How to serialize an array of a base class filled with its subclasses to XML?

I'm trying to serialize an array of Test objects that contains some TestC开发者_运维技巧hild objects.

public class Test
{
    public string SomeProperty { get; set; }
}

public class TestChild : Test
{
    public string SomeOtherProperty { get; set; }
}

class Program
{
    static void Main()
    {
        Test[] testArray = new[]
        {
            new TestChild { SomeProperty = "test1", SomeOtherProperty = "test2" },
            new TestChild { SomeProperty = "test3", SomeOtherProperty = "test4" },
            new TestChild { SomeProperty = "test5", SomeOtherProperty = "test6" },
        };

        XmlSerializer xs = new XmlSerializer(typeof(Test));

        using (XmlWriter writer = XmlWriter.Create("test.xml"))
            xs.Serialize(writer, testArray);
    }
}

I get and InvalidOperationException that says TestChild cannot be converted to Test.

This makes sense but is there a way to do it anyway?

The simplest way is to annotate the class so that the serializer anticipates the subclass:

[XmlInclude(typeof(TestChild))]
public class Test
{
    public string SomeProperty { get; set; }
}

Otherwise (if using the more complex constructors for XmlSerializer) you need to be very careful to cache and re-use the serializer instance - otherwise it will haemorrhage memory (it creates an assembly each time that cannot be garbage-collected; the simplest constructor taking just a Type handles this caching for you).

You could specify known types by using the proper constructor, also you are serializing a test array Test[] and not Test, hence the first argument of the constructor should be typeof(Test[]):

var xs = new XmlSerializer(typeof(Test[]), new Type[] { typeof(TestChild) });