C# Online Compiler | .NET Fiddle

using System;
using System.Collections.Generic;

public interface IEbook
{
	string Filename {get;}
	void Show();
}

public class RealEbook: IEbook
{
	private string filename;
	public string Filename {
		get
		{
			return filename;
		}
	}
	
	public RealEbook(string filename)
	{
		this.filename = filename;
		Load();
	}
	
	private void Load()
	{
		Console.WriteLine("loading file");
	}
	
	public void Show()
	{
		Console.WriteLine("showing file");
	}
}

public class EbookProxy: IEbook
{
	private RealEbook ebook;
	private string filename;
	
	public string Filename
	{
		get
		{
			return filename;
		}
	}
	
	public EbookProxy(string filename)
	{
		this.filename = filename;
	}
	
	public void Show()
	{
		if(ebook == null)
			ebook = new RealEbook(filename);
		
		ebook.Show();
	}
}

public class Library
{
	private Dictionary<string, IEbook> ebooks = new Dictionary<string, IEbook>();
	
	public void Add(IEbook ebook)
	{
		ebooks.Add(ebook.Filename, ebook);
	}
	
	public void OpenEbook(string filename)
	{
		var ebook = ebooks[filename];
		ebook.Show();
	}
}

public class Program
{
	public static void Main()
	{
		var library = new Library();
		string[] filenames = {"a", "b", "c"};
		
		foreach(var filename in filenames)
		{
			library.Add(new EbookProxy(filename));
		}
		library.OpenEbook("a");
	}
}

Cached Result

directory not found exception handled
Tasks Exception: ArgumentNullException Value cannot be null. (Parameter 'path')