Question

• I have a File class that has an Open() method.
• I have a subclass of File called TextFile that implements an IReadableFile interface, which requires the implementation of a Read() method.

If I declare a variable myFile as IReadableFile, I can’t call the Open() method on it. I want to be able to exercise the functionality of TextFile‘s base class (File) methods and its interface (IReadableFile) methods at once. Is this possible?

EDIT: I’m working in VB.NET (if that matters).

I’m trying to provide a minimum set of File I/O functionality via a File class and then provide extended capabilities for particular types of files by deriving from File and adding some additional methods (like Read, Write, etc.). I want the derived classes to be polymorphic – e.g., calling the Write method on a TextFile will simply write the text data to the filesystem, whereas calling the Write method on a BinaryFile might base 64 encode the binary data before writing it to the filesystem.

Answer 1

Create an IFile interface, make the File class implements IFile. The IReadableFile will inherits from IFile.