XML-serializer with ABSTRACT class

[MeisterBiber]

Hi ! I have a problem with serializing a class, which has a variable with an abstract-class-type. If I'm using the specific class Test2, everything works fine, but when I use the abstract class Test, I get the following error-message: An unhandled exception of type 'System.InvalidOperationException' occurred in system.xml.dll Additional information: There was an error generating the XML document. Does anybody know whats the matter here? using System; using System.Xml; using System.Xml.Serialization; using System.IO; namespace xmlTest { class Class1 { [STAThread] static void Main(string[] args) { myObject o = new myObject(); XmlSerializer mySerializer = new XmlSerializer(typeof(myObject)); StreamWriter myWriter = new StreamWriter("test.xml"); mySerializer.Serialize(myWriter, o); } } public class myObject { //public Test2 t = new Test2(); public Test t = new Test2(); } public abstract class Test { public string s = "Test"; } public class Test2:Test { new public string s="Test2"; } } Thank you !!!!

[Heath Stewart]

As the documentation states, serializable classes must have a publicly accessible default constructor because they are instantiated. Abstract classes cannot be instantiated, however. That is the problem. The solution is easy: don't hardcode the type (using typeof in your example) in your serializable code. For one, one of the benefits (one could say, the "point") of using abstract classes is that you can use a generic to refer to multiple types without caring about the specific Type of the object. Instead, use o.GetType(). Remember that in OO, even though a variable might be declared as Test or Object (pulling examples from your code), they will still be instances of myObject or Test2. So, using o.GetType() will still return the Type for myObject (even if it were declared as an Object). If you use that in the constructor for the XmlSerializer, you shouldn't have any problems. This way, also, you don't hardcode that the serializer only accepts myObject. You will want to, however, use a condition or something before serializing to make sure that the object to be serialized is a Test derivative:

public void SaveTest(Test t)
{
// In this case, the CLR verifies that the object is of type Test
// (or a derivative) because the parameter above is a Test type.
XmlSerializer serializer = new XmlSerializer(t.GetType());
StreamWriter writer = new StreamWriter("test.xml");
serializer.Serialize(writer, t);
}
public void Save(object o)
{
// In this case, we should check to make sure that the object is
// a Test type. For brevity, well check and pass to SaveTest.
if (o is Test)
SaveTest(o as Test);
}

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