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First of all apologize for long post nevertheless i wanted to highlight problem exactly and to be most readable and understandably. I am developing architecture of my program which will be responsible for files/databases data gather and face some architecture issues so far. All information step by step down below.

Let's consider following code below:

public interface IWatchService<TEntity> where TEntity : IEntity

 IList<TEntity> MatchingEntries get; set; 


public interface IWatchServiceDatabase<TEntity> : IWatchService<TEntity> where TEntity : IDatabaseEntity
 
public interface IWatchServiceFiles<TEntity> : IWatchService<TEntity> where TEntity : IFileEntity
 

class Database : IWatchServiceDatabase<DatabaseQuery>

 public IList<DatabaseQuery> MatchingEntries get; set; 


class Files : IWatchServiceFiles<CsvFile>

 public IList<CsvFile> MatchingEntries get; set; 


class Consumer

 public IWatchService<IEntity> WatchService get; set; 

 public Consumer(IWatchService<IEntity> watchService)
 
 WatchService = watchService;
 var newList = WatchService.MatchingEntries;
 


 public void AddNewEntries(IEntity entity) => WatchService.MatchingEntries.Add(entity);


class Program

 static void Main(string[] args)
 
 IWatchServiceDatabase<DatabaseQuery> db = new Database();
 IWatchServiceFiles<CsvFile> filesCsv = new Files();

 var dbConsumer = new Consumer(db); //cannot convert from 'IWatchServiceDatabase<DatabaseQuery>' to 'IWatchService<IEntity>' 
 var filesCsvConsumer = new Consumer(filesCsv); //cannot convert from 'IWatchServiceFiles<CsvFile>' to 'IWatchService<IEntity>'

 dbConsumer.AddNewEntries(new DatabaseQuery());
 dbConsumer.AddNewEntries(new CsvFile()); //illegal cause it's not FileConsumer !!!

 filesCsvConsumer.AddNewEntries(new CsvFile());
 filesCsvConsumer.AddNewEntries(new DatabaseQuery()); //illegal cause it's not DbConsumer !!!
 


public interface IEntity 

public interface IFileEntity : IEntity

 int Id get; set; 
 string Name get; set; 


public interface IDatabaseEntity : IEntity 

public class CsvFile : IFileEntity

 public int Id get; set; 
 public string Name get; set; 


public class XmlFile : IFileEntity

 public int Id get; set; 
 public string Name get; set; 


public class DatabaseQuery : IDatabaseEntity 

We have two errors there:

var dbConsumer = new Consumer(db); 
var filesCsvConsumer = new Consumer(filesCsv);

Errors:

cannot convert from 'IWatchServiceDatabase' to 'IWatchService'

cannot convert from 'IWatchServiceFiles' to 'IWatchService'

This seems to be understandable because otherwise "we would be able" to add CsvFile or XmlFile to dbConsumer where generic IDatabaseEntity is expected and CsvFile and XmlFile are in fact IFileEntity and from the other hand DatabaseQuery to filesConsumer which expects IFileEntity and DatabaseQuery is IDatabaseEntity

//Database related
dbConsumer.AddNewEntries(new DatabaseQuery());
dbConsumer.AddNewEntries(new CsvFile()); //illegal cause it's not FileConsumer !!!

//Files related
filesCsvConsumer.AddNewEntries(new CsvFile()); 
filesCsvConsumer.AddNewEntries(new DatabaseQuery()); //illegal cause it's not DbConsumer !!!

From my understanding this is the clue why compiler raise those errors and which is fine. Therefore I've decided to overcome it in this way:

public interface IWatchService<out TEntity> where TEntity : IEntity

 IEnumerable<TEntity> MatchingEntries get; 

As can be seen i marked generic parameter out and changed IList to IEnumerable because IEnumerable can be only foreached. Without possibility to modify the list.

Now having this there is no possibility to modify MatchingEntries e.g Add() on therefore we are now not able to add e.g CsvFile (IFileEntity) where IDatabaseEntity is expected and vice versa DatabaseQuery (IDatabaseEntity) where IFileEntity is expected. Fine and understandably.

At the end i have two main questions:

What is the benefit to have this: IEnumerable MatchingEntries get; since it's get; it cannot be initialized or populated with values therefore i would always get empty list when calling that property. Or i am in wrong? Can somebody explain showing based on my code what can be done with it?

Let's imagine i want to have possibility to Add items to this MatchingEntries list and in Consumer class i want still to be able to pass in ctor either Database or Files related classes based on interfaces. How this can be accomplished? Please also show an example based on current code.

Many thanks for your support and hope someone benefit from it as i saw a lot of confusions related to that topic.