Serialize a list of objects inheriting from class A to xml, so the names of the elements in the xml are B,C,D

Firstly DataContractSerializer does not have a mechanism to support collection item polymorphism by changing collection element name(s). It only supports the known type mechanism which uses the i:type attribute - which you indicate is not acceptable.

Since you are willing to switch to XmlSerializer, you could use the attribute XmlArrayItemAttribute.Type to specify element names for polymorphic types in lists:

public class AListObject
{
    [XmlArrayItem(typeof(B))]
    [XmlArrayItem(typeof(C))]
    public List<A> SerializedListObjects { get; set; }
}

However, you also indicate that the polymorphic subtypes cannot be declared statically at compile type because they exist in some other assembly.

As a result, you will need to use the XmlAttributeOverrides mechanism to specify all possible derived types for all List<A> properties in runtime, and manually construct an XmlSerializer using those overrides.

Here is a prototype solution. First, let's assume you have a root object that refers to an object containing a List<A> like so:

public class RootObject
{
    public AListObject AList { get; set; }
}

public class AListObject
{
    public List<A> SerializedListObjects { get; set; }
}

(The root object could be the object with the List<A> property, but doesn't need to be.) Let's also assume you know all such objects like AListObject that may contain List<A> properties.

With those assumptions, the following serializer factory can be used to generate an XmlSerializer for any root object that may refer to any instances of the known types containing a List<A> property:

public interface IXmlSerializerFactory
{
    XmlSerializer CreateSerializer(Type rootType);
}

public static class AListSerializerFactory
{
    static readonly XmlArrayItemTypeOverrideSerializerFactory instance;

    static AListSerializerFactory()
    {
        // Include here a list of all types that have a List<A> property.
        // You could use reflection to find all such public types in your assemblies.
        var declaringTypeList = new []
        {
            typeof(AListObject),
        };

        // Include here a list of all base types with a corresponding mapping 
        // to find all derived types in runtime.   Here you could use reflection
        // to find all such types in your assemblies, as shown in 
        // https://stackoverflow.com/questions/857705/get-all-derived-types-of-a-type
        var derivedTypesList = new Dictionary<Type,  Func<IEnumerable<Type>>>
        {
            { typeof(A), () => new [] { typeof(B), typeof(C) } },
        };
        instance = new XmlArrayItemTypeOverrideSerializerFactory(declaringTypeList, derivedTypesList);
    }

    public static IXmlSerializerFactory Instance { get { return instance; } }
}

public class XmlArrayItemTypeOverrideSerializerFactory : IXmlSerializerFactory
{
    // To avoid a memory & resource leak, the serializers must be cached as explained in
    // https://stackoverflow.com/questions/23897145/memory-leak-using-streamreader-and-xmlserializer

    readonly object padlock = new object();
    readonly Dictionary<Type, XmlSerializer> serializers = new Dictionary<Type, XmlSerializer>();
    readonly XmlAttributeOverrides overrides;

    public XmlArrayItemTypeOverrideSerializerFactory(IEnumerable<Type> declaringTypeList, IEnumerable<KeyValuePair<Type, Func<IEnumerable<Type>>>> derivedTypesList)
    {
        var completed = new HashSet<Type>();
        overrides = declaringTypeList
            .SelectMany(d => derivedTypesList.Select(p => new { declaringType = d, itemType = p.Key, derivedTypes = p.Value() }))
            .Aggregate(new XmlAttributeOverrides(), (a, d) => a.AddXmlArrayItemTypes(d.declaringType, d.itemType, d.derivedTypes, completed));
    }

    public XmlSerializer CreateSerializer(Type rootType)
    {
        lock (padlock)
        {
            XmlSerializer serializer;
            if (!serializers.TryGetValue(rootType, out serializer))
                serializers[rootType] = serializer = new XmlSerializer(rootType, overrides);
            return serializer;
        }
    }
}

public static partial class XmlAttributeOverridesExtensions
{
    public static XmlAttributeOverrides AddXmlArrayItemTypes(this XmlAttributeOverrides overrides, Type declaringType, Type itemType, IEnumerable<Type> derivedTypes)
    {
        return overrides.AddXmlArrayItemTypes(declaringType, itemType, derivedTypes, new HashSet<Type>());
    }

