Question

Is it possible to use LINQ to XML to create an array of a polymorphic type?

I have 3 classes, Base, Der1 and Der2. Base is base class for Der1 and Der2. I have an xml file in which I have nodes corresponding to Der1 and Der2 objets. What I’d like to do is to parse the file and fill in a List with Der1 and Der2 objects.

xml would look like this:

<nodes>
    <node type = "Der1" attr1="val1" />
    <node type = "Der2" attr2="val2" />
</nodes>

What I tried to do but what does not work is:

List<Base> PMList = 
(from der1node from xmlDoc.Descendants("nodes") 
where der1node.type == ("Der1") 
select new Der1()
{
    Attr1 = der1node.Attribute("attr1").Value
}
).Union<Base>
(from der2node from xmlDoc.Descendants("baseNode") 
where der2node.type == ("Der2") 
select new Der2()
{
    Attr2 = der2node.Attribute("attr2").Value
}
).ToList<Base>();

Here what I tried to do is to construct Der1 objects with type=Der1 and Der2 objects with type=Der2 and add them together into a List using Union.

However this does not work. How can I get objects of different type using LINQ to XML and put them in one polymorphic collection?

Answer 1

One way would be to cast the new instances to their base class before projecting them:

List<Base> PMList = (from baseNode from xmlDoc.Descendants("nodes") 
                     where baseNode.type == ("Der1") 
                     select (Base) new Der1() {
                         Attr1 = baseNode.Attribute("attr1").Value
                     }
                    ).Union(
                     from baseNode from xmlDoc.Descendants("baseNode") 
                     where baseNode.type == ("Der2") 
                     select (Base) new Der2() {
                         Attr2 = baseNode.Attribute("attr2").Value
                     }
                    ).ToList();