Composition? Inheritance? Best approach?

composition_concrete.cs

/*
Pro: 
  - Least amount of code. 
  - Least amount of mapping. 
  - Least amount of repeated boilerplate code. 
  - Has "AddContents" method.
  - "GetFriendlyString" functionality is available to ShoppingCartWithContents.

Con: 
  - It is not very clear what properties this tree of composed classes contains. To get to the Cart Id you have to dig through cartWithContents.ShoppingCart.Id.
  - If something is composed using ShoppingCartWithContents we'd look at something like: premiumShoppingCartWithContents.ShoppingCartWithContents.ShoppingCart.Id.
  
Other:
  - Does not have the ability to change, override or otherwise mess with "GetFriendlyString", which can be a good thing or a bad thing.
  
*/
void Main()
{
  var source = new ShoppingCartDatabaseModel
  {
    Id = 1,
    OwnerName = "Test",
    Value = 1.21M
  };
  
  var cart = new ShoppingCart(source);

  var contents = new ShoppingCartContent
  {
    ItemId = 1,
    Quantity = 3
  };
  
  var cartWithContents = cart.AddContents(contents);
  
  cartWithContents.ShoppingCart.GetFriendlyString().Dump();  
  Debug.Assert(cartWithContents.Content != null && cartWithContents.ShoppingCart.OwnerName != null);
}

public class ShoppingCart
{
  public int Id { get; set; }
  public string OwnerName { get; set; }
  public decimal Value { get; set; }

  public ShoppingCart(ShoppingCartDatabaseModel source) {
    Id = source.Id;
    OwnerName = source.OwnerName;
    Value = source.Value;
  }

  public ShoppingCartWithContents AddContents(ShoppingCartContent content)
  {
    return new ShoppingCartWithContents(this, content);
  }

  public string GetFriendlyString()
  {
     return $"{Id}: {OwnerName} {Value}";
  }
}

public class ShoppingCartWithContents
{
  public ShoppingCart ShoppingCart { get; set; }
  public ShoppingCartContent Content { get; set; }

  public ShoppingCartWithContents(ShoppingCart shoppingCart, ShoppingCartContent content)
  {
    ShoppingCart = shoppingCart;
    Content = content;
  }
}

public class ShoppingCartDatabaseModel
{
  public int Id { get; set; }
  public string OwnerName { get; set; }
  public decimal Value { get; set; }
}

public class ShoppingCartContent
{
  public int ItemId { get; set; }
  public int Quantity { get; set; }
}

composition_interface_bad.cs

/*
Pro: 
  - Not too much code.
  - Properties are surfaced directly.
  - Has "AddContents" method.
  
Con: 
  - "GetFriendlyString" ( stand in for business logic ) is not available to ShoppingCartWithContents, meaning business logic would have to be repeated.
  - This con is big enough to make this unusable in my opinion.
  
*/
void Main()
{
  var source = new ShoppingCartDatabaseModel
  {
    Id = 1,
    OwnerName = "Test",
    Value = 1.21M
  };
  
  var cart = new ShoppingCart(source);

  var contents = new ShoppingCartContent
  {
    ItemId = 1,
    Quantity = 3
  };
  
  var cartWithContents = cart.AddContents(contents);
  
  cartWithContents.GetFriendlyString().Dump();  
  Debug.Assert(cartWithContents.Content != null && cartWithContents.OwnerName != null);
}

public interface IShoppingCart
{
  int Id { get; set; }
  string OwnerName { get; set; }
  decimal Value { get; set; }
  string GetFriendlyString();
}

public class ShoppingCart : IShoppingCart
{
  public int Id { get; set; }
  public string OwnerName { get; set; }
  public decimal Value { get; set; }

  public ShoppingCart(ShoppingCartDatabaseModel source) {
    Id = source.Id;
    OwnerName = source.OwnerName;
    Value = source.Value;
  }

  public ShoppingCartWithContents AddContents(ShoppingCartContent content)
  {
    return new ShoppingCartWithContents(this, content);
  }

  public string GetFriendlyString()
  {
    return $"{Id}: {OwnerName} {Value}";
  }
}

public class ShoppingCartWithContents : IShoppingCart
{
  public int Id { get; set; }
  public string OwnerName { get; set; }
  public decimal Value { get; set; }
  
  public ShoppingCartContent Content { get; set; }

  public ShoppingCartWithContents(IShoppingCart shoppingCart, ShoppingCartContent content)
  {
    Id = shoppingCart.Id;
    OwnerName = shoppingCart.OwnerName;
    Value = shoppingCart.Value;
    Content = content;
  }