    public static XmlAttributeOverrides AddXmlArrayItemTypes(this XmlAttributeOverrides overrides, Type declaringType, Type itemType, IEnumerable<Type> derivedTypes, HashSet<Type> completedTypes)
    {
        if (overrides == null || declaringType == null || itemType == null || derivedTypes == null || completedTypes == null)
            throw new ArgumentNullException();
        XmlAttributes attributes = null;
        for (; declaringType != null && declaringType != typeof(object); declaringType = declaringType.BaseType)
        {
            // Avoid duplicate overrides.
            if (!completedTypes.Add(declaringType))
                break;
            foreach (var property in declaringType.GetProperties(BindingFlags.Public | BindingFlags.DeclaredOnly | BindingFlags.Instance))
            {
                // Skip the property if already ignored
                if (property.IsDefined(typeof(XmlIgnoreAttribute), false))
                    continue;

                // See if it is a list property, and if so, get its type.
                var propertyItemType = property.PropertyType.GetListType();
                if (propertyItemType == null)
                    continue;

                // OK, its a List<itemType>.  Add all the necessary XmlElementAttribute declarations.
                if (propertyItemType == itemType)
                {
                    if (attributes == null)
                    {
                        attributes = new XmlAttributes();
                        foreach (var derivedType in derivedTypes)
                            // Here we are assuming all the derived types have unique XML type names.
                            attributes.XmlArrayItems.Add(new XmlArrayItemAttribute { Type = derivedType });
                        if (itemType.IsConcreteType())
                            attributes.XmlArrayItems.Add(new XmlArrayItemAttribute { Type = itemType });
                    }
                    overrides.Add(declaringType, property.Name, attributes);
                }
            }
        }
        return overrides;
    }
}

public static class TypeExtensions
{
    public static bool IsConcreteType(this Type type)
    {
        return !type.IsAbstract && !type.IsInterface;
    }

    public static Type GetListType(this Type type)
    {
        while (type != null)
        {
            if (type.IsGenericType)
            {
                var genType = type.GetGenericTypeDefinition();
                if (genType == typeof(List<>))
                    return type.GetGenericArguments()[0];
            }
            type = type.BaseType;
        }
        return null;
    }
}

Then, you can serialize and deserialize instances of RootObject to and from XML as follows:

var root = new RootObject
{
    AList = new AListObject
    {
        SerializedListObjects = new List<A> { new B(), new C() },
    },
};

var serializer = AListSerializerFactory.Instance.CreateSerializer(root.GetType());

var xml = root.GetXml(serializer);
var root2 = xml.LoadFromXml<RootObject>(serializer);

Using the extension methods:

public static class XmlSerializationHelper
{
    public static T LoadFromXml<T>(this string xmlString, XmlSerializer serial = null)
    {
        serial = serial ?? new XmlSerializer(typeof(T));
        using (var reader = new StringReader(xmlString))
        {
            return (T)serial.Deserialize(reader);
        }
    }

    public static string GetXml<T>(this T obj, XmlSerializer serializer = null)
    {
        using (var textWriter = new StringWriter())
        {
            var settings = new XmlWriterSettings() { Indent = true }; // For cosmetic purposes.
            using (var xmlWriter = XmlWriter.Create(textWriter, settings))
                (serializer ?? new XmlSerializer(obj.GetType())).Serialize(xmlWriter, obj);
            return textWriter.ToString();
        }
    }
}

And the result is:

<RootObject xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
  <AList>
    <SerializedListObjects>
      <B />
      <C />
    </SerializedListObjects>
  </AList>
</RootObject>

Notes:

• As explained in Memory Leak using StreamReader and XmlSerializer, you must statically cache any XmlSerializer constructed with XmlAttributeOverrides to avoid a severe memory leak. The documentation suggests using a Hashtable, however XmlAttributeOverrides does not override Equals() or GetHashCode(), and does not provide enough access to its internal data for applications developers to write their own. Thus it's necessary to hand-craft some sort of static caching scheme whenever XmlAttributeOverrides is used.

• Given the complexity of finding and overriding the XmlArrayItem attributes of all List<A> properties, you might consider sticking with the existing i:type mechanism. It's simple, works well, is supported by both DataContractSerializer and XmlSerializer, and is standard.

• I wrote the class XmlArrayItemTypeOverrideSerializerFactory in a generic way, which adds to the apparent complexity.

Working sample .Net fiddle here.