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));