using System;

using System.Collections.Generic;

using System.Linq;

using System.IO;

​

namespace MySandbox;

​

public class Program

{

    public static void Main()

    {

        var path = @"X:\some dir\another folder\filename.eml";

        var emailModel = EmailModelFactory.CreateEmailModel(path);

        Console.WriteLine(emailModel.SayHello());

        Console.WriteLine(emailModel.SayExtension());

        int i = 0;

        foreach(var a in emailModel.Attachments){

            i++;

            Console.WriteLine($"Attachment {i}: {a}");

        }

        Console.WriteLine("Done");

    }

}

​

public static class EmailModelFactory

{

    private const string EmlExt = ".eml";

    private const string MsgExt = ".msg";

    private static bool IgnoreCaseEquals(string a, string b) =>

        a.Equals(b, StringComparison.InvariantCultureIgnoreCase);

    private static bool IsEml(string ext) => IgnoreCaseEquals(ext, EmlExt);

    private static bool IsMsg(string ext) => IgnoreCaseEquals(ext, MsgExt);

    public static IEmailModel CreateEmailModel(string path) 

    {

        var ext = Path.GetExtension(path);

        return true switch {

            _ when IsEml(ext) => new EmailEmlModel(path),

            _ when IsMsg(ext) => new EmailMsgModel(path),

            _ => throw new NotImplementedException($"IEmailModel not implemented for extension: {ext}")

        };

    }

}

​

public interface IEmailModel

{

    string SayExtension();

    string SayHello();

    IEnumerable<string> Attachments{get;}   

}

​

public abstract class EmailModelBase : IEmailModel

{

    private readonly string _ext;

    public EmailModelBase(string path)

    {

        _ext = Path.GetExtension(path);

    }

​

    public string SayExtension(){

        return _ext;

    }

​

    public string SayHello()

    {

        return "Hello World";

    }

    public abstract IEnumerable<string> Attachments {get;}

}

​

public class EmailEmlModel : EmailModelBase

{

    private List<string> _attachments;

    public EmailEmlModel(string path) : base(path)

    {

        _attachments = path.Split(@"\").ToList();

    }

    public override IEnumerable<string> Attachments => _attachments;

}

​

public class EmailMsgModel : EmailModelBase 

{

    private List<string> _attachments;

    public EmailMsgModel(string path) : base(path)

    {

        _attachments = path.Split(@"\").ToList();

    }

    public override IEnumerable<string> Attachments => _attachments;

}