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I'm saving into a FILE : super.txt
I'm saving into a FILE : extra.txt
I'm saving into a DATABASE : connection=superdb;integrated security=true
I'm saving into a WEB LOCATION : http://test.com/saveData?
I'm saving into a FILE : super.txt
I'm saving into a FILE : extra.txt
I'm saving into a DATABASE : connection=superdb;integrated security=true
I'm saving into a WEB LOCATION : http://test.com/saveData?

void Main()
{
    List<IPersistable<IConfigurable>> allItems = new List<IPersistable<IConfigurable>>();
	FileConfig fc = new FileConfig("super.txt");
	IPersistable<IConfigurable> item = new FileSaver(fc);
    
	List<IPersistable<IConfigurable>> fakeData = new List<IPersistable<IConfigurable>>();
	fakeData.Add(new FileSaver(new FileConfig("extra.txt")));
	fakeData.Add(new DatabaseSaver(new DatabaseConfig("connection=superdb;integrated security=true")));
	fakeData.Add(new TcpSaver(new TcpConfig("http://test.com/saveData?")));
	
	allItems.Add(item);
    foreach (IPersistable<IConfigurable> ic in fakeData){
        allItems.Add(ic);            
    } 

    foreach (IPersistable<IConfigurable> ip in allItems)
    {
        ip.Save();
    }
	
	foreach (var ip in allItems)
    {
        ip.Save();
    }
}

interface IPersistable<T> where T : IConfigurable {
    bool Save();
}

interface IConfigurable {
}

class FileSaver : IPersistable<IConfigurable>{
	
    protected FileConfig config;
    public FileSaver(FileConfig config){
        this.config = config;
    }
    virtual public bool Save(){
        Console.WriteLine(String.Format("I'm saving into a FILE : {0}",config.FileName));
        return true;
    }
}

class FileConfig : IConfigurable{

    public string FileName{get;set;}
    public FileConfig(String fileName=null){
        FileName = fileName;
    }

}

class DatabaseConfig : IConfigurable{
    public string ConnectionString{get;set;}
    public DatabaseConfig(String ConnectionString=null){
        this.ConnectionString = ConnectionString;
    }
}

class TcpConfig : IConfigurable{
    public string Uri {get;set;}
	public TcpConfig(String uri){
		Uri = uri;
	}
}

class DatabaseSaver : IPersistable<IConfigurable>{
    protected DatabaseConfig config;
    public DatabaseSaver(DatabaseConfig config){
        this.config = config;
    }
    public bool Save(){
        Console.WriteLine(String.Format("I'm saving into a DATABASE : {0}", config.ConnectionString));
        return true;
    }
}

class TcpSaver : IPersistable<IConfigurable>{
    TcpConfig config;
    public TcpSaver(TcpConfig config){
        this.config = config;
    }
    public bool Save(){
        Console.WriteLine(String.Format("I'm saving into a WEB LOCATION : {0}", config.Uri));
        return true;
    }
}

• It can be be easily extended by anyone, but not easily enhanced.
• It can be easily swapped out and replaced with a different implementation without needing the programs using that library to be recompiled, redeployed or changed in any way.
• Because it is a library, it will only be for a single type of task (no one would even think of putting unrelated functionality into a compression library, but I've seen devs happily put in unrelated stuff into a compression class).

fwiw, the code I am modifying has not changed in 10 years. So, why bother making it general?

NelsonGoncalves wrote:

Usually because I decide to make a class for something that will only have one object instance.