ellipse
}
public sealed class Shape
{
[XmlElement("Line", typeof(Line))]
[XmlElement("Triangle", typeof(Triangle))]
[XmlElement("Ellipse", typeof(Ellipse))]
[XmlChoiceIdentifier("ItemType")]
public DrawingObject[] DrawingObjects
{
get;
set;
}
// Do not serialize this next field:
[XmlIgnore]
public ShapeChoiceType[] ItemType;
}
public sealed class Device
{
[XmlElement("Shape")]
public ListShapes
{
get;
set;
}
}
