unable to pas derived List<>

I have

class A
{}

class B : A
{}

I also have a method that expects a List parameter

void AMethod(List<A> parameter)
{}

Why can't I

List<B> bs = new List<B>();
AMethod(bs);

And secondly what is the most elegant way to make this work?

regards

Contrary to other answers, this isn't supported in .NET 4.0. Only interfaces and delegates support generic variance. However, .NET 4.0 would allow you to do this:

void AMethod(IEnumerable<A> parameter) {}
...
List<B> list = new List<B>();
AMethod(list);

In .NET 3.5 you can get much the same thing to work with the aid of Cast:

void AMethod(IEnumerable<A> parameter) {}
...
List<B> list = new List<B>();
AMethod(list.Cast<A>());

Another alternative is to make AMethod generic:

void AMethod<T>(List<T> parameter) where T : A
...
List<B> list = new List<B>();
AMethod(list); // Implicitly AMethod<B>(list);

That may or may not do what you need - it depends on what you do within AMethod. If you need to add new items of type A, you'll have problems - and rightly so. If you only need to get items out of the list, that would be fine.

You could make the method signature generic like this:

void AMethod<T>(List<T> parameter) where T : A
{}

Or, you could wait for .NET 4 where this scenario is supported for IEnumerable<>

Have a look at the Covariance and Contravariance FAQ. There are some good explanations and examples (for the upcoming c# 4.0).

The reason is that generics do not support covariance; I have read that this is coming in 4.0

That is why you are not able to pass the list where base type is expected

http://www.boyet.com/Articles/CSharpCovarianceOne.html