Question

I have two classes as follows, and Class A inherits from Class B.

public class A
{
     public string Title { get; set; }
}

public class B: A
{

}

Then I have a function as follows which takes in a list of items of class A:

public static void Get(List<A> values)
{
    // Do something
}

Now when I try to pass a list of items of Class B, into the function as follows,

private function void Test()
{
     var k = new List<B>();
     Get(k);
}

I get the following error message

cannot convert from ‘System.Collections.Generic.List<B>’ to ‘System.Collections.Generic.List<A>’

Should this not work as B just inherits from A and will have the properties that A has?

I have already spent hours trying to look this up, but I have not been to figure out what I an doing wrong. How can I fix it?

Answer 1

No, it should not. It is potentially unsafe:

class C : A {

}

public static void Get(List<A> values)
{
    values.Add(new C());
}

If you were to call Get with new List<B>(), it would be unsafe, as you’d be inserting instances of C in List<B>.

It would be safe, however, if List<T> was a “read-only” interface, in which case covariance, which is what you are looking for, would be safe. You can change List<A> to IEnumerable<A> instead and it’ll work.