What about types assignable to each other?

As an example, I often do this:

```c#
public class Grault
{
    private readonly IEnumerable<string> bribbets;

    public Grault(params string[] bribbets)
    {
        this.bribbets = bribbets;
    }

    public IEnumerable<string> Bribbets
    {
        get { return this.bribbets; }
    }
}
```