Adding a setter to a derived interface

Question

Is it possible somehow to achieve this behavior in C#:

public interface IReadOnly {     Data Value { get; } }  internal interface IWritable : IReadOnly  {     Data Value { get; set; } } 

I want to be able to expose a readonly interface to outside assemblies, but use a writable interface internally (which I could also implement in different ways).

I know I can use an abstract class which implements IReadOnly but adds setters, but that forces me to derive all internal implementations from that class.

Answer 1

This isn't a problem:

public interface IReadOnly {     Data Value { get; } }  internal interface IWritable : IReadOnly {     new Data Value { get; set; } }  internal class Impl : IWritable {     public Data Value { get; set; } } 

The Impl.Value property implementation takes care of both IReadOnly.Value and IWritable.Value, as demonstrated in this test snippet:

        var obj = new Data();         var target = new Impl();         var irw = (IWritable)target;         irw.Value = obj;         var iro = (IReadOnly)target;         System.Diagnostics.Debug.Assert(Object.ReferenceEquals(iro.Value, obj));