  public string GetFriendlyString()
  {
    return $"{Id}: {OwnerName} {Value} With Items!";
  }
}

public class ShoppingCartDatabaseModel
{
  public int Id { get; set; }
  public string OwnerName { get; set; }
  public decimal Value { get; set; }
}

public class ShoppingCartContent
{
  public int ItemId { get; set; }
  public int Quantity { get; set; }
}

composition_interface_behavior.cs

/*
Pro: 
  - Properties are surfaced directly.
  - "GetFriendlyString" business logic is a modular piece that can be re-used, modified and expanded.
Con: 
  - Lots of code.
  - Id, OwnerName and Value properties are repeated 6 times, not counting the DB Model.
  - 4 Identical mapping code blocks, this id = that id, this ownername = that ownername etc.
  - No "AddContents" method. 
  
*/
void Main()
{
  var source = new ShoppingCartDatabaseModel
  {
    Id = 1,
    OwnerName = "Test",
    Value = 1.21M
  };
  
  var cart = new ShoppingCart(source);

  var contents = new ShoppingCartContent
  {
    ItemId = 1,
    Quantity = 3
  };
  
  var cartWithContents = new ShoppingCartWithContents(cart, contents);
  
  cartWithContents.FriendlyStringCreator.GetFriendlyString().Dump();
  Debug.Assert(cartWithContents.Content != null && cartWithContents.OwnerName != null);
}

public interface IShoppingCart
{
  int Id { get; set; }
  string OwnerName { get; set; }
  decimal Value { get; set; }

  IFriendlyStringCreator FriendlyStringCreator { get; set; }
}

public interface IFriendlyStringCreator
{
  int Id { get; set; }
  string OwnerName { get; set; }
  decimal Value { get; set; }
  
  string GetFriendlyString();
}

public class FriendlyStringCreator : IFriendlyStringCreator
{
  public int Id { get; set; }
  public string OwnerName { get; set; }
  public decimal Value { get; set; }
  
  public FriendlyStringCreator(int id, string ownerName, decimal value)
  {
    Id = id;
    OwnerName = ownerName;
    Value = value;
  }

  public string GetFriendlyString()
  {
    return $"{Id}: {OwnerName} {Value}";
  }
}

public class FriendlyStringCreatorForContents : IFriendlyStringCreator
{
  public int Id { get; set; }
  public string OwnerName { get; set; }
  public decimal Value { get; set; }

  public FriendlyStringCreatorForContents(int id, string ownerName, decimal value)
  {
    Id = id;
    OwnerName = ownerName;
    Value = value;
  }

  public string GetFriendlyString()
  {
    return $"{Id}: {OwnerName} {Value} With Items";
  }
}

public class ShoppingCart : IShoppingCart
{
  public int Id { get; set; }
  public string OwnerName { get; set; }
  public decimal Value { get; set; }
  public IFriendlyStringCreator FriendlyStringCreator { get; set;}

  public ShoppingCart(ShoppingCartDatabaseModel source) {
    Id = source.Id;
    OwnerName = source.OwnerName;
    Value = source.Value;
    FriendlyStringCreator = new FriendlyStringCreator(Id, OwnerName, Value);
  }
}

public class ShoppingCartWithContents : IShoppingCart
{
  public int Id { get; set; }
  public string OwnerName { get; set; }
  public decimal Value { get; set; }
  public IFriendlyStringCreator FriendlyStringCreator { get; set;}
  
  public ShoppingCartContent Content { get; set; }

  public ShoppingCartWithContents(IShoppingCart shoppingCart, ShoppingCartContent content)
  {
    Id = shoppingCart.Id;
    OwnerName = shoppingCart.OwnerName;
    Value = shoppingCart.Value;
    Content = content;
    FriendlyStringCreator = new FriendlyStringCreatorForContents(Id, OwnerName, Value);
  }
}

public class ShoppingCartDatabaseModel
{
  public int Id { get; set; }
  public string OwnerName { get; set; }
  public decimal Value { get; set; }
}

public class ShoppingCartContent
{
  public int ItemId { get; set; }
  public int Quantity { get; set; }
}

composition_interface_inheritance.cs

/*
Pro: 
  - Properties are surfaced directly.
  - "AddContents" method.
  - "GetFriendlyString" business logic is a modular piece that can be re-used, modified and expanded.
  - IShoppingCartWithItems and IShoppingCartWithoutItems can be used to target specific cart implementations
  
Con: 
  - Ungodly amount of code.
  - Id, OwnerName and Value properties are repeated 6 times, not counting the DB Model.
  - 4 Identical mapping code blocks, this id = that id, this ownername = that ownername etc. 
  
