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

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; }   } } 

Answer 2

If the end goal is to use this with dependency injection, what's wrong with injecting them into each other instead of nesting?

public interface ICountry {     ICountryInfo Info { get; } }  public interface ICountryInfo {     int Population { get; set; }     string Note { get; set; } } 

and implement as:

public class Country : ICountry {     private readonly ICountryInfo _countryInfo;      public Country(ICountryInfo countryInfo)     {         _countryInfo = countryInfo;     }      public ICountryInfo Info     {         get { return _countryInfo; }     } }  public class CountryInfo : ICountryInfo {     public int Population { get; set; }     public string Note { get; set;} } 

Then once you set up your bindings for ICountry & ICountryInfo, CountryInfo will inject into Country whenever Country is injected.

You could then restrict the binding, if you wanted, to only inject CountryInfo into Country and nowhere else. Example in Ninject:

Bind<ICountry>().To<Country>(); Bind<ICountryInfo>().To<CountryInfo>().WhenInjectedInto<Country>();