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

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; }
  }
}