C# XML serialization of derived classes

Question

Hi I am trying to serialize an array of objects which are derived from a class and I keep hitting the same error using c#. Any help is much appreciated.

obviously this example has been scaled down for the purpose of this post in the real world Shape would contain a plethora of different shapes.

Program.cs

namespace XMLInheritTests
{
    class Program
    {
        static void Main(string[] args)
        {
            Shape[] a = new Shape[1] { new Square(1) };

            FileStream fS = new FileStream("C:\\shape.xml",
                                        FileMode.OpenOrCreate);
            XmlSerializer xS = new XmlSerializer(a.GetType());
            Console.WriteLine("writing");
            try
            {
                xS.Serialize(fS, a);
            }
            catch (Exception e)
            {
                Console.WriteLine(e.InnerException.ToString());
                Console.ReadKey();
            }
            fS.Close();
            Console.WriteLine("Fin");
        }
    }
}

Shape.cs

namespace XMLInheritTests
{
    public abstract class Shape
    {
        public Shape() { }
        public int area;
        public int edges;
    }
}

Square.cs

namespace XMLInheritTests
{
    public  class  Square : Shape
    {
        public int iSize;
        public Square() { }

        public Square(int size)
        {
            iSize = size;
            edges = 4;
            area = size * size;
        }
    }
}

Error: System.InvalidOperationException: The type XMLInheritTests.Square was not expected. Use the XmlInclude or SoapInclude attribute to specify types that are not known statically.

at Microsoft.Xml.Serialization.GeneratedAssembly.XmlSerializationWriterShapeA rray.Write2_Shape(String n, String ns, Shape o, Boolean isNullable, Boolean need Type)

at Microsoft.Xml.Serialization.GeneratedAssembly.XmlSerializationWriterShapeA rray.Write3_ArrayOfShape(Object o)

Many Thanks

As a side-not, public fields are *usually* to be avoided; a `public int Size {get;set;}` would be preferable. — Marc Gravell, Jul 24 '10 at 19:28
You could *also* have the edges and size as virtual `get` properties, so that you don't need to **store** them. — Marc Gravell, Jul 24 '10 at 19:28

score 29 · Accepted Answer · answered Jul 24 '10 at 19:15

29

[XmlInclude(typeof(Square))]
public abstract class Shape {...}

(repeat for all known subtypes)

If the types are only known at runtime, you can supply them to the XmlSerializer constructor, but: then it is important to cache and reuse that serializer instance; otherwise you will haemorrhage dynamically created assemblies. It does this automatically when you use the constructor that just takes a Type, but not for the other overloads.

answered Jul 24 '10 at 19:15

Marc Gravell

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Awesome, now how would I do this for multiple inherited classes. (Example keeping with the theme of shapes) Shape->Square->ColoredSquare? – Robert Snyder Feb 22 '13 at 18:29
You wrote: `but: **then** it is important to cache and reuse that serializer instance...` (my emphasis). Your reasoning depends on the constructor, not on the `Type` being supplied dynamically. Or did I miss something? – ispiro Dec 14 '13 at 17:57
@ispiro yes, the choice of constructor is key. The single parameter overload will reuse the generated assembly for the same type. Many of the other overloads generate a dynamic assembly *every time it is called*, even if identical to the one used a few momenta ago. – Marc Gravell Dec 14 '13 at 18:02

score 3 · Answer 2 · answered Jul 24 '10 at 19:36

Solution:

class Program
    {
        static void Main(string[] args)
        {
            Shape[] a = new Shape[2] { new Square(1), new Triangle() };

            FileStream fS = new FileStream("C:\\shape.xml",FileMode.OpenOrCreate);

            //this could be much cleaner
            Type[] t = { a[1].GetType(), a[0].GetType() };


            XmlSerializer xS = new XmlSerializer(a.GetType(),t);
            Console.WriteLine("writing");
            try
            {
                xS.Serialize(fS, a);
            }
            catch (Exception e)
            {
                Console.WriteLine(e.InnerException.ToString());
                Console.ReadKey();
            }
            fS.Close();
            Console.WriteLine("Fin");
        }
    }

namespace XMLInheritTests
{
    [XmlInclude(typeof(Square))]
    [XmlInclude(typeof(Triangle))]
    public abstract class Shape
    {
        public Shape() { }
        public int area;
        public int edges;
    }
}

Thanks; I no doubt will have another problem very soon :S

C# XML serialization of derived classes

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