Question

I am learning to use polymorphism in C#, but cannot figure out this one. I’m trying to write a class that allows me to get a filtered list of files from a repository.

The repository could be a file system folder or the embedded resources in an arbitrary already-loaded assembly (but it is not the one currently executing).

Any suggestions?

Answer 1

You could define pair of interfaces like this:

public interface IReadableFile {     Stream OpenRead(); }  public interface IRepository {     IEnumerable<IReadableFile> Search(string pattern); } 

And have two different implementations of them:

public class FolderFile : IReadableFile {     readonly private string _name;      public FolderFile(string name)     {         _name = name;     }      #region IFile Members      public Stream OpenRead()     {         return File.OpenRead(_name);     }      #endregion }  public class FolderRepository : IRepository {     readonly private string _directory;      public FolderRepository(string directory)     {         _directory = directory;     }      #region IRepository Members      public IEnumerable<IReadableFile> Search(string pattern)     {         return Array.ConvertAll(Directory.GetFiles(_directory, pattern), name => new FolderFile(name));     }      #endregion }  public class AssemblyFile : IReadableFile {     readonly private Assembly _assembly;     readonly private string _name;      public AssemblyFile(Assembly assembly, string name)     {         _assembly = assembly;         _name = name;     }      #region IReadableFile Members      public Stream OpenRead()     {         return _assembly.GetManifestResourceStream(_name);     }      #endregion }  public class AssemblyRepository : IRepository {     readonly private Assembly _assembly;      public AssemblyRepository(Assembly assembly)     {         _assembly = assembly;     }      #region IRepository Members      public IEnumerable<IReadableFile> Search(string pattern)     {         return _assembly.GetManifestResourceNames().Where(name => name.Contains(pattern)).Select(name => new AssemblyFile(_assembly, name)).ToArray();     }      #endregion } 

And then you can write your algorithms dependant on only these interfaces and not on their implementations.