How to add XmlInclude attribute dynamically

Question

I have the following classes

[XmlRoot]
public class AList
{
   public List<B> ListOfBs {get; set;}
}

public class B
{
   public string BaseProperty {get; set;}
}

public class C : B
{
    public string SomeProperty {get; set;}
}

public class Main
{
    public static void Main(string[] args)
    {
        var aList = new AList();
        aList.ListOfBs = new List<B>();
        var c = new C { BaseProperty = "Base", SomeProperty = "Some" };
        aList.ListOfBs.Add(c);

        var type = typeof (AList);
        var serializer = new XmlSerializer(type);
        TextWriter w = new StringWriter();
        serializer.Serialize(w, aList);
    }    
}

Now when I try to run the code I got an InvalidOperationException at last line saying that

The type XmlTest.C was not expected. Use the XmlInclude or SoapInclude attribute to specify types that are not known statically.

I know that adding a [XmlInclude(typeof(C))] attribute with [XmlRoot] would solve the problem. But I want to achieve it dynamically. Because in my project class C is not known prior to loading. Class C is being loaded as a plugin, so it is not possible for me to add XmlInclude attribute there.

I tried also with

TypeDescriptor.AddAttributes(typeof(AList), new[] { new XmlIncludeAttribute(c.GetType()) });

before

var type = typeof (AList);

but no use. It is still giving the same exception.

Does any one have any idea on how to achieve it?

Answer 1

Have a look at the documentation of XmlSerializer. There is a constructor which expects known types as the second parameter. That should work fine for you use case.

Answer 2

I'm don't think attributes can be applied at runtime, as they are used to create Meta-data at the CIL code.

Answer 3

Two options; the simplest (but giving odd xml) is:

        XmlSerializer ser = new XmlSerializer(typeof(AList),
            new Type[] {typeof(B), typeof(C)});

With example output:

<AList xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema"&gt;
  <ListOfBs>
    <B />
    <B xsi:type="C" />
  </ListOfBs>
</AList>

The more elegant is:

        XmlAttributeOverrides aor = new XmlAttributeOverrides();
        XmlAttributes listAttribs = new XmlAttributes();
        listAttribs.XmlElements.Add(new XmlElementAttribute("b", typeof(B)));
        listAttribs.XmlElements.Add(new XmlElementAttribute("c", typeof(C)));
        aor.Add(typeof(AList), "ListOfBs", listAttribs);

        XmlSerializer ser = new XmlSerializer(typeof(AList), aor);

With example output:

<AList xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema"&gt;
  <b />
  <c />
</AList>

In either case you must cache and re-use the ser instance; otherwise you will haemorrhage memory from dynamic compilation.