This has been introduced in C# 4.0, but is there a way to achieve this in c# 3.0?

For e.g., consider the following code:

class Base
{
}

class Derived1 : Base
{
}

class Derived2 : Base
{
}

class User<T> where T : Base
{
}

class User1 : User<Derived1>
{
}

Now, I would like to have a list of User<T>, in which I can store User<Derived1> as well as User<Derived2>, but the following code fails to compile in C# 3.0:

List<User<Base>> users = new List<User<Base>>();
users.Add(new User1());

Any ideas?