What you want to do is not possible as of C# 3. You may want to read Eric Lippert's series of articles on adding covariance and contravariance to a hypothetical version of C#[^]. In the meantime, here are some work arounds: -Use a non-generic list. -Make IMyItemProvider implement a non-generic interface, call it IProvidesItemsBase, then make your list be of type List<IProvidesItemsBase>. This will work, of course when you call LoadItems, you'd get back an array of AbstractMyItems. Life, family, faith: Give me a visit. From my latest post: "A lot of Christians struggle, perhaps at a subconscious level, about the phrase "God of Israel". After all, Israel's God is the God of Judaism, is He not? And the God of Christianity is not the God of Judaism, right?" Judah Himango

We use sourcecontrol for the different config files. We maintain a version for each environment, e.g DEV, UAT, TEST, PROD, and as part of our automated deployment process, we pick up the appropriate config file. It's relatively easy to do as long as you can script your particular source control system. Deja View - the feeling that you've seen this post before.