Alternatives to nested interfaces (not possible in C#)

Question

I'm using interfaces in this case mostly as a handle to an immutable instance of an object. The problem is that nested interfaces in C# are not allowed. Here is the code:

public interface ICountry
{
    ICountryInfo Info { get; }

    // Nested interface results in error message:
    // Error    13  'ICountryInfo': interfaces cannot declare types
    public interface ICountryInfo
    {
        int Population { get; }
        string Note { get; }
    }
}


public class Country : ICountry
{
    CountryInfo Info { get; set; }

    public class CountryInfo : ICountry.ICountryInfo
    {
        int Population { get; set; }
        string Note { get; set; }
        .....
    }
    .....
}

I'm looking for an alternative, anybody would have a solution?

Answer 1

This will work just fine, no need to nest:

public interface ICountry
{
    ICountryInfo Info { get; }
}

public interface ICountryInfo
{
    int Population { get; }
    string Note { get; }
}

Answer 2

If ICountryInfo has no reason to exist outside ICountry, then why shouldn't you just put the properties of ICountryInfo in ICountry and dismiss the idea of nested interfaces?

An interface that hasn't a meaning of its own without another interface doesn't make sense to me, as an interface on itself is useless if not implemented by a class.

Answer 3

VB.NET allows this. So, you can create a VB.NET assembly only with the interface definitions that you need:

Public Interface ICountry
  ReadOnly Property Info() As ICountryInfo

  Public Interface ICountryInfo
    ReadOnly Property Population() As Integer
    ReadOnly Property Note() As String
  End Interface
End Interface

As for the implementation, C# does not support covariant return types, so you must declare your class like this:

public class Country : ICountry {
  // this property cannot be declared as CountryInfo
  public ICountry.ICountryInfo Info { get; set; }

  public class CountryInfo : ICountry.ICountryInfo {
    public string Note { get; set; }
    public int Population { get; set; }
  }
}