Question

I have a base class, called NodeUpgrade, which have several child types. An example of a specific child class is FactoryUpgrade.

I have a list of NodeUpgrades, which can be a mix of different child types. How do I write a linq query to retrieve a type of NodeUpgrade and cast to that specific type?

My working query looks something like this:

var allFactories = (from Node n in assets.Nodes
                    from FactoryUpgrade u in n.NodeUpgrades
                    where u.ClassID == NodeUpgradeTypes.Factory
                    select u)

This, of course, doesn’t work. Can I specify the final type of the output?

Answer 1

If you are sure that every type in a sequence is a given type, you can use the Cast<T>() extension method. If there can be multiple types in the list and you only want one of them, you can use OfType<T>() to filter the sequence.

List<Animal> animals = ...

// assumes all animals are cats 
var cats = animals.Cast<Cat>();
// var cats = (from animal in animals where ... select animal).Cast<Cat>();

// or maybe animals can contain dogs, but you don't want them
var cats = animals.OfType<Cat>();

The difference is that Cast will throw an exception if an animal isn’t a cat, whereas OfType will perform a type check before actually trying the conversion. I would favor Cast over OfType when you are confident of the uniform type. (Also note that these do not perform user-defined conversions. If you have defined an implicit or explicit conversion, those will not be supported by these methods.)

The resulting sequence in each case will be IEnumerable<Cat>, which you can do further query operations on (filters, groupings, projections, ToList(), etc.)