*/
void Main()
{
  var source = new ShoppingCartDatabaseModel
  {
    Id = 1,
    OwnerName = "Test",
    Value = 1.21M
  };
  
  var cart = new ShoppingCart(source);

  var contents = new ShoppingCartContent
  {
    ItemId = 1,
    Quantity = 3
  };
  
  var cartWithContents = cart.AddContents(contents);
  
  cartWithContents.FriendlyStringCreator.GetFriendlyString().Dump();
  Debug.Assert(cartWithContents.Content != null && cartWithContents.OwnerName != null);
}

public interface IShoppingCart
{
  int Id { get; set; }
  string OwnerName { get; set; }
  decimal Value { get; set; }
  
  IFriendlyStringCreator FriendlyStringCreator { get; set; }
}

public interface IFriendlyStringCreator
{
  int Id { get; set; }
  string OwnerName { get; set; }
  decimal Value { get; set; }

  string GetFriendlyString();
}

public class FriendlyStringCreator : IFriendlyStringCreator
{
  public int Id { get; set; }
  public string OwnerName { get; set; }
  public decimal Value { get; set; }

  public FriendlyStringCreator(int id, string ownerName, decimal value)
  {
    Id = id;
    OwnerName = ownerName;
    Value = value;
  }

  public string GetFriendlyString()
  {
    return $"{Id}: {OwnerName} {Value}";
  }
}

public class FriendlyStringCreatorForContents : IFriendlyStringCreator
{
  public int Id { get; set; }
  public string OwnerName { get; set; }
  public decimal Value { get; set; }
  public IFriendlyStringCreator FriendlyStringCreator { get; set;}

  public FriendlyStringCreatorForContents(int id, string ownerName, decimal value)
  {
    Id = id;
    OwnerName = ownerName;
    Value = value;
  }

  public string GetFriendlyString()
  {
    return $"{Id}: {OwnerName} {Value} With Items";
  }
}


public interface IShoppingCartWithoutContents : IShoppingCart
{
  IShoppingCartWithContents AddContents(IShoppingCartContent content);
}

public interface IShoppingCartWithContents : IShoppingCart
{
  IShoppingCartContent Content { get; set; }
}

public class ShoppingCart : IShoppingCartWithoutContents
{
  public int Id { get; set; }
  public string OwnerName { get; set; }
  public decimal Value { get; set; }
  public IFriendlyStringCreator FriendlyStringCreator { get; set;}

  public ShoppingCart(ShoppingCartDatabaseModel source) {
    Id = source.Id;
    OwnerName = source.OwnerName;
    Value = source.Value;
    FriendlyStringCreator = new FriendlyStringCreator(Id, OwnerName, Value);
  }

  public IShoppingCartWithContents AddContents(IShoppingCartContent content)
  {
    return new ShoppingCartWithContents(this, content);
  }
}

public class ShoppingCartWithContents : IShoppingCartWithContents
{
  public int Id { get; set; }
  public string OwnerName { get; set; }
  public decimal Value { get; set; }
  public IFriendlyStringCreator FriendlyStringCreator { get; set;}
  
  public IShoppingCartContent Content { get; set; }

  public ShoppingCartWithContents(IShoppingCart shoppingCart, IShoppingCartContent content)
  {
    Id = shoppingCart.Id;
    OwnerName = shoppingCart.OwnerName;
    Value = shoppingCart.Value;
    Content = content;
    FriendlyStringCreator = new FriendlyStringCreatorForContents(Id, OwnerName, Value);
  }
}

public class ShoppingCartDatabaseModel
{
  public int Id { get; set; }
  public string OwnerName { get; set; }
  public decimal Value { get; set; }
}

public interface IShoppingCartContent
{
  int ItemId { get; set; }
  int Quantity { get; set; }
}

public class ShoppingCartContent : IShoppingCartContent
{
  public int ItemId { get; set; }
  public int Quantity { get; set; }
}

inheritance.cs

/*
Pro: 
  - Not too much code.
  - Properties are surfaced directly.
  - "AddContents" method.
  - "GetFriendlyString" business logic can be overriden.
  
Con: 
  - "GetFriendlyString" business logic is tied directly to ShoppingCartBase and can not be re-used in other scenarios that require the same business logic.
  - ShoppingCartBase needs to know about ShoppingCartDatabaseModel.
  
