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

Answer 1

[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.