How to implement a readonly (immutable) object interface in C#

Question

What I need is to make sure that in most scenarios objects are used via "readonly interface", which is a subset of the full interface.

• If I were in C++, I would just return a const object, for instance.
• If I could use interfaces, I would just implement a readonly interface and use it everywhere, however, I need operator overloading, which is not allowed for interfaces, that's why I have to use abstract base class.
• But if I define abstract base class, I am not allowed to change accessibility in the derived type..

Any ideas? Thanx

Answer 1

You could implement the readonly behaviour via state in the object, and throw an exception if a modifying method is called?

Answer 2

How about if you placed your write operations in an interface and then implement in explicitly on the abstract base class? It would not be a 100% perfect solution (you could still cast the object to the modification interface), but for most part it would prevent anyone from accidentally calling the modifying methods.

Answer 3

Do you really NEED operator overloading? We are talking about syntax sugar here. Plus, not all languages utilize operator overloading the same. If you use it you are effectively making your code language specific.

I would implement the readonly interface and drop the operator overloading requirement.

Answer 4

If anyone is interested in what I did, I finally went for an abstract class instead of interface, and I did hide the method in the derived class to get the right accessors:

Like, in the base abstract class (readonly interface):

protected double accuracy;
public double Accuracy { get { return accuracy; } }

In the derived class:

public new double Accuracy
{
    get { return accuracy; }
    set { accuracy = value; }
}

Of course, ugly with the "new" keyword, but in this case it will do for me.