Say I wanted to customize an OpenFileDialog and change, for example, the way the filter for file extensions work, as in the case of this question. After I pointed out to the author of said question that the OpenFileDialog is not inheritable, I got a comment with the following: Even though the OpenFileDialog is sealed (not inheritable), you may use it as a nested type. For instance, using a property that will get the NativeDialog. Then, you write your method always using the NativeDialog property and you're done. My question is, can someone provide me with an example code on how would I proceed on doing something like that? I'm kind of new to the concept of nested types so I'm having a hard time figuring that out by myself, and I searched around the web and couldn't find anything too concrete about it. Thanks!