Cannot convert from List<DerivedClass> to List<BaseClass>

Solution 1:

It is because List<T> is in-variant, not co-variant, so you should change to IEnumerable<T> which supports co-variant, it should work:

IEnumerable<BaseClass> bcl = new List<DerivedClass>();
public void doSomething(IEnumerable<BaseClass> bc)
{
    // do something with bc
}

Information about co-variant in generic

Solution 2:

Explanations that I have found have simply said that it violates type safety somehow, but I'm not seeing it. What is the risk of the compiler allowing conversion from List<DerivedClass> to List<BaseClass>?

This question is asked almost every day.

A List<Mammal> cannot be converted to a List<Animal> because you can put a lizard into a list of animals. A List<Mammal> cannot be converted to a List<Giraffe> because there might be a tiger in the list already.

Therefore List<T> has to be invariant in T.

However, List<Mammal> can be converted to IEnumerable<Animal> (as of C# 4.0) because there is no method on IEnumerable<Animal> that adds a lizard. IEnumerable<T> is covariant in T.