dotnet: how to handle calls/references to subsclasses and/or implementors of interfaces?

[williballenthin]

within dotnet, objects of a concrete type may implement interfaces or be subclasses of more generic functionality. how should we extract method features in these cases? should we reference the concrete class? or the interface/superclass? or both?

[williballenthin]

need concrete example @mike-hunhoff

[mike-hunhoff]

example:

C#:

FtpWebResponse resp = (FtpWebResponse)req.GetResponse();

IL:

IL_0000: callvirt  instance class [System]System.Net.WebResponse [System]System.Net.WebRequest::GetResponse()
IL_0005: castclass [System]System.Net.FtpWebResponse