*/

void Main()
{
  var source = new ShoppingCartDatabaseModel
  {
    Id = 1,
    OwnerName = "Test",
    Value = 1.21M
  };
  
  var cart = new ShoppingCart(source);

  var contents = new ShoppingCartContent
  {
    ItemId = 1,
    Quantity = 3
  };
  
  var cartWithContents = cart.AddContents(contents);
  
  cartWithContents.GetFriendlyString().Dump();  
  Debug.Assert(cartWithContents.Content != null && cartWithContents.OwnerName != null);
}

public class ShoppingCart
{
  public int Id { get; set; }
  public string OwnerName { get; set; }
  public decimal Value { get; set; }

  public ShoppingCart(ShoppingCartDatabaseModel source) {
    Id = source.Id;
    OwnerName = source.OwnerName;
    Value = source.Value;
  }

  public ShoppingCart(ShoppingCart shoppingCart)
  {
    Id = shoppingCart.Id;
    OwnerName = shoppingCart.OwnerName;
    Value = shoppingCart.Value;
  }

  public ShoppingCartWithContents AddContents(ShoppingCartContent content)
  {
    return new ShoppingCartWithContents(this, content);
  }

  public virtual string GetFriendlyString()
  {
    return $"{Id}: {OwnerName} {Value}";
  }
}

public class ShoppingCartWithContents : ShoppingCart
{
  public ShoppingCartContent Content { get; set; }

  public ShoppingCartWithContents(ShoppingCart shoppingCart, ShoppingCartContent content) : base(shoppingCart)
  {
    Content = content;
  }

  public override string GetFriendlyString()
  {
    return string.Format("{0} With Items", base.GetFriendlyString());
  }
}

public class ShoppingCartDatabaseModel
{
  public int Id { get; set; }
  public string OwnerName { get; set; }
  public decimal Value { get; set; }
}

public class ShoppingCartContent
{
  public int ItemId { get; set; }
  public int Quantity { get; set; }
}

inheritance_abstract.cs

/*
Pro: 
  - Not too much code.
  - Properties are surfaced directly.
  - "AddContents" method.
  - "GetFriendlyString" business logic can be overriden.
  - ShoppingCartBase does not need any knowledge of ShoppingCartDatabaseModel.
  
Con: 
  - "GetFriendlyString" business logic is tied directly to ShoppingCartBase and can not be re-used in other scenarios that require the same business logic.
  
*/

void Main()
{
  var source = new ShoppingCartDatabaseModel
  {
    Id = 1,
    OwnerName = "Test",
    Value = 1.21M
  };
  
  var cart = new ShoppingCart(source);

  var contents = new ShoppingCartContent
  {
    ItemId = 1,
    Quantity = 3
  };
  
  var cartWithContents = cart.AddContents(contents);
  
  cartWithContents.GetFriendlyString().Dump();
  Debug.Assert(cartWithContents.Content != null && cartWithContents.OwnerName != null);
}

public abstract class ShoppingCartBase
{
  public int Id { get; set; }
  public string OwnerName { get; set; }
  public decimal Value { get; set; }

  public ShoppingCartBase(int id, string ownerName, decimal value)
  {
    Id = id;
    OwnerName = ownerName;
    Value = value;
  }

  public ShoppingCartBase(ShoppingCartBase shoppingCartBase)
  {
    Id = shoppingCartBase.Id;
    OwnerName = shoppingCartBase.OwnerName;
    Value = shoppingCartBase.Value;
  }

  public virtual string GetFriendlyString()
  {
    return $"{Id}: {OwnerName} {Value}";
  }
}

public class ShoppingCart : ShoppingCartBase
{
  public ShoppingCart(ShoppingCartDatabaseModel source) : base(source.Id, source.OwnerName, source.Value) { }
  public ShoppingCartWithContents AddContents(ShoppingCartContent content)
  {
    return new ShoppingCartWithContents(this, content);
  }
}

public class ShoppingCartWithContents : ShoppingCartBase
{
  public ShoppingCartContent Content { get; set; }

  public ShoppingCartWithContents(ShoppingCartBase shoppingCart, ShoppingCartContent content) : base(shoppingCart)
  {
    Content = content;
  }

  public override string GetFriendlyString()
  {
    return string.Format("{0} With Items", base.GetFriendlyString());
  }
}

public class ShoppingCartDatabaseModel
{
  public int Id { get; set; }
  public string OwnerName { get; set; }
  public decimal Value { get; set; }
}

public class ShoppingCartContent
{
  public int ItemId { get; set; }
  public int Quantity { get; set; }
}